Jan Ehrenwerth
Harry A. Seifert
Key Points
1. A basic principle of electricity is known as Ohm's law (Voltage = Current × Resistance).
2. To have the completed circuit necessary for current flow, a closed loop must exist and a voltage source must drive the current through the impedance.
3. To receive a shock, one must contact the electrical circuit at two points, and there must be a voltage source that causes the current to flow through an individual.
4. In electrical terminology, grounding is applied to two separate concepts: the grounding of electrical power and the grounding of electrical equipment.
5. To provide an extra measure of safety from gross electrical shock (macroshock), the power supplied to most operating rooms (ORs) is ungrounded.
6. The line isolation monitor is a device that continuously monitors the integrity of an isolated power system.
7. The ground fault circuit interrupter is a popular device used to prevent individuals from receiving an electrical shock in a grounded power system.
8. An electrically susceptible patient (i.e., one who has a direct, external connection to the heart) they may be at risk from very small currents; this is called microshock.
9. Problems can arise if the electrosurgical return plate is improperly applied to the patient or if the cord connecting the return plate to the electrosurgical unit (ESU) is damaged or broken.
10. Fires in the OR are just as much a danger today as they were 100 years ago when patients were anesthetized with flammable anesthetic agents.
11. The necessary components for a fire consist of the triad of heat or an ignition source, a fuel, and an oxidizer.
12. The two major ignition sources for OR fires are the ESU and the laser.
13. It is known that desiccated carbon dioxide absorbent can, in rare circumstances, react with sevoflurane to produce a fire.
14. All OR personnel should be familiar with the location and operation of the fire extinguishers.
The myriad of electrical and electronic devices in the modern operating room (OR) greatly improve patient care and safety. However, these devices also subject both the patient and OR personnel to increased risks. To reduce the risk of electrical shock, most ORs have electrical systems that incorporate special safety features. It is incumbent upon the anesthesiologist to have a thorough understanding of the basic principles of electricity and an appreciation of the concepts of electrical safety applicable to the OR environment.
Principles of Electricity
A basic principle of electricity is known as Ohm's law, which is represented by the equation:
E = I × R
where E is electromotive force (in volts), I is current (in amperes), and R is resistance (in ohms). Ohm's law forms the basis for the physiologic equation BP = CO × SVR; that is, blood pressure (BP) is equal to the cardiac output (CO) times the systemic vascular resistance (SVR). In this case, the blood pressure of the vascular system is analogous to voltage, the cardiac output to current, and the systemic vascular resistance to the forces opposing the flow of electrons. Electrical power is measured in watts. Wattage (W) is the product of the voltage (E) and the current (I), as defined by the formula:
W = E × I
The amount of electrical work done is measured in watts multiplied by a unit of time. The watt-second (a joule, J) is a common designation for electrical energy expended in doing work. The energy produced by a defibrillator is measured in watt-seconds (or joules). The kilowatt-hour is used by electrical utility companies to measure larger quantities of electrical energy.
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Wattage can be thought of as a measure not only of work done but also of heat produced in any electrical circuit. Substituting Ohm's law in the formula:
W = E × I
W = (I × R) × I
W = I2 × R
Thus, wattage is equal to the square of the current I2 (amperage) times the resistance R. Using these formulas, it is possible to calculate the number of amperes and the resistance of a given device if the wattage and the voltage are known. For example, a 60-watt light bulb operating on a household 120-volt circuit would require 0.5 ampere of current for operation. Rearranging the formula so that:
I = W / E
we have:
I = (60 watts)/(120 volts)
I = 0.5 ampere
Using this in Ohm's law:
R = E/I
the resistance can be calculated to be 240 ohms:
R = (120 volts)/(0.5 ampere)
R = 240 ohms
It is obvious from the previous discussion that 1 volt of electromotive force (EMF) flowing through a 1-ohm resistance will generate 1 ampere of current. Similarly, 1 ampere of current induced by 1 volt of electromotive force will generate 1 watt of power.
Direct and Alternating Currents
Any substance that permits the flow of electrons is called a conductor. Current is characterized by electrons flowing through a conductor. If the electron flow is always in the same direction, it is referred to as direct current (DC). However, if the electron flow reverses direction at a regular interval, it is termed alternating current (AC). Either of these types of current can be pulsed or continuous in nature.
The previous discussion of Ohm's law is accurate when applied to DC circuits. However, when dealing with AC circuits, the situation is more complex because the flow of the current is opposed by a more complicated form of resistance, known as impedance.
Impedance
Impedance, designated by the letter Z, is defined as the sum of the forces that oppose electron movement in an AC circuit. Impedance consists of resistance (ohms) but also takes capacitance and inductance into account. In actuality, when referring to AC circuits, Ohm's law is defined as:
E = I × Z
An insulator is a substance that opposes the flow of electrons. Therefore, an insulator has a high impedance to electron flow, whereas a conductor has a low impedance to electron flow.
In AC circuits the capacitance and inductance can be important factors in determining the total impedance. Both capacitance and inductance are influenced by the frequency (cycles per second or hertz, Hz) at which the AC current reverses direction. The impedance is directly proportional to the frequency (f) times the inductance (IND):
Zα (f × IND)
and the impedance is inversely proportional to the product of the frequency (f) and the capacitance (CAP):
Zα1/(f × CAP)
As the AC current increases in frequency, the net effect of both capacitance and inductance increases. However, because impedance and capacitance are inversely related, total impedance decreases as the product of the frequency and the capacitance increases. Thus, as frequency increases, impedance falls and more current is allowed to pass.
Capacitance
A capacitor consists of any two parallel conductors that are separated by an insulator (Fig. 8-1). A capacitor has the ability to store charge. Capacitance is the measure of that substance's ability to store charge. In a DC circuit the capacitor plates are charged by a voltage source (i.e., a battery) and there is only a momentary current flow. The circuit is not completed and no further current can flow unless a resistance is connected between the two plates and the capacitor is discharged.
In contrast to DC circuits, a capacitor in an AC circuit permits current flow even when the circuit is not completed by a resistance. This is because of the nature of AC circuits, in which the current flow is constantly being reversed. Because current flow results from the movement of electrons, the capacitor plates are alternately charged—first positive and then negative with every reversal of the AC current direction—resulting in an effective current flow as far as the remainder of the circuit is concerned, even though the circuit is not completed.
Because the effect of capacitance on impedance varies directly with the AC frequency in hertz, the greater the AC frequency, the lower the impedance. Therefore, high-frequency currents (0.5 to 2 million Hz), such as those used by electrosurgical units (ESUs), will cause a marked decrease in impedance.
Electrical devices use capacitors for various beneficial purposes. There is, however, a phenomenon known as stray capacitance—capacitance that was not designed into the system but is incidental to the construction of the equipment. All AC-operated equipment produces stray capacitance. An ordinary power cord, for example, consisting of two insulated wires running next to each other will generate significant capacitance simply by being plugged into a 120-volt circuit, even though the piece of equipment is not turned on. Another example of stray capacitance is found in electric motors. The circuit wiring in electric motors generates stray capacitance to the metal housing of the motor. The clinical importance of capacitance will be emphasized later in the chapter.
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Figure 8-1. A capacitor consists of two parallel conductors separated by an insulator. The capacitor is capable of storing charge supplied by a voltage source. |
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Inductance
Whenever electrons flow in a wire, a magnetic field is induced around the wire. If the wire is coiled repeatedly around an iron core, as in a transformer, the magnetic field can be very strong. Inductance is a property of AC circuits in which an opposing EMF can be electromagnetically generated in the circuit. The net effect of inductance is to increase impedance. Because the effect of inductance on impedance also depends on AC frequency, increases in frequency will increase the total impedance. Therefore, the total impedance of a coil will be much greater than its simple resistance.
Electrical Shock Hazards
Alternating and Direct Currents
Whenever an individual contacts an external source of electricity, an electrical shock is possible. An electrical current can stimulate skeletal muscle cells to contract, and thus can be used therapeutically in devices such as pacemakers or defibrillators. However, casual contact with an electrical current, whether AC or DC, can lead to injury or death. Although it takes approximately 3 times as much DC as AC to cause ventricular fibrillation, this by no means renders DC harmless. Devices such as an automobile battery or a DC defibrillator can be sources of direct current shocks.
In the United States, utility companies supply electrical energy in the form of alternating currents of 120 volts at a frequency of 60 Hz. The 120 volts of EMF and 1 ampere of current are the effective voltage and amperage in an AC circuit. This is also referred to as RMS (root-mean-square). It takes 1.414 amperes of peak amperage in the sinusoidal curve to give an effective amperage of 1 ampere. Similarly, it takes 170 volts (120 × 1.414) at the peak of the AC curve to get an effective voltage of 120 volts. The 60 Hz refers to the number of times in 1 second that the current reverses its direction of flow. Both the voltage and current waveforms form a sinusoidal pattern (Fig. 8-2).
To have the completed circuit necessary for current flow, a closed loop must exist and a voltage source must drive the current through the impedance. If current is to flow in the electrical circuit, there has to be a voltage differential, or a drop in the driving pressure across the impedance. According to Ohm's law, if the resistance is held constant, then the greater the current flow, the larger the voltage drop must be.
The power company attempts to maintain the line voltage constant at 120 volts. Therefore, by Ohm's law the current flow is inversely proportional to the impedance. A typical power cord consists of two conductors. One, designated as hot carries the current to the impedance; the other is neutral, and it returns the current to the source. The potential difference between the two is effectively 120 volts (Fig. 8-3). The amount of current flowing through a given device is frequently referred to as the load. The load of the circuit depends on the impedance. A very high impedance circuit allows only a small current to flow and thus has a small load. A very low impedance circuit will draw a large current and is said to be a large load. A short circuit occurs when there is a zero impedance load with a very high current flow.1
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Figure 8-2. Sine wave flow of electrons in a 60-Hz alternating current. |
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Figure 8-3. A typical alternating current (AC) circuit where there is a potential difference of 120 volts between the hot and neutral sides of the circuit. The current flows through a resistance, which in AC circuits is more accurately referred to as impedance, and then returns to the electrical power company. |
Source of Shocks
Electrical accidents or shocks occur when a person becomes part of, or completes, an electrical circuit. To receive a shock, one must contact the electrical circuit at two points, and there must be a voltage source that causes the current to flow through an individual (Fig. 8-4).
When an individual contacts a source of electricity, damage occurs in one of two ways. First, the electrical current can disrupt the normal electrical function of cells. Depending on its magnitude, the current can contract muscles, alter brain function, paralyze respiration, or disrupt normal heart function, leading to ventricular fibrillation. The second mechanism involves the dissipation of electrical energy throughout the body's tissues. An electrical current passing through any resistance raises the temperature of that substance. If enough thermal energy is released, the
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temperature will rise sufficiently to produce a burn. Accidents involving household currents usually do not result in severe burns. However, in accidents involving very high voltages (i.e., power transmission lines), severe burns are common.
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Figure 8-4. An individual can complete an electric circuit and receive a shock by coming in contact with the hot side of the circuit (point A). This is because he or she is standing on the ground (point B) and the contact point A and the ground point B provide the two contact points necessary for a completed circuit. The severity of the shock that the individual receives depends on his or her skin resistance. |
The severity of an electrical shock is determined by the amount of current (number of amperes) and the duration of the current flow. For the purposes of this discussion, electrical shocks are divided into two categories. Macroshock refers to large amounts of current flowing through a person, which can cause harm or death. Microshock refers to very small amounts of current and applies only to the electrically susceptible patient. This is an individual who has an external conduit that is in direct contact with the heart. This can be a pacing wire or a saline-filled catheter such as a central venous or pulmonary artery catheter. In the case of the electrically susceptible patient, even minute amounts of current (microshock) may cause ventricular fibrillation.
Table 8-1 shows the effects typically produced by various currents following a 1-second contact with a 60-Hz current. When an individual contacts a 120-volt household current, the severity of the shock will depend on his or her skin resistance, the duration of the contact, and the current density. Skin resistance can vary from a few thousand to 1 million ohms. If a person with a skin resistance of 1,000 ohms contacts a 120-volt circuit, he or she would receive 120 milliamperes (mA) of current, which would probably be lethal. However, if that same person's skin resistance is 100,000 ohms, the current flow would be 1.2 mA, which would barely be perceptible.
I = E/R = (120 volts)/(1,000 ohms) = 120 mA
I = E/R = (120 volts)/(100,000 ohms) = 1.2 mA
The longer an individual is in contact with the electrical source, the more dire the consequences because more energy will be released and more tissue damaged. Also, there will be a greater chance of ventricular fibrillation from excitation of the heart during the vulnerable period of the electrocardiogram (ECG) cycle.
Current density is a way of expressing the amount of current that is applied per unit area of tissue. The diffusion of current in the body tends to be in all directions. The greater the current or the smaller the area to which it is applied, the higher the current density. In relation to the heart, a current of 100 mA (100,000 µA) is generally required to produce ventricular fibrillation when applied to the surface of the body. However, only 100 µA (0.1 mA) is required to produce ventricular fibrillation when that minute current is applied directly to the myocardium through an instrument having a very small contact area, such as a pacing wire electrode. In this case, the current density is 1,000-fold greater when applied directly to the heart; therefore, only 1/1,000 of the energy is required to cause ventricular fibrillation. In this case, the electrically susceptible patient can be electrocuted with currents well below 1 mA, which is the threshold of perception for humans. The frequency at which the current reverses is also an important factor in determining the amount of current an individual can safely contact. Utility companies in the United States produce electricity at a frequency of 60 Hz. They use 60 Hz because higher frequencies cause greater power loss through transmission lines and lower frequencies cause a detectable flicker from light sources.2 The “let-go” current is defined as that current above which sustained muscular contraction occurs and at which an individual would be unable to let go of an energized wire. The let-go current for a 60-Hz AC power is 10 to 20 mA,1,3,4 whereas at a frequency of 1 million Hz, up to 3 amperes (3,000 mA) is generally considered safe. It should be noted that very high frequency currents do not excite contractile tissue; consequently, they do not cause cardiac dysrhythmias.
It can be seen that Ohm's law governs the flow of electricity. For a completed circuit to exist, there must be a closed loop with a driving pressure to force a current through a resistance, just as in the cardiovascular system there must be a blood pressure to drive the cardiac output through the peripheral resistance. Figure 8-5 illustrates that a hot wire carrying a 120-volt pressure through the resistance of a 60-watt light bulb produces a current flow of 0.5 ampere. The voltage in the neutral wire is approximately 0 volts, while the current in the neutral wire remains at 0.5 ampere. This correlates with our cardiovascular analogy, where a mean blood pressure decrease of 80 mm Hg between the aortic root and the right atrium forces a cardiac output of 6 L/min through a systemic vascular resistance of 13.3 resistance units. However, the flow (in this case, the cardiac output, or in the case of the electrical model, the current) is still the same everywhere in the circuit. That is, the cardiac output on the arterial side is the same as the cardiac output on the venous side.
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Table 8-1 Effects of 60-Hz Current on an Average Human for A 1-Second Contact |
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Grounding
To fully understand electrical shock hazards and their prevention, one must have a thorough knowledge of the concepts of grounding. These concepts of grounding probably constitute the most confusing aspects of electrical safety because the same term is used to describe several different principles. In electrical terminology, grounding is applied to two separate concepts. The first is the grounding of electrical power, and the second is the grounding of electrical equipment. Thus, the concepts that (1) power can be grounded or ungrounded and that (2) power can supply electrical devices that are themselves grounded or ungrounded are not mutually exclusive. It is vital to understand this point as the basis of electrical safety (Table 8-2). Whereas electrical power is grounded in the home, it is usually ungrounded in the OR. In the home, electrical equipment may be grounded or ungrounded, but it should always be grounded in the OR.
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Figure 8-5. A 60-watt light bulb has an internal resistance of 240 ohms and draws 0.5 ampere of current. The voltage drop in the circuit is from 120 in the hot wire to 0 in the neutral wire, but the current is 0.5 ampere in both the hot and neutral wires. |
Electrical Power: Grounded
Electrical utilities universally provide power that is grounded (by convention, the earth-ground potential is zero, and all voltages represent a difference between potentials). That is, one of the wires supplying the power to a home is intentionally connected to the earth. The utility companies do this as a safety measure to prevent electrical charges from building up in their wiring during electrical storms. This also prevents the very high voltages used in transmitting power by the utility from entering the home in the event of an equipment failure in their high-voltage system.
The power enters the typical home via two wires. These two wires are attached to the main fuse or the circuit breaker box at the service entrance. The “hot” wire supplies power to the “hot” distribution strip. The neutral wire is connected to the neutral distribution strip and to a service entrance ground (i.e., a pipe buried in the earth; Fig. 8-6). From the fuse box, three wires leave to supply the electrical outlets in the house. In the United States, the hot wire is color-coded black and carries a voltage 120 volts above ground potential. The second wire is the neutral wire color-coded white; the third wire is the ground wire, which is either color-coded green or is uninsulated (bare wire). The ground and the neutral wires are attached at the same point in the circuit breaker box and then further connected to a cold-water pipe (Figs. 8-7 and 8-8). Thus, this grounded power system is also referred to as a neutral grounded power system. The black wire is not connected to the ground, as this would create a short circuit. The black wire is attached to the hot (i.e., 120 volts above ground) distribution strip on which the circuit breakers or fuses are located. From here, numerous branch circuits supply electrical power to the outlets in the house. Each branch circuit is protected by a circuit breaker or fuse that limits current to a specific maximum amperage. Most electrical circuits in the house are 15- or 20-ampere circuits. These typically supply power to the electrical outlets and lights in the house. Several higher amperage circuits are also provided for devices such as an electric stove or an electric clothes dryer. These devices are powered by 240-volt circuits, which can draw from 30 to 50 amperes of current. The circuit breaker or fuse will interrupt the flow of current on the hot side of the line in the event of a short circuit or if the demand placed on that circuit is too high. For example, a 15-ampere branch circuit will be capable of supporting 1,800 watts of power.
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Table 8-2 Differences Between Power and Equipment Grounding in the Home and the Operating Room |
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W = E/I
W = 120 volts × 15 amperes
W = 1,800 watts
Therefore, if two 1,500-watt hair dryers were simultaneously plugged into one outlet, the load would be too great for a 15-ampere circuit, and the circuit breaker would open (trip) or the fuse would melt. This is done to prevent the supply wires in the circuit from melting and starting a fire. The amperage of the circuit breaker on the branch circuit is determined by the thickness of the wire that it supplies. If a 20-ampere breaker is used with wire rated for only 15 amperes, the wire could melt and start a fire before the circuit breaker would trip. It is important to note that a 15-ampere circuit breaker does not protect an individual from lethal shocks. The 15 amperes of current that would trip the circuit breaker far exceeds the 100 to 200 mA that will produce ventricular fibrillation.
The wires that leave the circuit breaker supply the electrical outlets and lighting for the rest of the house. In older homes the electrical cable consists of two wires, a hot and a neutral, which supply power to the electrical outlets (Fig. 8-9). In newer homes, a third wire has been added to the electrical cable (Fig. 8-10). This third wire is either green or uninsulated (bare) and serves as a ground wire for the power receptacle
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(Fig. 8-11). On one end, the ground wire is attached to the electrical outlet (Fig. 8-12); on the other, it is connected to the neutral distribution strip in the circuit breaker box along with the neutral (white) wires (Fig. 8-13).
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Figure 8-6. In a neutral grounded power system, the electric company supplies two lines to the typical home. The neutral wire is connected to ground by the power company and again connected to a service entrance ground when it enters the fuse box. Both the neutral and ground wires are connected together in the fuse box at the neutral bus bar, which is also attached to the service entrance ground. |
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Figure 8-7. Inside a fuse box with the circuit breakers removed. The arrowheads indicate the hot wires energizing the strips where the circuit breakers are located. The arrowspoint to the neutral bus bar where the neutral and ground wires are connected. |
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Figure 8-8. The arrowhead indicates the ground wire from the fuse box attached to a cold-water pipe. |
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Figure 8-9. An older style electrical outlet consisting of just two wires (a hot and a neutral). There is no ground wire. |
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Figure 8-10. Modern electrical cable in which a third, or ground, wire has been added. |
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Figure 8-11. Modern electrical outlet in which the ground wire is present. The arrowhead points to the part of the receptacle where the ground wire connects. |
It should be realized that in both the old and new situations, the power is grounded. That is, a 120-volt potential exists between the hot (black) and the neutral (white) wire and between the hot wire and ground. In this case, the ground is the earth (Fig. 8-14). In modern home construction, there is still a 120-volt potential difference between the hot (black) and the neutral (white) wire as well as a 120-volt difference between the equipment ground wire (which is the third wire), and between the hot wire and earth (Fig. 8-15).
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Figure 8-12. Detail of modern electrical power receptacle. The arrow points to the ground wire, which is attached to the grounding screw on the power receptacle. |
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Figure 8-13. The ground wires from the power outlet are run to the neutral bus bar, where they are connected with the neutral wires (arrowheads). |
A 60-watt light bulb can be used as an example to further illustrate this point. Normally, the hot and neutral wires are connected to the two wires of the light bulb socket, and throwing the switch will illuminate the bulb (Fig. 8-16). Similarly, if the hot wire is connected to one side of the bulb socket and the other wire from the light bulb is connected to the equipment ground wire, the bulb will still illuminate. If there is no equipment ground wire, the bulb will still light if the second wire is connected to any grounded metallic object such as a water pipe or a faucet. This illustrates the fact that the 120-volt potential difference exists not only between the hot and the neutral wires but also between the hot wire and any grounded object. Thus, in a grounded power system, the current will flow between the hot wire and any conductor with an earth ground.
As previously stated, current flow requires a closed loop with a source of voltage. For an individual to receive an electric shock, he or she must contact the loop at two points. Because we may be standing on ground or be in contact with an object that is referenced to ground, only one additional contact point is necessary to complete the circuit and thus receive an electrical shock. This is an unfortunate and inherently dangerous consequence of grounded power systems. Modern wiring systems have added the third wire, the equipment ground wire, as a safety measure to reduce the severity of a potential electrical shock. This is accomplished by providing an alternate, low-resistance pathway through which the current can flow to ground.
Over time the insulation covering wires may deteriorate. It is then possible for a bare, hot wire to contact the metal case or frame of an electrical device. The case would then become energized and constitute a shock hazard to someone coming in contact with it. Figure 8-17 illustrates a typical short circuit, where the individual has come in contact with the hot case of an instrument. This illustrates the type of wiring found in older homes. There is no ground wire in the electrical outlet, nor is the electrical apparatus equipped with a ground wire. Here, the individual completes the circuit and receives a severe
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shock. Figure 8-18 illustrates a similar example, except that now the equipment ground wire is part of the electrical distribution system. In this example, the equipment ground wire provides a pathway of low impedance through which the current can travel; therefore, most of the current would travel through the ground wire. In this case, the person may get a shock, but it is unlikely to be fatal.
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Figure 8-14. Diagram of a house with older style wiring that does not contain a ground wire. A 120-volt potential difference exists between the hot and the neutral wires, as well as between the hot wire and the earth. |
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Figure 8-15. Diagram of a house with modern wiring in which the third, or ground, wire has been added. The 120-volt potential difference exists between the hot and neutral wires, the hot and the ground wires, and the hot wire and the earth. |
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Figure 8-16. A simple light bulb circuit in which the hot and neutral wires are connected with the corresponding wires from the light bulb fixture. |
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Figure 8-17. When a faulty piece of equipment without an equipment ground wire is plugged into an electrical outlet not containing a ground wire, the case of the instrument will become hot. An individual touching the hot case (point A) will receive a shock because he or she is standing on the earth (point B) and completes the circuit. The current (dashed line) will flow from the instrument through the individual touching the hot case. |
The electrical power supplied to homes is always grounded. A 120-volt potential always exists between the hot conductor and ground or earth. The third or equipment ground wire used in modern electrical wiring systems does not normally have current flowing through it. In the event of a short circuit, an electrical device with a three-prong plug (i.e., a ground wire connected to its case) will conduct the majority of the short-circuited or “fault” current through the ground wire and away from the individual. This provides a significant safety benefit to someone accidentally contacting the defective device. If a large enough fault current exists, the ground wire also will provide a means to complete the short circuit back to the circuit breaker or fuse, and this will either melt the fuse or trip the circuit breaker. Thus, in a grounded power system, it is possible to have either grounded or ungrounded equipment, depending on when the wiring was installed and whether the electrical device is equipped with a three-prong plug containing a ground wire. Obviously, attempts to bypass the safety system of the equipment ground should be avoided. Devices such as a “cheater plug” (Fig. 8-19) should never be used because they defeat the safety feature of the equipment ground wire.
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Figure 8-18. When a faulty piece of equipment containing an equipment ground wire is properly connected to an electrical outlet with a grounding connection, the current (dashed line) will preferentially flow down the low-resistance ground wire. An individual touching the case (point A) while standing on the ground (point B) will still complete the circuit; however, only a small part of the current will go through the individual. |
Electrical Power: Ungrounded
Numerous electronic devices, together with power cords and puddles of saline solutions on the floor, make the OR an electrically hazardous environment for both patients and personnel. Bruner et al.5 found that 40% of electrical accidents in hospitals occurred in the OR. The complexity of electrical equipment in the modern OR demands that electrical safety be a factor of paramount importance. To provide an extra measure of safety from macroshock, the power supplied to most ORs is ungrounded. In this ungrounded power system, the current is isolated from ground potential. The 120-volt potential difference exists only between the two wires of the isolated power system, but no circuit exists between the ground and either of the isolated power lines.
Supplying ungrounded power to the OR requires the use of an isolation transformer (Fig. 8-20). This device uses electromagnetic induction to induce a current in the ungrounded or secondary winding of the transformer from energy supplied to the primary winding. There is no direct electrical connection between the power supplied by the utility company on the primary side and the power induced by the transformer on the ungrounded or secondary side. Thus, the power supplied to the OR is isolated from ground (Fig. 8-21). Because the 120-volt potential exists only between the two wires of the isolated circuit, neither wire is hot or neutral with reference to ground. In this case, they are simply referred to as line 1 and
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line 2 (Fig. 8-22). Using the example of the light bulb, if one connects the two wires of the bulb socket to the two wires of the isolated power system, the light will illuminate. However, if one connects one of the wires to one side of the isolated power and the other wire to ground, the light will not illuminate. If the wires of the isolated power system are connected, the short circuit will trip the circuit breaker. In comparing the two systems, the standard grounded power has a direct connection to ground, whereas the isolated system imposes a very high impedance to any current flow to ground. The added safety of this system can be seen in Figure 8-23. In this case, a person has come in contact with one side of the isolated power system (point A). Because standing on ground (point B) does not constitute a part of the isolated circuit, the individual does not complete the loop and will not receive a shock. This is because the ground is part of the primary circuit (solid lines), and the person is contacting only one side of the isolated secondary circuit (cross-hatched lines). The person does not complete either circuit (i.e., have two contact points); therefore, this situation does not pose an electric shock hazard. Of course, if the person contacts both lines of the isolated power system (an unlikely event), he or she would receive a shock.
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Figure 8-19. Right. A “cheater plug” that converts a three-prong power cord to a two-prong cord. Left. The wire attached to the cheater plug is rarely connected to the screw in the middle of the outlet. This totally defeats the purpose of the equipment ground wire. |
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Figure 8-20. A. Isolated power panel showing circuit breakers, line isolation monitor, and isolation transformer (arrow). B. Detail of an isolation transformer with the attached warning lights. The arrow points to ground wire connection on the primary side of the transformer. Note that no similar connection exists on the secondary side of the transformer. |
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Figure 8-21. In the operating room, the isolation transformer converts the grounded power on the primary side to an ungrounded power system on the secondary side of the transformer. A 120-volt potential difference exists between line 1 and line 2. There is no direct connection from the power on the secondary side to ground. The equipment ground wire, however, is still present. |
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Figure 8-22. Detail of the inside of a circuit breaker box in an isolated power system. The bottom arrow points to ground wires meeting at the common ground terminal. Arrows 1 and 2 indicate lines 1 and 2 from the isolated power circuit breaker. Neither line 1 nor line 2 is connected to the same terminals as the ground wires. This is in marked contrast to Figure 8-13, where the neutral and ground wires are attached at the same point. |
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Figure 8-23. A safety feature of the isolated power system is illustrated. An individual contacting one side of the isolated power system (point A) and standing on the ground (point B) will not receive a shock. In this instance, the individual is not contacting the circuit at two points and thus is not completing the circuit. Point A (cross-hatched lines) is part of the isolated power system, and point B is part of the primary or grounded side of the circuit (solid lines). |
If a faulty electrical appliance with an intact equipment ground wire is plugged into a standard household outlet, and the home wiring has a properly connected ground wire, then the amount of electrical current that will flow through the individual is considerably less than what will flow through the low-resistance ground wire. Here, an individual would be fairly well protected from a serious shock. However, if that ground wire were broken, the individual might receive a lethal shock. No shock would occur if the same faulty piece of equipment were plugged into the isolated power system, even if the equipment ground wire were broken. Thus, the isolated power system provides a significant amount of protection from macroshock. Another feature of the isolated power system is that the faulty piece of equipment, even though it may be partially short-circuited, will not usually trip the circuit breaker. This is an important feature because the faulty piece of equipment may be part of a life-support system for a patient. It is important to note that even though the power is isolated from ground, the case or frame of all electrical equipment is still connected to an equipment ground. The third wire (equipment ground wire) is necessary for a total electrical safety program.
Figure 8-24 illustrates a scenario involving a faulty piece of equipment connected to the isolated power system. This does not represent a hazard; it merely converts the isolated power back to a grounded power system as exists outside the OR. In fact, a second fault is necessary to create a hazard.
The previous discussion assumes that the isolated power system is perfectly isolated from ground. Actually, perfect isolation is impossible to achieve. All AC-operated power systems and electrical devices manifest some degree of capacitance. As previously discussed, electrical power cords, wires, and electrical motors exhibit capacitive coupling to the ground wire and metal conduits and “leak” small amounts of current to ground (Fig. 8-25). This so-called leakage current partially ungrounds the isolated power system. This does not usually amount to more than a few milliamperes in an OR. So an individual coming in contact with one side of the isolated power system would receive only a very small shock (1 to 2 mA). Although this amount of current would be perceptible, it would not be dangerous.
The Line Isolation Monitor
The line isolation monitor (LIM) is a device that continuously monitors the integrity of an isolated power system. If a faulty
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piece of equipment is connected to the isolated power system, this will, in effect, change the system back to a conventional grounded system. Also, the faulty piece of equipment will continue to function normally. Therefore, it is essential that a warning system be in place to alert the personnel that the power is no longer ungrounded. The LIM continuously monitors the isolated power to ensure that it is indeed isolated from ground, and the device has a meter that displays a continuous indication of the integrity of the system (Fig. 8-26). The LIM is actually measuring the impedance to ground of each side of the isolated power system. As previously discussed, with perfect isolation, impedance would be infinitely high and there would be no current flow in the event of a first fault situation (Z = E/I; if I = 0, then Z = ∞). Because all AC wiring and all AC-operated electrical devices have some capacitance, small leakage currents are present that partially degrade the isolation of the system. The meter of the LIM will indicate (in milliamperes) the total amount of leakage in the system resulting from capacitance, electrical wiring, and any devices plugged into the isolated power system.
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Figure 8-24. A faulty piece of equipment plugged into the isolated power system does not present a shock hazard. It merely converts the isolated power system into a grounded power system. The figure inset illustrates that the isolated power system is now identical to the grounded power system. The dashed line indicates current flow in the ground wire. |
The reading on the LIM meter does not mean that current is actually flowing; rather, it indicates how much current would flow in the event of a first fault. The LIM is set to alarm at 2 or 5 mA, depending on the age and brand of the system. Once this preset limit is exceeded, visual and audible alarms are triggered to indicate that the isolation from ground has been degraded beyond a predetermined limit (Fig. 8-27). This does not necessarily mean that there is a hazardous situation, but rather that the system is no longer totally isolated from ground. It would require a second fault to create a dangerous situation.
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Figure 8-25. The capacitance that exists in alternating current (AC) power lines and AC-operated equipment results in small “leakage currents” that partially degrade the isolated power system. |
For example, if the LIM were set to alarm at 2 mA, using Ohm's law, the impedance for either side of the isolated power system would be 60,000 ohms:
Z = E/I
Z = (120 volts)/(0.002 ampere)
Z = 60,000 ohms
Therefore, if either side of the isolated power system had less than 60,000 ohms impedance to ground, the LIM would trigger an alarm. This might occur in two situations. In the first situation, a faulty piece of equipment is plugged into the isolated power system. In this case, a true fault to ground exists
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from one line to ground. Now the system would be converted to the equivalent of a grounded power system. This faulty piece of equipment should be removed and serviced as soon as possible. However, this piece of equipment could still be used safely if it were essential for the care of the patient. It should be remembered, however, that continuing to use this faulty piece of equipment would create the potential for a serious electrical shock. This would occur if a second faulty piece of equipment were simultaneously connected to the isolated power system.
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Figure 8-26. The meter of the line isolation monitor (LIM) is calibrated in milliamperes. If the isolation of the power system is degraded such that >2 mA (5 mA in newer systems) of current could flow, the hazard light will illuminate and a warning buzzer will sound. Note the button for testing the hazard warning system. A. Older LIM that will trigger an alarm at 2 mA. B. Newer LIM that will trigger an alarm at 5 mA. |
The second situation involves connecting many perfectly normal pieces of equipment to the isolated power system. Although each piece of equipment has only a small amount of leakage current, if the total leakage exceeds 2 mA, the LIM will trigger an alarm. Assume that in the same OR there are 30 electrical devices, each having 100 µA of leakage current. The total leakage current (30 × 100 µA) would be 3 mA. The impedance to ground would still be 40,000 ohms (120/0.003). The LIM alarm would sound because the 2-mA set point was violated. However, the system is still safe and represents a state significantly different from that in the first situation. For this reason, the newer LIMs are set to alarm at 5 mA instead of 2 mA.
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Figure 8-27. When a faulty piece of equipment is plugged into the isolated power system, it will markedly decrease the impedance from line 1 or line 2 to ground. This will be detected by the line isolation monitor, which will sound an alarm. |
The newest LIMs are referred to as third-generation monitors. The first-generation monitor, or static LIM, was unable to detect balanced faults (i.e., a situation in which there are equal faults to ground from both line 1 and line 2). The second-generation, or dynamic, LIM did not have this problem but could interfere with physiologic monitoring. Both of these monitors would trigger an alarm at 2 mA, which led to annoying “false” alarms. The third-generation LIM corrects the problems of its predecessors and has the alarm threshold set at 5 mA.6 Proper functioning of the LIM depends on having both intact equipment ground wires
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as well as its own connection to ground. First- and second-generation LIMs could not detect the loss of the LIM ground connection. The third-generation LIM can detect this loss of ground to the monitor. In this case the LIM alarm would sound and the red hazard light would illuminate, but the LIM meter would read zero. This condition will alert the staff that the LIM needs to be repaired. However, the LIM still cannot detect broken equipment ground wires. An example of the third-generation LIM is the Iso-Gard made by the Square D Company (Monroe, NC).
The equipment ground wire is again an important part of the safety system. If this wire is broken, a faulty piece of equipment that is plugged into an outlet would operate normally, but the LIM would not alarm. A second fault could therefore cause a shock, without any alarm from the LIM. Also, in the event of a second fault, the equipment ground wire provides a low-resistance path to ground for most of the fault current (see Fig. 8-24). The LIM will only be able to register leakage currents from pieces of equipment that are connected to the isolated power system and have intact ground wires.
If the LIM alarm is triggered, the first thing to do is to check the gauge to determine if it is a true fault. The other possibility is that too many pieces of electrical equipment have been plugged in and the 2-mA limit has been exceeded. If the gauge is between 2 and 5 mA, it is probable that too much electrical equipment has been plugged in. If the gauge reads >5 mA, most likely there is a faulty piece of equipment present in the OR. The next step is to identify the faulty equipment, which is done by unplugging each piece of equipment until the alarm ceases. If the faulty piece of equipment is not of a life-support nature, it should be removed from the OR. If it is a vital piece of life-support equipment, it can be safely used. However, it must be remembered that the protection of the isolated power system and the LIM is no longer operative. Therefore, if possible, no other electrical equipment should be connected during the remainder of the case, or until the faulty piece of equipment can be safely removed.
Ground Fault Circuit Interrupter
The ground fault circuit interrupter (GFCI, or occasionally abbreviated as GFI) is another popular device used to prevent individuals from receiving an electrical shock in a grounded power system. Electrical codes for most new construction require that a GFCI circuit be present in potentially hazardous (e.g., wet) areas such as bathrooms, kitchens, or outdoor electrical outlets. The GFCI may be installed as an individual power outlet (Fig. 8-28) or may be a special circuit breaker to which all the individual protected outlets are connected at a single point. The special GFCI circuit breaker is located in the main fuse/circuit breaker box and can be distinguished by its red test button (Fig. 8-29). As Figure 8-5demonstrates, the current flowing in both the hot and neutral wires is usually equal. The GFCI monitors both sides of the circuit for the equality of current flow; if a difference is detected, the power is immediately interrupted. If an individual should contact a faulty piece of equipment such that current flowed through the individual, an imbalance between the two sides of the circuit would be created, which would be detected by the GFCI. Because the GFCI can detect very small current differences (in the range of 5 mA), the GFCI will open the circuit in a few milliseconds, thereby interrupting the current flow before a significant shock occurs. Thus, the GFCI provides a high level of protection at a very modest cost.
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Figure 8-28. A ground fault circuit interrupter electrical outlet with integrated test and reset buttons. |
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Figure 8-29. Special ground fault circuit interrupter circuit breaker. The arrowhead points to the distinguishing red test button. |
The disadvantage of using a GFCI in the OR is that it interrupts the power without warning. A defective piece of equipment could no longer be used, which might be a problem if it were of a life-support nature, whereas if the same faulty piece of equipment were plugged into an isolated power system, the LIM would alarm but the equipment could still be used.
Double Insulation
There is one instance in which it is acceptable for a piece of equipment to have only a two-prong and not a three-prong plug. This is permitted when the instrument has what is termed double insulation. These instruments have two layers of insulation and usually have a plastic exterior. Double insulation is found in many home power tools and is seen in hospital equipment such as infusion pumps. Double-insulated equipment is permissible in the OR with isolated power systems. However, if water or saline should get inside the unit, there could be a hazard because the double insulation is bypassed. This is even more serious if the OR has no isolated power or GFCIs.7
Microshock
As previously discussed, macroshock involves relatively large amounts of current applied to the surface of the body. The current is conducted through all the tissues in proportion to their conductivity and area in a plane perpendicular to the current. Consequently, the “density” of the current (amperes per meter squared) that reaches the heart is considerably less than what is applied to the body surface. However, an electrically susceptible patient (i.e., one who has a direct, external connection to
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the heart, such as through a central venous pressure catheter or transvenous cardiac pacing wires) may be at risk from very small currents; this is called microshock.8 The catheter orifice or electrical wire with a very small surface area in contact with the heart produces a relatively large current density at the heart.9 Stated another way, even very small amounts of current applied directly to the myocardium will cause ventricular fibrillation. Microshock is a particularly difficult problem because of the insidious nature of the hazard.
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Figure 8-30. The electrically susceptible patient is protected from microshock by the presence of an intact equipment ground wire. The equipment ground wire provides a low-impedance path in which the majority of the leakage current (dashed lines) can flow. R, resistance. |
In the electrically susceptible patient, ventricular fibrillation can be produced by a current that is below the threshold of human perception. The exact amount of current necessary to cause ventricular fibrillation in this type of patient is unknown. Whalen et al.10 were able to produce fibrillation with 20 µA of current applied directly to the myocardium of dogs. Raftery et al.11 produced fibrillation with 80 µA of current in some patients. Hull12 used data obtained by Watson et al.13 to show that 50% of patients would fibrillate at currents of 200 µA. Because 1,000 µA (1 mA) is generally regarded as the threshold of human perception with 60-Hz AC, the electrically susceptible patient can be electrocuted with one-tenth the normally perceptible currents. This is not only of academic interest but also of practical concern because many cases of ventricular fibrillation from microshock have been reported.14,15,16,17,18
The stray capacitance that is part of any AC-powered electrical instrument may result in significant amounts of charge buildup on the case of the instrument. If an individual simultaneously touches the case of an instrument where this has occurred and the electrically susceptible patient, he or she may unknowingly cause a discharge to the patient that results in ventricular fibrillation. Once again, the equipment ground wire constitutes the major source of protection against microshock for the electrically susceptible patient. In this case, the equipment ground wire provides a low-resistance path by which most of the leakage current is dissipated instead of stored as a charge.
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Figure 8-31. A broken equipment ground wire results in a significant hazard to the electrically susceptible patient. In this case, the entire leakage current can be conducted to the heart and may result in ventricular fibrillation. R, resistance. |
Figure 8-30 illustrates a situation involving a patient with a saline-filled catheter in the heart with a resistance of ~500 ohms. The ground wire with a resistance of 1 ohm is connected to the instrument case. A leakage current of 100 µA will divide according to the relative resistances of the two paths. In this case, 99.8 µA will flow through the equipment ground wire and only 0.2 µA will flow through the fluid-filled catheter. This extremely small current does not endanger the patient. However, if the equipment ground wire were broken, the electrically susceptible patient would be at great risk because all 100 µA of leakage current could flow through the catheter and cause ventricular fibrillation (Fig. 8-31). Currently, electronic equipment is permitted 100 µA of leakage current.
Modern patient monitors incorporate another mechanism to reduce the risk of microshock for electrically susceptible patients.19 This mechanism involves electrically isolating all direct patient connections from the power supply of the monitor by placing a very high impedance between the patient and
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any device. This limits the amount of internal leakage through the patient connection to a very small value. The standard currently is <10 µA. For instance, the output of an ECG monitor's power supply is electrically isolated from the patient by placing a very high impedance between the monitor and the patient's ECG leads.20 Isolation techniques are designed to inhibit hazardous electrical pathways between the patient and the monitor while allowing the passage of the physiologic signal.
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Figure 8-32. A. A hospital-grade plug that can be visually inspected. The arrow points to the equipment ground wire whose integrity can be readily verified. B. A hospital-grade plug that can be easily disassembled for inspection. Note that the prong for the ground wire (arrow) is longer than the hot or neutral prong, so that it is the first to enter the receptacle. C. The arrow points to the green dot denoting a hospital-grade power outlet. |
An intact equipment ground wire is probably the most important factor in preventing microshock. There are, however, other things that the anesthesiologist can do to reduce the incidence of microshock. One should never simultaneously touch an electrical device and a saline-filled central catheter or external pacing wires. Whenever one is handling a central catheter or pacing wires, it is best to insulate oneself by wearing rubber gloves. Also, one should never let any external current source, such as a nerve stimulator, come into contact with the catheter or wires. Finally, one should be alert to potential sources of energy that can be transmitted to the patient. Even stray radiofrequency current from the ESU (cautery) can, with the right conditions, be a source of microshock.21 It must be remembered that the LIM is not designed to provide protection from microshock. The microampere currents involved in microshock are far below the LIM threshold of protection. In addition, the LIM does not register the leakage of individual monitors, but rather indicates the status of the total system. The LIM reading indicates the total amount of leakage current resulting from the entire capacitance of the system. This is the amount of current that would flow to ground in the event of a first-fault situation.
The essence of electrical safety is a thorough understanding of all the principles of grounding. The objective of electrical safety is to make it difficult for electrical current to pass through people. For this reason, both the patient and the anesthesiologist should be isolated from ground as much as possible. That is, their resistance to current flow should be as high as is technologically feasible. In the inherently unsafe electrical environment of an OR, several measures can be taken to help protect against contacting hazardous current flows. First, the grounded power provided by the utility company can be converted to ungrounded power by means of an isolation transformer. The LIM will continuously monitor the status of this isolation from ground and warn that the isolation of the power (from ground) has been lost in the event that a defective piece of equipment is plugged into one of the isolated circuit outlets. In addition, the shock that an individual could receive from a faulty piece of equipment is determined by the capacitance of the system and is limited to a few milliamperes. Second, all equipment plugged into the isolated power system has an equipment ground wire that is attached to the case of the instrument. This equipment ground wire provides an alternative low-resistance pathway enabling potentially dangerous currents to flow to ground. Thus, the patient and the anesthesiologist should be as insulated from ground as possible and all electrical equipment should be grounded.
The equipment ground wire serves three functions. First, it provides a low-resistance path for fault currents to reduce the risk of macroshock. Second, it dissipates leakage currents that are potentially harmful to the electrically susceptible patient. Third, it provides information to the LIM on the status of the ungrounded power system. If the equipment ground wire is broken, a significant factor in the prevention of electrical shock is lost. Additionally, the isolated power system will appear safer than it actually is because the LIM is unable to detect broken equipment ground wires.
Because power cord plugs and receptacles are subjected to greater abuse in the hospital than in the home, the Underwriters Laboratories (Melville, NY) has issued a strict specification for special “hospital-grade” plugs and receptacles (Fig. 8-32). The plugs and receptacles that conform to this specification are marked by a green dot.22 The hospital-grade plug is one that can be visually inspected or easily disassembled to ensure the integrity of the ground wire connection. Molded opaque plugs are not acceptable. Edwards23reported that of 3,000 nonhospital-grade receptacles installed in a new hospital building, 1,800 (60%) were defective after 3 years. When 2,000 of the nonhospital-grade receptacles were replaced with ones of hospital grade, no failures had occurred after 18 months of use.
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Electrosurgery
On that fateful October day in 1926 when Dr. Harvey W. Cushing first used an electrosurgical machine invented by Professor William T. Bovie to resect a brain tumor, the course of modern surgery and anesthesia was forever altered.24 The ubiquitous use of electrosurgery attests to the success of Professor Bovie's invention. However, this technology was not adopted without a cost. The widespread use of electrocautery has, at the very least, hastened the elimination of explosive anesthetic agents from the OR. In addition, as every anesthesiologist is aware, few things in the OR are immune to interference from the “Bovie.” The high-frequency electrical energy generated by the ESU interferes with everything from the ECG signal to cardiac output computers, pulse oximeters, and even implanted cardiac pacemakers.25
The ESU operates by generating very-high-frequency currents (radiofrequency range) of anywhere from 500,000 to 1 million Hz. Heat is generated whenever a current passes through a resistance. The amount of heat (H) produced is proportional to the square of the current and inversely proportional to the area through which the current passes (H = I2/A).26 By concentrating the energy at the tip of the “Bovie pencil,” the surgeon can produce either a cut or a coagulation at any given spot. This very-high-frequency current behaves differently from the standard 60-Hz AC current and can pass directly across the precordium without causing ventricular fibrillation.26 This is because high-frequency currents have a low tissue penetration and do not excite contractile cells.
The large amount of energy generated by the ESU can pose other problems to the operator and the patient. Dr. Cushing became aware of one such problem. He wrote, “Once the operator received a shock which passed through a metal retractor to his arm and out by a wire from his headlight, which was unpleasant to say the least.”27 The ESU cannot be safely operated unless the energy is properly routed from the ESU through the patient and back to the unit. Ideally, the current generated by the active electrode is concentrated at the ESU tip, constituting a very small surface area. This energy has a high current density and is able to generate enough heat to produce a therapeutic cut or coagulation. The energy then passes through the patient to a dispersive electrode of large surface area that returns the energy safely to the ESU (Fig. 8-33).
One unfortunate quirk in terminology concerns the return (dispersive) plate of the ESU. This plate, often incorrectly referred to as a ground plate, is actually a dispersive electrode of large surface area that safely returns the generated energy to the ESU via a low current density pathway. When inquiring whether the dispersive electrode has been attached to the patient, OR personnel frequently ask, “Is the patient grounded?” Because the aim of electrical safety is to isolate the patient from ground, this expression is worse than erroneous; it can lead to confusion. Because the area of the return plate is large, the current density is low; therefore, no harmful heat is generated and no tissue destruction occurs. In a properly functioning system, the only tissue effect is at the site of the active electrode that is held by the surgeon.
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Figure 8-33. A properly applied electrosurgical unit (ESU) return plate. The current density at the return plate is low, resulting in no danger to the patient. |
Problems can arise if the electrosurgical return plate is improperly applied to the patient or if the cord connecting the return plate to the ESU is damaged or broken. In these instances, the high-frequency current generated by the ESU will seek an alternate return pathway. Anything attached to the patient, such as ECG leads or a temperature probe, can provide this alternate return pathway. The current density at the ECG pad will be considerably higher than normal because its surface area is much less than that of the ESU return plate. This may result in a serious burn at this alternate return site. Similarly, a burn may occur at the site of the ESU return plate if it is not properly applied to the patient or if it becomes partially dislodged during the operation (Fig. 8-34). This is not merely a theoretical possibility but is evidenced by the numerous case reports involving patients who have received ESU burns.28,29,30,31,32,33
The original ESUs were manufactured with the power supply connected directly to ground by the equipment ground wire. These devices made it extremely easy for ESU current to return by alternate pathways. The ESU would continue to operate normally even without the return plate connected to the patient. In most modern ESUs, the power supply is isolated from ground to protect the patient from burns. It was hoped that by isolating the return pathway from ground, the only route for current flow would be via the return electrode. Theoretically, this would eliminate alternate return pathways and greatly reduce the incidence of burns. However, Mitchell34 found two situations in which the current could return via alternate pathways, even with the isolated ESU circuit. If the return plate were left either on top of an uninsulated ESU cabinet or in contact with the bottom of the OR table, then the ESU could operate fairly normally and the current would return via alternate pathways. It will be recalled that the impedance is inversely proportional to the capacitance times the current frequency. The ESU operates at 500,000 to >1,000,000 Hz, which greatly enhances the effect of capacitive coupling and causes a marked reduction in impedance. Therefore, even with isolated ESUs, the decrease in impedance allows the current to return to the ESU by alternate pathways. In addition, the isolated ESU does not protect the patient from burns if the return electrode does not make proper contact with the patient. Although the isolated ESU does provide additional
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patient safety, it is by no means foolproof protection against the patient receiving a burn.
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Figure 8-34. An improperly applied electrosurgical unit (ESU) return plate. Poor contact with the return plate results in a high current density and a possible burn to the patient. |
Preventing patient burns from the ESU is the responsibility of all professional staff in the OR. Not only the circulating nurse, but also the surgeon and the anesthesiologist must be aware of proper techniques and be vigilant to potential problems. The most important factor is the proper application of the return plate. It is essential that the return plate has the appropriate amount of electrolyte gel and an intact return wire. Reusable return plates must be properly cleaned after each use, and disposable plates must be checked to ensure that the electrolyte has not dried out during storage. In addition, it is prudent to place the return plate as close as possible to the site of the operation. ECG pads should be placed as far from the site of the operation as is feasible. OR personnel must be alert to the possibility that pools of flammable “prep” solutions such as alcohol and acetone can ignite when the ESU is used. If the ESU must be used on a patient with a demand pacemaker, the return electrode should be located below the thorax, and preparations for treating potential dysrhythmias should be available, including a magnet to convert the pacemaker to a fixed rate, a defibrillator, and an external pacemaker. It is best to keep the pacemaker out of the path between the surgical site and the dispersal plate.
The ESU has also caused other problems in patients with pacemakers, including reprogramming and microshock.35,36 If the surgeon requests higher than normal power settings on the ESU, this should alert both the circulating nurse and the anesthesiologist to a potential problem. The return plate and cable must be immediately inspected to ensure that it is functioning and properly positioned. If this does not correct the problem, the return plate should be replaced.37,38 If the problem remains, the entire ESU should be taken out of service. Finally, an ESU that is dropped or damaged must be removed immediately from the OR and thoroughly tested by a qualified biomedical engineer. Following these simple safety steps will prevent most patient burns from the ESU.
The previous discussion concerned only unipolar ESUs. There is a second type of ESU, in which the current passes only between the two blades of a pair of forceps. This type of device is referred to as a bipolar ESU. Because the active and return electrodes are the two blades of the forceps, it is not necessary to attach another dispersive electrode to the patient, unless a unipolar ESU is also being used. The bipolar ESU generates considerably less power than the unipolar and is mainly used for ophthalmic and neurologic surgery.
In 1980 Mirowski et al.39 reported the first human implantation of a device to treat intractable ventricular tachydysrhythmias. This device, known as the automatic implantable cardioverter-defibrillator (AICD), is capable of sensing ventricular tachycardia and ventricular fibrillation and then automatically defibrillating the patient. Since 1980 thousands of patients have received AICD implants.40,41 Because some of these patients may present for noncardiac surgery, it is important that the anesthesiologist be aware of potential problems.42 The use of a unipolar ESU may cause electrical interference that could be interpreted by the AICD as a ventricular tachydysrhythmia. This would trigger a defibrillation pulse to be delivered to the patient and would likely cause an actual episode of ventricular tachycardia or ventricular fibrillation. The patient with an AICD is also at risk for ventricular fibrillation during electroconvulsive therapy.42 In both cases, the AICD should be disabled by placing a magnet over the device or by use of a specific protocol to shut it off. Therefore, it is best to consult with someone experienced with the device before starting surgery. The device can be reactivated by reversing the process. Also, an external defibrillator and a noninvasive pacemaker should be in the OR whenever a patient with an AICD is anesthetized.
Electrical safety in the OR is a matter of combining common sense with some basic principles of electricity. Once OR personnel understand the importance of safe electrical practice, they are able to develop a heightened awareness to potential problems. All electrical equipment must undergo routine maintenance, service, and inspection to ensure that it conforms to designated electrical safety standards. Records of these test results must be kept for future inspection because human error can easily compound electrical hazards. Starmer et al.43 cited one case concerning a newly constructed laboratory where the ground wire was not attached to a receptacle. In another study Albisser et al.44 found a 14% (198/1,424) incidence of improperly or incorrectly wired outlets. Furthermore, potentially hazardous situations should be recognized and corrected before they become a problem. For instance, electrical power cords are frequently placed on the floor where they can be crushed by various carts or the anesthesia machine. These cords could be located overhead or placed in an area of low traffic flow. Multiple-plug extension boxes should not be left on the floor where they can come in contact with electrolyte solutions. These could easily be mounted on a cart or the anesthesia machine. Pieces of equipment that have been damaged or have obvious defects in the power cord must not be used until they have been properly repaired. If everyone is aware of what constitutes a potential hazard, dangerous situations can be prevented with minimal effort.
Sparks generated by the ESU may provide the ignition source for a fire with resulting burns to the patient and OR personnel. This is a particular risk when the ESU is used in
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an oxygen-enriched environment as may be present in the patient's airway or in close proximity to the patient's face. The administration of high-flow nasal oxygen to a sedated patient during procedures on the face and eye is particularly hazardous. Most plastics such as tracheal tubes and components of the anesthetic breathing system that would not burn in room air will ignite in the presence of oxygen and/or nitrous oxide. Tenting of the drapes to allow dispersion of any accumulated oxygen and/or its dilution by room air or use of a circle anesthesia breathing system with minimal to no leak of gases around the anesthesia mask will decrease the risk of ignition from a spark generated by a nearby ESU.
Conductive Flooring
In past years, conductive flooring was mandated for ORs where flammable anesthetic agents were being administered. This would minimize the buildup of static charges that could cause a flammable anesthetic agent to ignite. The standards have now been changed to eliminate the necessity for conductive flooring in anesthetizing areas where flammable agents are no longer used.
Environmental Hazards
There are a number of potential electrically related hazards in the OR that are of concern to the anesthesiologist. There is the potential for electrical shock not only to the patient but also to OR personnel. In addition, cables and power cords to electrical equipment and monitoring devices can become hazardous. Finally, all OR personnel should have a plan of what to do in the event of a power failure.
In today's OR there are literally dozens of pieces of electrical equipment. It is not uncommon to have numerous power cords lying on the floor, where they are vulnerable to damage. If the insulation on the power cable becomes damaged, it is fairly easy for the hot wire to come in contact with a piece of metal equipment. If the OR did not have isolated power, that piece of equipment would become energized and a potential electrical shock hazard.45 Having isolated power minimizes the risk to the patient and OR personnel. Clearly, getting electrical power cords off the floor is desirable. This can be accomplished by having electrical outlets in the ceiling or by having ceiling-mounted articulated arms that contain electrical outlets. Also, the use of multi-outlet extension boxes that sit on the floor can be hazardous. These can be contaminated with fluids, which could easily trip the circuit breaker. In one case, it apparently tripped the main circuit breaker for the entire OR, resulting in a loss of all electrical power except for the overhead lights.46
Modern monitoring devices have many safety features incorporated into them. Virtually all of them have isolated the patient input from the power supply of the device. This was an important feature that was lacking from the original ECG monitors. In the early days, patients could actually become part of the electrical circuit of the monitor. There have been relatively few problems with patients and monitoring devices since the advent of isolated inputs. However, between 1985 and 1994, the Food and Drug Administration (FDA) received approximately 24 reports in which infants and children had received an electrical shock, including five children who died by electrocution.47,48 These electrical accidents occurred because the electrode lead wires from either an ECG monitor or an apnea monitor were plugged directly into a 120-volt electrical outlet instead of the appropriate patient cable. In 1997, the FDA issued a new performance standard for electrode lead wires and patient cables that requires that the exposed male connector pins from the electrode lead wires be eliminated. Therefore, the lead wires must have female connections and the connector pins must be housed in a protected patient cable (Fig. 8-35). This effectively eliminates the possibility of the patient being connected directly to an alternating current source since there are no exposed connector pins on the lead wires.
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Figure 8-35. The current standard for patient lead wires (left) requires a female connector. The patient cable (right) has shielded connector pins that the lead wires plug into. |
All health care facilities are required to have a source of emergency power. This generally consists of one or more electrical generators. These generators are configured to start up automatically and provide power to the facility within 10 seconds after detecting the loss of power from the utility company. The facility is required to test these generators on a regular basis. However, in the past, not all health care facilities tested them under actual load. There are numerous anecdotal reports of generators not functioning properly during an actual power failure. If the generators are not tested under actual load, it is possible that many years will pass before a real power outage puts a severe demand on the generator. If the facility has several generators and one of them fails, the increased demand on the others may be enough to cause them to fail in rapid succession. Hospitals (under the current National Fire Protection Association [NFPA] 99 standards) must test their emergency power supply systems (generators) under connected load once a month for at least 30 minutes. If the generator is oversized for the application and cannot be loaded to at least 30% of its rating, it must be load-banked and run for a total of 2 hours every year. A fairly recent requirement is for emergency power supply systems to be tested once every 3 years for 4 continuous hours, with a recommendation this be performed during peak usage of the system.49,50
It is vitally important that each OR have a contingency plan for a power failure. In most cases, the emergency generator will take over, but that is not always going to happen. There should be a supply of battery-operated light sources available in each OR. A laryngoscope can serve as a readily available source of light that allows one to find flashlights and other pieces of equipment. The overhead lights in the OR should also be connected to some sort of battery-operated lighting system. A supply of battery-operated monitoring devices and pneumatically powered ventilators and anesthesia machines would enable life-support functions to continue. The cost of these contingencies is relatively small but the benefits can be incomparable in an emergency.
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Electromagnetic Interference
Rapid advances in technology have led to an explosion in the number of wireless communication devices in the marketplace. These devices include cellular telephones, cordless telephones, walkie-talkies, and wireless Internet access devices. All of these devices have something in common: they emit electromagnetic interference (EMI). This most commonly manifests itself when traveling on airplanes. Most airlines require that these devices be turned off when the plane is taking off or landing or, in some cases, during the entire flight. There is concern that the EMI emitted by these devices may interfere with the plane's navigation and communication equipment.
In recent years, the number of people who own these devices has increased exponentially. Indeed, in some hospitals, they form a vital link in the regular or emergency communication system. It is not uncommon for physicians, nurses, paramedics, and other personnel to have their own cellular telephones. In addition, patients and visitors may also have cellular telephones and other types of communication devices. Hospital maintenance and security personnel frequently have walkie-talkie–type radios and some hospitals have even instituted an in-house cellular telephone network that augments or replaces the paging system. There has been concern that the EMI emitted by these devices may interfere with implanted pacemakers or various types of monitoring devices and ventilators in critical care areas.51 One case of a patient death has been reported when a ventilator malfunctioned secondary to EMI.52
Several studies have been done to find out if cellular telephones cause problems with cardiac pacemakers. One report by Hayes et al.53 studied 980 patients with five different types of cellular telephones. They conducted more than 5,000 tests and found that in more than 20% of the cases they could detect some interference from the cellular telephone. Patients were symptomatic in 7.2% of the cases, and clinically significant interference occurred in 6.6% of the cases. When the telephone was held in the normal position over the ear, clinically significant interference was not detected. In fact, the interference that caused clinical symptoms occurred only if the telephone was directly over the pacemaker. Other studies have demonstrated changes such as erroneous sensing and pacer inhibition.54,55 Again, these occurred only when the telephone was close to the pacemaker. The changes were temporary, and the pacemaker reverted to normal when the cellular telephone was moved to a safe distance. Currently, the FDA guidelines are that the cellular telephones be kept at least 6 inches from the pacemaker. Therefore, a patient with a pacemaker should not carry a cellular telephone in the shirt pocket, which is adjacent to the pacemaker. There appears to be little risk if hospital personnel carry a cellular telephone and if they ensure that it is kept at a reasonable distance from patients with a pacemaker.
AICDs comprise another group of devices of concern to biomedical engineers. Fetter et al.56 conducted a study of 41 patients who had AICDs. They concluded that the cellular telephones did not interfere with the AICDs. They did, however, recommend keeping the cellular telephone at least 6 inches from the device.
EMI extends well beyond that of cellular telephones. Walkie-talkies, which are frequently used by hospital maintenance and security personnel, paging systems, police radios, and even televisions all emit EMI, which could potentially interfere with medical devices of any nature. Although there are many anecdotal reports, the amount of available scientific information on this problem is scant. Reports of interference include ventilator and infusion pumps that have been shut down or reprogrammed, interference with ECG monitors, and even an electronic wheelchair that was accidentally started because of EMI. It is a difficult problem to study because there are many different types of devices that emit EMI and a vast array of medical equipment that has the potential to interact with these devices. Even though a device may seem “safe” in the medical environment, if two or three cellular telephones or walkie-talkies are brought together in the same area at the same time, there may be unanticipated problems or interference.
Any time a cellular telephone is turned on, it is actually communicating with the cellular network, even though a call is not in progress. Therefore, the potential to interfere with devices exists. The Emergency Care Research Institute (ECRI) reported in October 1999 that walkie-talkies were far more likely to cause problems with medical devices than cellular telephones.57 This is because they operate on a lower frequency than cellular telephones and have a higher power output. The ECRI recommends that cellular telephones be maintained at a distance of 1 meter from medical devices, while walkie-talkies be kept at a distance of 6 to 8 meters.
Some hospitals have made restrictive policies on the use of cellular telephones, particularly in critical care areas.58 These policies are supported by little scientific documentation and are nearly impossible to enforce. The ubiquitous presence of cellular telephones carried by hospital personnel and visitors makes enforcing a ban virtually impossible. Even when people try to comply with the ban, failure is nearly inevitable because the general public is usually unaware that a cellular telephone in the standby mode is still communicating with the tower and generating EMI.
The real solution is to “harden” devices against EMI. This is difficult to do because of the many different frequencies on which these devices operate. Education of medical personnel is essential. When working in an OR or critical care area, all personnel must be alert to the fact that electronic devices and pacemakers can be interfered with by EMI. Creating a restrictive policy would certainly irritate personnel and visitors, and, in some cases, may actually compromise emergency communications.59
Construction of New Operating Rooms
Frequently, an anesthesiologist is asked to consult with hospital administrators and architects in designing new, or remodeling older, ORs. In the past a strict electrical code was enforced because of the use of flammable anesthetic agents. This code included a requirement for isolated power systems and LIMs. The NFPA revised its standard for health care facilities in 1984 (NFPA 99-1984). These standards do not require isolated power systems or LIMs in areas designated for use of nonflammable anesthetic agents only.60,61 Although not mandatory, NFPA standards are usually adopted by local authorities when revising their electrical codes.
This change in the standard creates a dilemma. The NFPA 99—Standard for Health Care Facilities, 2005 edition, mandates that “wet location patient care areas be provided with special protection against electrical shock.” Section 3-4.1.2.6 further states that “this special protection shall be provided by a power distribution system that inherently limits the possible ground fault current due to a first fault to a low value, without interrupting the power supply; or by a power distribution system in which the power supply is interrupted if the ground fault current does, in fact, exceed a value of 6 milliamperes.”
The decision of whether to install isolated power hinges on two factors. The first is whether or not the OR is considered a
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wet location, and, if so, whether an interruptible power supply is tolerable. Where power interruption is tolerable, a GFCI is permitted as the protective means. However, the standard also states that “the use of an isolated power system (IPS) shall be permitted as a protective means capable of limiting ground fault current without power interruption.”
Most people who have worked in an OR would attest to its being a wet location. The presence of blood, body fluids, and saline solutions spilled on the floor all contribute to making this a wet environment. The cystoscopy suite serves as a good example.
Once the premise that the OR is a wet location is accepted, it must be determined whether a GFCI can provide the means of protection. The argument against using GFCIs in the OR is illustrated by the following example. Assume that during an open heart procedure the cardiopulmonary bypass pump and the patient monitors are plugged into outlets on the same branch circuit. Also assume that during bypass, the circulating nurse now plugs in a faulty headlight. If there is a GFCI protecting the circuit, the fault will be detected and the GFCI will interrupt all power to the pump and the monitors. This undoubtedly would cause a great deal of confusion and consternation among the OR personnel and may place the patient at risk for injury. The pump would have to be manually operated while the problem was being resolved. In addition, the GFCI could not be reset (and power restored) until the headlight was identified as the cause of the fault and unplugged from the outlet. However, if the OR were protected with an isolated power system and LIM, the same scenario would cause the LIM to alarm, but the pump and patient monitors would continue to operate normally. There would be no interruption of power and the problem could be resolved without risk to the patient.
It should be realized that a GFCI is an active system. That is, a potentially hazardous current is already flowing and must be actively interrupted, whereas the isolated power system (with LIM) is designed to be safe during a first-fault situation. Thus, it is a passive system because no mechanical action is required to activate the protection.62
It is likely that hospital administrators may want to eliminate isolated power systems in new OR construction as a cost-saving measure. Others, however, have advocated the retention of isolated power systems.62,63,64 Not to do this would be a short-sighted, foolhardy measure. This is especially true because the cost of adding isolated power is estimated to be 1% of the cost of constructing an OR.62 Although not perfect,65 the isolated power system and LIM do provide both the patient and OR personnel with a significant amount of protection in an electrically hazardous environment. Isolated power systems provide clean stable voltages, which is important for sensitive diagnostic equipment.66 Also, modern LIMs, which are microprocessor-based, require only yearly instead of monthly testing.
The value of the isolated power system is illustrated in a report by Day67 in 1994. He reported four instances of electrical shock to OR personnel in a 1-year period. The operating suite had been renovated and the isolated power system removed, and it was not until the OR personnel received a shock that a problem was discovered.
Anesthesiologists need to be aware of this cost-saving attitude and strongly encourage that new ORs be constructed with isolated power systems. The relatively small cost savings that the alternative would represent do not justify the elimination of such a useful safety system. The use of GFCIs in the OR environment can be acceptable if carefully planned and engineered. In order to avoid the loss of power to multiple instruments and monitors at one time, each outlet must be an individual GFCI. If that is done, then a fault will result in only one piece of equipment losing power. Using GFCIs also precludes the use of multiple plug strips in the OR.
Electrical safety should be the concern of everyone in the OR. Accidents can be prevented only if proper installation and maintenance of the appropriate safety equipment in the OR have occurred and the OR personnel understand the concepts of electrical safety and are vigilant in their efforts to detect new hazards.68
Fire Safety
Fires in the OR are just as much a danger today as they were 100 years ago when patients were anesthetized with flammable anesthetic agents.69,70 Because the potential consequences of a fire or explosion with ether or cyclopropane were well known and potentially devastating, OR fire safety practices were routinely followed.71,72
Today, the risk of an OR fire is probably as great or greater than the days when ether and cyclopropane were used, in part because of the routine use of potential sources of ignition (including electrosurgical cauteries) in an environment rich in fuel sources (i.e., flammable materials) and oxidizers (e.g., oxygen and nitrous oxide). Although the number of OR fires that occur annually in the United States is unknown, some estimates suggest that there are 50 to 200 fires each year, with as many as 20% associated with serious injury or death. In contrast to the era of flammable anesthetics, there currently appears to be a lack of awareness of the potential for an OR fire. In response to the risks presented by this situation, in 2008 the American Society of Anesthesiologists released a Practice Advisory on the Prevention and Management of Operating Room Fires73 (Table 8-3).
For a fire to start, three components are necessary. The limbs of the “fire triad” are a heat or ignition source, fuel, and an oxidizer.74 A fire occurs when there is a chemical reaction of a fuel rapidly combining with an oxidizer to release energy in the form of heat and light. In the OR, there are many heat or ignition sources, such as the ESU, lasers, and the ends of fiberoptic light cords. The main oxidizers in the OR are air, oxygen, and nitrous oxide. Oxygen and nitrous oxide function equally well as oxidizers, so a combination of 50% oxygen and 50% nitrous oxide would support combustion, as would 100% oxygen. Fuel for a fire can be found everywhere in the OR. Paper drapes, which have largely replaced cloth drapes, are much easier to ignite and can burn with greater intensity.75,76 Other sources of fuel include gauze dressings, endotracheal tubes, gel mattress pads, and even facial or body hair77(Table 8-4).
Fire prevention is accomplished by not allowing all three of the elements of the fire triad to come together at the same time.78 The challenge in the OR is that frequently each of the limbs of the fire triad is controlled by a different individual. For instance, the surgeon is frequently in charge of the ignition source, the anesthesiologist is usually administering the oxidizer, and the OR nurse frequently controls the fuel sources. It is not always evident to any one individual that all of these elements may be coming together at the same time. This is especially true in any case in which there is the possibility of oxygen or an oxygen-nitrous oxide mixture being delivered around the surgical site. In these circumstances, the risk of an OR fire is markedly increased and the need for communication among the surgeon, the anesthesiologist, and the OR nurses throughout the procedure is essential.
There are several dangers that may result from an OR fire. The most obvious is that the patient and OR personnel can suffer severe burns. However, a less obvious but potentially more deadly risk can be posed by the products of combustion (called toxicants). When materials, such as plastics burn, a variety of injurious compounds can be produced. These include carbon monoxide, ammonia, hydrogen chloride, and
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even cyanide. Toxicants can produce injury by damaging airways and lung tissue, and can cause asphyxia. OR fires can often produce significant amounts of smoke and toxicants, but may not cause enough heat to activate overhead sprinkler systems. If enough smoke is produced, the OR personnel may have to evacuate the area. Thus, it is essential to have a prethought-out evacuation plan for both the OR personnel and the patient.
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Table 8-3 Recommendations for the Prevention and Management of Operating Room Fires |
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Table 8-4 Fuel Sources Commonly Found in the Operating Room |
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OR fires can be divided into two different types. The more common type of fire occurs in or on the patient, especially during high-risk procedures in which an ignition source is used in an oxidizer-rich environment. These would include airway fires (including endotracheal tube fires, fires in the oropharynx, which may occur during a tonsillectomy, and fires in the breathing circuit), and fires during laparoscopy. Fires occurring on the patient mainly involve head and neck surgery done under regional anesthesia or monitored anesthesia care when the patient is receiving high flows of supplemental oxygen. Because these fires occur in an oxygen-enriched environment, items such as surgical towels, drapes, or even the body hair can be readily ignited and produce a severe burn. The other type of OR fire is one that is remote from the patient. This would include an electrical fire in a piece of equipment, or a carbon dioxide (CO2) absorber fire.
The two major ignition sources for OR fires are the ESU and the laser. However, the ends of some fiberoptic light cords can also become hot enough to start a fire if they are placed on paper drapes. Although the ESU is responsible for igniting the majority of the fires,79 it is the laser that has generated the most attention and research. Laser is the acronym forlight amplification by stimulated emission of radiation. A laser consists of an energy source and material that the energy excites to emit light.80,81,82 The material that the energy excites is called the lasing medium and provides the name of the particular type of laser. The important property of laser light is that it is coherent, meaning that is monochromatic (or even of a single wavelength). This coherent light can be focused into very small spots that have very-high-power density.
There are many different types of medical lasers, and each has a specific application. The argon laser is used in eye and dermatologic procedures because it is absorbed by hemoglobin and has a modest tissue penetration of between 0.05 and 2.0 mm. The potassium-titanyl-phosphate (KTP) or frequency-doubled yttrium aluminum garnet (YAG) lasers are also absorbed by hemoglobin and have tissue penetrations similar to that of the argon laser. The Dilaser has a wavelength that is easily changed and can be used in different applications, particularly in dermatologic procedures. The neodymium-doped yttrium aluminum garnet (Nd:YAG) laser is the most powerful of the medical lasers. Since the tissue penetration is between 2 and 6 mm, it can be used for tumor debulking, particularly in the trachea and main stem bronchi, or in the upper airway. The energy can be transmitted through a fiberoptic cable that is placed down the suction port of a fiberoptic bronchoscope. The laser can then be used in a contact mode to treat a tumor mass. The CO2 laser has very little tissue penetration and can be used where great precision is needed. It is also absorbed by water, so that minimal heat is dispersed to surrounding tissues. The CO2 laser is used primarily for procedures in the oropharynx and in and around the vocal cords. The helium-neon laser (He-Ne) produces an intense red light and thus can be used for aiming the CO2 and the Nd:YAG lasers. It has very low power and thus will present no significant danger to OR personnel.
One of the most devastating types of OR fires occurs when an endotracheal tube is ignited in the patient.83,84,85,86,87,88 If the patient is being ventilated with oxygen and/or nitrous oxide, the endotracheal tube will essentially emit a blowtorch type of flame that can result in severe injury to the trachea, lungs, and surrounding tissues. Red rubber, polyvinyl chloride, and silicone endotracheal tubes all have oxygen-flammability indices (defined as the minimum O2 fraction in N2 that will just support a candlelike flame for a given fuel source using a standard ignition source)89 of <26%.90 Historically, anesthesiologists attempted to improve the safety of these tubes by wrapping red rubber or polyvinyl chloride tubes with some sort of reflective tape. However, taped-wrapped tubes often became kinked, gaps in the tape exposed areas of the tube to the laser, and non–laser-resistant tape was sometimes unintentionally used. To prevent these problems during high-risk procedures, “laser-resistant” endotracheal tubes have been developed.91,92,93 Anesthesiologists can now use an endotracheal tube that is designed to be resistant to ignition by the specific type of laser that will be used during surgery. For instance, when using the CO2 laser, the LaserFlex (Mallinckrodt, Pleasanton, CA) is an excellent choice. This is a flexible metal tube that has two cuffs that can be inflated with saline colored with methylene blue. The methylene blue enables the surgeon to easily recognize if he or she has accidentally penetrated one of the cuffs. The LaserFlex™ tube is highly resistant to being struck by the laser. If the Nd:YAG laser is being used, then the Lasertubus™ (Rüsch Inc., Duluth, GA) can be used. The Lasertubus™ has a soft rubber shaft that is covered by a corrugated silver foil that is in turn covered in a Merocel sponge jacket. In order to provide maximum protection, the Merocel must be kept moist with saline.
Another potential source of ignition for an OR fire is the ESU.94,95 A typical example of how an ESU could cause ignition would be during a tonsillectomy in a child in whom the anesthesiologist was using an uncuffed, flammable endotracheal tube. In this case, the oxygen or oxygen-nitrous oxide mixture could leak around the endotracheal tube and pool at the operative site, providing an oxider-enriched environment. When the surgeon uses the ESU (or laser) to cauterize the tonsil bed, the combination of a high concentration of oxidizer (oxygen or oxygen-nitrous oxide mixture), fuel (endotracheal tube), and ignition source (the ESU or laser) could easily start a fire.96,97
The best way to prevent this type of fire is to take steps to prevent the three legs of the fire triad from coming together. For example, mixing the oxygen with air will keep the inspired oxygen concentration as low as possible, thus reducing the available oxidizer. Another possibility would be to place wet pledgets around the endotracheal tube, which would prevent the escape of oxygen or oxygen-nitrous oxide mixture from the trachea into the operative field. This reduces the available oxidizer and would keep the endotracheal tube and tissues from becoming desiccated, thus reducing their suitability as fuel sources. However, the pledgets must be kept moist, lest they dry out and become an additional source of fuel for a fire.
A related situation that requires a different solution can arise when a critically ill patient requires a tracheostomy.98,99 These patients may require very high concentrations of inspired oxygen to maintain tissue oxygenation so that any decrease in inspired oxygen concentration or interruption of ventilation would not be tolerated. In this circumstance, the best option for preventing a fire would be to avoid the use of electrocautery (ignition source) when the surgeon enters the trachea.
The Nd:YAG laser can be used to treat tumors of the lower trachea and main stem bronchi. Most commonly, the surgeon will use a fiberoptic bronchoscope (FOB) and pass the laser fiber through the suction port of the bronchoscope. The fiberoptic bronchoscope can be used in conjunction with a rigid metal bronchoscope or passed through an 8.5- or 9.0-mm polyvinyl chloride endotracheal tube. A special laser-resistant tube would not be used in this circumstance because the FOB and laser fiber pass through the endotracheal tube and focus on tissue distal to the tube. Fire safety precautions
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available in this setting include titrating the concentration of inspired oxygen to as low a concentration as the patient can tolerate while maintaining a saturation of between 90 and 95% (ideally keeping the inspired oxygen below 30%), keeping the tip of the endotracheal tube and FOB away from the site of surgery and out of the “line of fire” of the laser, and removing charred and desiccated tissue from the surgical field.
The use of a rigid metal bronchoscope instead of an endotracheal tube will eliminate the possibility of setting the tube on fire but does not eliminate the possibility of setting the FOB on fire. This would also necessitate the use of a jet venturi system to ventilate the patient, which would, in turn, deliver an inspired oxygen concentration of between 40 and 60%.
There are a number of basic safety precautions that should be taken whenever a laser is used in surgery. Since laser light can be reflected off any metal surface, it is important that all OR personnel wear protective goggles that are specific to the type of laser being used. The anesthesiologist needs to be aware that the laser goggles may make it difficult to read certain monitor displays. In addition, it is important that the patient's eyes be covered with wet gauze or eye packs. OR personnel should also wear high filtration masks because the laser “plume” may contain vaporized virus particles or chemical toxins. Finally, all doors to the OR should have warning signs that a laser is in use, and all windows should be covered with black window shades.
Laparoscopic surgery in the abdomen is another potential risk for a surgically related fire. Ordinarily, the abdomen is inflated with CO2, which does not support combustion. It is important to verify that, indeed, only CO2 is being used, as erroneous inclusion of oxygen can be disastrous.100 Also, nitrous oxide administered to the patient as part of the anesthetic can, over 30 minutes, diffuse into the abdominal cavity and attain a concentration that could support combustion.101 In fact, when sampling the abdominal gas contents after 30 minutes, the mean nitrous oxide concentration was 36%; however, in certain patients it reached a concentration of 47%. Both methane and hydrogen are flammable gases that are frequently present in bowel gas in significant concentrations. Methane concentration in bowel gas can be up to 56% and hydrogen has been reported as high as 69%. With the maximum abdominal concentration of 47% nitrous oxide mixed with CO2, it would require the maximum of 56% of methane to be flammable. Therefore, this represents a relatively small hazard. In contrast, a concentration of 69% hydrogen is flammable if the nitrous oxide concentration is >29%. Therefore, a fire is possible if the surgeon, while using the ESU, enters the bowel with a high concentration of hydrogen and the intra-abdominal nitrous oxide content is >29%.
In recent years, fires on the patient seem to have become the most frequent type of OR fire. These cases occur most often during surgery in and around the head and neck, where the patient is receiving monitored anesthesia care and supplemental oxygen is being administered by either a face mask or nasal cannulae.102,103,104,105,106 In these cases, the oxygen can collect under the drapes if not properly vented, and when the surgeon uses the ESU or the laser, a fire can easily start. There are many things that can act as fuel, such as the surgical towels, paper drapes, disinfecting preparation solutions, sponges, plastic tubing from the oxygen face mask, and even the body hair. These fires start very quickly and can turn into an intense blaze in only a few seconds. Even if the fire is quickly extinguished, the patient will usually sustain a significant burn.
The most important principle that the anesthesiologist has to keep in mind to minimize the risk of fire is to titrate the inspired oxygen to the lowest amount necessary to keep patient's oxygenation within safe levels. If the anesthesia machine has the ability to deliver air, then the nasal cannula or face mask can be attached to the anesthesia circuit by using a small no. 3 or no. 4, 15-mm endotracheal tube adapter. This is attached to the right-angle elbow of the circuit. If the anesthesia machine is equipped with an auxiliary oxygen flowmeter that has a removable nipple adapter, then a humidifier can be installed in place of the nipple adapter. The humidifier has a Venturi mechanism through which room air is entrained and thus the oxygen concentration that is delivered to the face mask can be varied from 28 to 100%. Finally, if this machine has a common gas outlet that is easily accessible, a nasal cannula or face mask can be attached at this point using the same small 3- or 4-mm endotracheal tube adaptor. If it is not possible to dilute the oxygen with air, then it is important that the drapes be arranged in such a manner that there is no oxygen buildup beneath them. Tenting the drapes and having the surgeon use an adhesive sticky drape that seals the operative site from the oxygen flow are steps that will help reduce the risk of a fire.
It is potentially possible to discontinue the use of oxygen before the surgeon plans to use the electrocautery or laser. This would have to be done several minutes beforehand in order to allow any oxygen that has built up to dissipate. If the surgeon is planning to use the electrocautery or laser during the entire case, this may not be practical.
Some newer surgical preparation solutions can contribute to surgically related fires. These solutions typically come prepackaged in a “paint stick” applicator with a sponge on the end (e.g., DuraPrep™, St. Paul, MN). It consists of Iodophor mixed with 74% isopropyl alcohol. This is highly flammable and can easily be the fuel for an OR fire. In 2001, Barker and Polson102 reported just such a case. In a laboratory re-creation, they found that if the DuraPrep™ had been allowed to dry completely (4 to 5 minutes), the fire did not occur (Fig. 8-36). The other problem with these types of preparation solutions is that small pools of the solution can accumulate if the person doing the preparation is not careful. The alcohol in these small puddles will continue to evaporate for a period of time, and the alcohol vapors are also extremely flammable. Flammable skin preparation solutions should be allowed to dry and puddles removed before the site is draped (Fig. 8-37).
It is important to bear in mind that halogenation of hydrocarbon anesthetics confers relative, but not absolute, resistance to combustion. Even the newer, “nonflammable” volatile anesthetics can, under certain circumstances, present fire hazards. For example, sevoflurane is nonflammable in air, but can serve as a fuel at concentrations as low as 11% in oxygen and 10% in nitrous oxide.107 In addition, sevoflurane and desiccated CO2 absorbent (either soda lime or Baralyme) can undergo exothermic chemical reactions that have been implicated in several fires that involved the anesthesia breathing circuit.108,109,110,111 In 2003, the manufacturer of sevoflurane published a “Dear Health Care Provider” letter and advisory alert.112 To prevent futures fires, the manufacturer of sevoflurane has recommended that anesthesiologists employ several measures, including avoiding the use of desiccated CO2 absorbent and monitoring the temperature of the absorbers and the inspired concentration of sevoflurane; if elevated temperature or an inspired sevoflurane concentration that differed unexpectedly from the vaporizer setting is detected, it is recommended that the patient be disconnected from the anesthesia circuit and monitored for signs of thermal or chemical injury, and that the CO2 absorbent is removed from the circuit and/or replaced.
Another way to prevent this type of fire is to use a CO2 absorbent that does not contain a strong alkali, as do soda lime and Baralyme™ (Chemetron Medical Division, Allied Healthcare Products, St. Louis, Missouri). Amsorb™ (Amstrong Medical Limited, Coleraine, Northern Ireland) is a CO2 absorbent that contains calcium hydroxide and calcium chloride, but no strong alkali.113 In experimental studies, it was found that Amsorb is unreactive with currently used volatile anesthetics
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and does not produce carbon monoxide or Compound A with desiccated absorbent. Therefore, it would not interact with sevoflurane and undergo an exothermic chemical reaction.
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Figure 8-36. Simulation of fire caused by ESU electrode during surgery. A. Mannequin prepared and draped for surgery. Electrosurgical unit monopolar pencil electrode applied to operative site at start of surgery. B. Six seconds after electrosurgical unit application. Smoke appears from under the drapes. C. Fourteen seconds after electrosurgical unit application. Flames burst through the drapes. D. Twenty-four seconds after electrosurgical unit application. Entire patient head and drapes in flames. (From Barker SJ, Polson JS: Fire in the operating room: A case report and laboratory study. Anesth Analg 2001; 93: 960, with permission.) |
If a fire does occur, it is important to extinguish it as soon as possible. The first step is to interrupt the fire triad by removing one component. This is usually best accomplished by removing the oxidizer from the fire. Therefore, if an endotracheal tube is on fire, disconnecting the circuit from the tube or disconnecting the inspiratory limb of the circuit will usually result in the fire immediately going out. Simultaneously the surgeon should remove the burning endotracheal tube. Once the fire is extinguished, the airway inspected via bronchoscopy, and the patient reintubated.
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Figure 8-37. A demonstration of the intense heat and flame that is present in an alcohol fire. (Photograph courtesy of Marc Bruley of Emergency Care Research Institute. Reprinted with permission, Copyright 2009, ECRI Institute. www.ecri.org.) |
If the fire is on the patient, then extinguishing it with a basin of saline may be the most rapid and effective method to deal with this type of fire. There is also a method to use a sheet or towel to extinguish the fire. If the drapes are burning, particularly if they are paper drapes, then they must be removed and placed on the floor. Paper drapes are impervious to water; thus, throwing water or saline on them will do little to extinguish the fire. Once the burning drapes are removed from the patient, the fire can then be extinguished with a fire extinguisher. In most OR fires, the sprinkler system is not activated. This is because the sprinklers are not located directly over the OR table and because OR fires seldom get hot enough to activate the sprinklers.
All OR personnel should receive OR fire safety education, which should include training in institutional fire safety protocols and learning the location and operation of the fire extinguishers. Fire safety education, including fire drills, allows each member of the OR team to learn and practice what his or her responsibilities and actions should be if a fire were to
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occur. Fire drills are an important part of the plan and can help personnel become familiar with the exits, evacuation routes, location of fire extinguishers, and how to shut off medical gas and electrical supplies. Although institutional fire safety protocols vary, the general principles of responding to an OR fire can be summarized by the mnemonic ERASE:extinguish, rescue, activate, shut, and evaluate. In sequence: First, the team should generally attempt to extinguish a fire on, in, or near the patient. Depending on the situation, this may include the use of saline or a CO2 fire extinguisher (see later discussion). If the initial attempts at extinguishing the fire are unsuccessful, the patient and all other persons at risk should be rescued and the OR evacuated, if possible, and the fire alarm should be activated. Once the OR is emptied of personnel, the doors should be shut and the medical gas supply to the room should be shut off. The patient should then be evaluated and any injuries should be appropriately managed.
Fire extinguishers are divided into three classes, termed A, B, and C, based on the types of fires for which they are best suited. Class A extinguishers are used on paper, cloth, and plastic materials; Class B extinguishers are used for fires when liquids or grease are involved; Class C extinguishers are used for energized electrical equipment. A single fire extinguisher may be useful for any one, two, or all three types of fires. Probably the best fire extinguisher for the OR is the CO2 extinguisher. This can be used on Class B and C fires and some Class A fires. Other extinguishers are water mist and new environmentally friendly fluorocarbons that replaced the Halon fire extinguisher. Finally, many ORs are equipped with a fire hose that supplies pressurized water at a rate of 50 gallons per minute. Such equipment is best left to the fire department to use, unless there is a need to rescue someone from a fire. In order to effectively use a fire extinguisher, the acronym “PASS” can be used. This stands for pull the pin to activate the fire extinguisher, aim at the base of the fire,squeeze the trigger, and sweep the extinguisher back and forth across the base of the fire. When responding to a fire, the acronym RACE is useful. This stands for rescue; alarm;confine; extinguish. Clearly, having a plan that everyone is familiar with will greatly facilitate extinguishing the fire and minimize the harm to the patient and equipment.
However, neither fire drills nor the presence and use of fire extinguishers should be relied on to provide a fire-safe operating environment. Only through heightened awareness, continuing education, and ongoing communication can the legs of the fire triad be kept apart and the risk of an OR fire minimized.
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Editors: Barash, Paul G.; Cullen, Bruce F.; Stoelting, Robert K.; Cahalan, Michael K.; Stock, M. Christine
