As discussed in Chapter 1 , the heart produces electrical currents similar to those of the familiar dry cell battery. The strength or voltage of these currents and the way they are distributed throughout the body over time can be measured by a suitable recording instrument such as an electrocardiograph.
The body acts as a conductor of electricity. Therefore recording electrodes placed some distance from the heart, such as on the arms, legs, or chest wall, are able to detect the voltages of the cardiac currents conducted to these locations. The usual way of recording these voltages from the heart is with the 12 standard ECG leads. The leads actually show the differences in voltage (potential) between electrodes placed on the surface of the body.
Taking an ECG is like recording an event such as a baseball game with a video camera. Multiple camera angles are necessary to capture the event completely. One view is not enough. Similarly, multiple ECG leads must be recorded to describe the dynamic electrical activity of the heart adequately. Figure 3-1 shows the ECG patterns that are obtained when electrodes are placed at various points on the chest. Notice that each lead presents a different pattern.
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FIGURE 3-1 Multiple chest leads give a three-dimensional view of cardiac electrical activity. |
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Figure 3-2 is an ECG illustrating the 12 leads. The leads can be subdivided into two groups: the six limb (extremity) leads (shown in the left two columns) and the six chest (precordial) leads (shown in the right two columns).
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FIGURE 3-2 A, Sample ECG showing the 12 standard leads. B, A lead II rhythm strip recorded simultaneously. What is the approximate heart rate? (Answer: about 80 beats/min) |
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The six limb leads—I, II, III, aVR, aVL, and aVF—record voltage differences by means of electrodes placed on the extremities. They can be further divided into two subgroups based on their historical development: three standard “bipolar” limb leads (I, II, and III), and three augmented “unipolar” limb leads (aVR, aVL, and aVF).
The six chest leads—V1, V2, V3, V4, V5, and V6—record voltage differences by means of electrodes placed at various positions on the chest wall.
The 12 ECG leads can also be viewed as 12 “channels.” In contrast to television channels (which can be tuned to different events), however, the 12 ECG channels (leads) are all tuned to the same event (the P-QRS-T cycle), with each lead viewing the event from a different angle.
LIMB (EXTREMITY) LEADS
STANDARD LIMB LEADS: I, II, AND III
The limb (extremity) leads are recorded first. In connecting a patient to an electrocardiograph, first place metal electrodes on the arms and legs. The right leg electrode functions solely as an electrical ground, so you need concern yourself with it no further. As shown in Figure 3-3 , the arm electrodes are attached just above the wrist and the leg electrodes are attached above the ankles.
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FIGURE 3-3 Metal electrodes are used to take an ECG. The right leg (RL) electrode functions solely as a ground to prevent alternating-current interference. LA, left arm; LL, left leg; RA, right arm. |
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The electrical voltages of the heart are conducted through the torso to the extremities. Therefore an electrode placed on the right wrist detects electrical voltages equivalent to those recorded below the right shoulder. Similarly, the voltages detected at the left wrist or anywhere else on the left arm are equivalent to those recorded below the left shoulder. Finally, voltages detected by the left leg electrode are comparable to those at the left thigh or near the groin. In clinical practice, the electrodes are attached to the wrists and ankles simply for convenience.[*]
As mentioned, the limb leads consist of standard “bipolar” (I, II, and III) and augmented (aVR, aVL, and aVF) leads. The bipolar leads were so named historically because they record the differences in electrical voltage between two extremities.
Lead I, for example, records the difference in voltage between the left arm (LA) and right arm (RA) electrodes:
Lead I = LA – RA
Lead II records the difference between the left leg (LL) and right arm (RA) electrodes:
Lead II = LL – RA
Lead III records the difference between the left leg (LL) and left arm (LA) electrodes:
Lead III = LL – LA
Consider what happens when you turn on the electrocardiograph to lead I. The LA electrode detects the electrical voltages of the heart that are transmitted to the left arm. The RA electrode detects the voltages transmitted to the right arm. Inside the electrocardiograph, the RA voltages are subtracted from the LA voltages, and the difference appears at lead I. When lead II is recorded, a similar situation occurs between the voltages of LL and RA. When lead III is recorded, the same situation occurs between the voltages of LL and LA.
Leads I, II, and III can be represented schematically in terms of a triangle, called Einthoven's triangle after the Dutch physiologist who invented the electrocardiograph in the early 1900s. At first the ECG consisted only of recordings from leads I, II, and III. Einthoven's triangle ( Fig. 3-4 ) shows the spatial orientation of the three standard limb leads (I, II, and III). As you can see, lead I points horizontally. Its left pole (LA) is positive and its right pole (RA) is negative. Therefore lead I = LA – RA. Lead II points diagonally downward. Its lower pole (LL) is positive and its upper pole (RA) is negative. Therefore lead II = LL – RA. Lead III also points diagonally downward. Its lower pole (LL) is positive and its upper pole (LA) is negative. Therefore lead III = LL – LA.
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FIGURE 3-4 Orientation of leads I, II, and III. Lead I records the difference in electrical potentials between the left arm and right arm. Lead II records it between the left leg and right arm. Lead III records it between the left leg and left arm. |
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Einthoven, of course, could have hooked the leads up differently. Yet because of the way he arranged them, the bipolar leads are related by the following simple equation:
Lead I + Lead III = Lead II
In other words, add the voltage in lead I to that in lead III and you get the voltage in lead II.[*] You can test this equation by looking at Figure 3-2 . Add the voltage of the R wave in lead I (+9 mm) to the voltage of the R wave in lead III (+4 mm) and you get +13 mm, the voltage of the R wave in lead II. You can do the same with the voltages of the P waves and T waves.
It is a good practice to scan leads I, II, and III rapidly when you first look at a mounted ECG. If the R wave in lead II does not seem to be the sum of the R waves in leads I and II, this may be a clue that the leads have been recorded incorrectly or mounted improperly.
Einthoven's equation is simply the result of the way the bipolar leads are recorded; that is, the LA is positive in lead I and negative in lead III and thus cancels out when the two leads are added:
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Thus, in electrocardiography, one plus three equals two.
In summary, leads I, II, and III are the standard (bipolar) limb leads, which, historically, were the first invented. These leads record the differences in electrical voltage between selected extremities.
In Figure 3-5 , Einthoven's triangle has been redrawn so that leads I, II, and III intersect at a common central point. This was done simply by sliding lead I downward, lead II rightward, and lead III leftward. The result is the triaxial diagram in Figure 3-5 B. This diagram, a useful way of representing the three bipolar leads, is employed in Chapter 5 .
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FIGURE 3-5 A, Einthoven's triangle. B, The triangle is converted to a triaxial diagram by shifting leads I, II, and III so that they intersect at a common point. |
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* Obviously, if you are taking an ECG on an amputee or someone with a cast, you have to place the electrodes below or near the shoulders or groin, depending on the circumstance.
* This rule is only approximate. It is exact when the three standard limb leads are recorded simultaneously, using a three-channel electrocardiograph, because the peaks of the R waves in the three leads do not occur simultaneously. The exact rule is as follows: The voltage at the peak of the R wave (or at any point) in lead II equals the sum of the voltages in leads I and III at points occurring simultaneously.
AUGMENTED LIMB (EXTREMITY) LEADS: aVR, aVL, AND aVF
Nine leads have been added to the original three “bipolar” extremity leads. In the 1930s, Dr. Frank N. Wilson and his colleagues at the University of Michigan invented the “unipolar” limb leads and also introduced the six “unipolar” chest leads, V1 through V6. A short time later, Dr. Emanuel Goldberger invented the three augmented unipolar limb leads: aVR, aVL, and aVF. The abbreviation a refers toaugmented; V tovoltage; R, L, and F to right arm, left arm, and left foot (leg), respectively. Today 12 leads are routinely employed, consisting of the six limb leads (I, II, III, aVR, aVL, and aVF) and the six precordial leads (V1 to V6).
A so-called unipolar lead records the electrical voltages at one location relative to an electrode with close to zero potential rather than relative to the voltages at another single extremity, as in the case of the bipolar limb leads.[*] The zero potential is obtained inside the electrocardiograph by joining the three extremity leads to a central terminal. Because the sum of the voltages of RA, LA, and LL equals zero, the central terminal has a zero voltage. The aVL and aVF leads are derived in a slightly different way because the voltages recorded by the electrocardiograph have been augmented 50% over the actual voltages detected at each extremity. This augmentation is also done electronically inside the electrocardiograph.[†]
Just as Einthoven's triangle represents the spatial orientation of the three standard limb leads, the diagram in Figure 3-6 represents the spatial orientation of the three augmented limb leads. Notice that each of these “unipolar” leads can also be represented by a line (axis) with a positive and negative pole. Because the diagram has three axes, it is also called a triaxial diagram.
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FIGURE 3-6 Triaxial lead diagram showing the relationship of the three augmented (“unipolar”) leads (aVR, aVL, and aVF). Notice that each lead is represented by an axis with a positive and negative pole. The term unipolar was used to mean that the leads record the voltage in one location relative to about zero potential, instead of relative to the voltage in one other extremity. |
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As would be expected, the positive pole of lead aVR, the right arm lead, points upward and to the patient's right arm. The positive pole of lead aVL points upward and to the patient's left arm. The positive pole of lead aVF points downward toward the patient's left foot.
Furthermore, just as leads I, II, and III are related by Einthoven's equation, so leads aVR, aVL, and aVF are related:
aVR + aVL + aVF = 0
In other words, when the three augmented limb leads are recorded, their voltages should total zero. Thus the sum of the P wave voltages is zero, the sum of the QRS voltages is zero, and the sum of the T wave voltages is zero. Using Figure 3-2 , test this equation by adding the QRS voltages in the three unipolar limb leads (aVR, aVL, and aVF).
It is also a good practice to scan leads aVR, aVL, and aVF rapidly when you first look at a mounted ECG. If the sum of the waves in these three leads does not equal zero, the leads may have been recorded incorrectly or mounted improperly.
The 12 ECG leads have two major features, which have already been described. They have both a specific orientation and a specific polarity.
Thus the axis of lead I is oriented horizontally, and the axis of lead aVR points diagonally downward. The orientation of the standard (bipolar) leads is shown in Einthoven's triangle (see Fig. 3-5 ), and the orientation of the augmented (unipolar) limb leads is diagrammed in Figure 3-6 .
The second major feature of the ECG leads, their polarity, can be represented by a line (axis) with a positive and a negative pole. (The polarity and spatial orientation of the leads are discussed further inChapters 4 and 5 when the normal ECG patterns seen in each lead are considered and the concept of electrical axis is explored.)
Do not be confused by the difference in meaning between ECG electrodes and ECG leads. An electrode is simply the metal plate used to detect the electrical currents of the heart in any location. An ECGlead shows the differences in voltage detected by electrodes. For example, lead I presents the differences in voltage detected by the left and right arm electrodes. Therefore a lead is a means of recording the differences in cardiac voltages obtained by different electrodes.
* Although so-called unipolar leads (like bipolar leads) are represented by axes with positive and negative poles, the historical term unipolar does not refer to these poles; rather it refers to the fact that unipolar leads record the voltage in one location relative to an electrode with close to zero potential.
† Augmentation was developed to make the complexes more readable.
RELATIONSHIP BETWEEN LIMB (EXTREMITY) LEADS
Einthoven's triangle in Figure 3-4 shows the relationship of the three standard limb leads (I, II, and III). Similarly, the triaxial diagram in Figure 3-7 shows the relationship of the three augmented limb leads (aVr, aVl, and aVF). For convenience, these two diagrams can be combined so that the axes of all six limb leads intersect at a common point. The result is the hexaxial lead diagram shown in Figure 3-7 . The hexaxial diagram shows the spatial orientation of the six extremity leads (I, II, III, aVR, aVL, and aVF).
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FIGURE 3-7 A, Triaxial diagram of the so-called bipolar leads (I, II, and III). B, Triaxial diagram of the augmented limb leads (aVR, aVL, and aVF). C, The two triaxial diagrams can be combined into a hexaxial diagram that shows the relationship of all six limb leads. The negative pole of each lead is now indicated by a dashed line. |
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The exact relationships among the three augmented extremity leads and the three standard extremity leads can also be described mathematically. For present purposes, however, the following simple guidelines allow you to get an overall impression of the similarities between these two sets of leads.
As you might expect by looking at the hexaxial diagram, the pattern in lead aVL usually resembles that in lead I. The positive poles of lead aVR and lead II, on the other hand, point in opposite directions. Therefore the P-QRS-T pattern recorded by lead aVR is generally the reverse of that recorded by lead II. For example, when lead II shows a qR pattern
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lead aVR usually shows an rS pattern
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Finally, the pattern shown by lead aVF usually but not always resembles that shown by lead III.
CHEST (PRECORDIAL) LEADS
The chest leads (V1 to V6) show the electrical currents of the heart as detected by electrodes placed at different positions on the chest wall. The precordial leads used today are also considered “unipolar” leads in that they measure the voltage in any one location relative to about zero potential ( Box 3-1 ). These chest leads are recorded simply by means of electrodes at six designated locations on the chest wall[*] ( Fig. 3-8 ). Two points are worth mentioning here:
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The fourth intercostal space can be located by placing your finger at the top of the sternum and moving it slowly downward. After you move your finger down about 1½ inches, you can feel a slight horizontal ridge. This is called the angle of Louis, which is located where the manubrium joins the body of the sternum (see Fig. 3-8 ). The second intercostal space is just below and lateral to this point. Move down two more spaces. You are now in the fourth interspace and ready to place lead V4. |
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Chest lead placement in females is complicated by breast tissue, which may result in misplacement of the chest leads. In taking ECGs on women, you must remember to place the electrodeunder the breast for leads V3 to V6. If, as often happens, the electrode is placed on the breast, electrical voltages from higher interspaces are recorded. Also, never use the nipples to locate the position of any of the chest lead electrodes, even in men, because nipple location varies greatly. |
BOX 3-1
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Conventional Placement of ECG Chest Leads |
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Lead V1 is recorded with the electrode in the fourth intercostal space just to the right of the sternum. Lead V2 is recorded with the electrode in the fourth intercostal space just to the left of the sternum. Lead V3 is recorded on a line midway between leads V2 and V4. Lead V4 is recorded in the midclavicular line in the fifth interspace. Lead V5 is recorded in the anterior axillary line at the same level as lead V4. Lead V6 is recorded in the midaxillary line at the same level as lead V4. |
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FIGURE 3-8 Locations of the electrodes for the chest (precordial) leads. |
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The chest leads, like the six limb leads, can be represented diagrammatically ( Fig. 3-9 ). Like the other leads, each chest lead has a positive and negative pole. The positive pole of each chest lead points anteriorly, toward the front of the chest. The negative pole of each chest lead points posteriorly, toward the back (see the dashed lines in Fig. 3-9 ).
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FIGURE 3-9 The positive poles of the chest leads point anteriorly, and the negative poles (dashed lines) point posteriorly. |
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* Sometimes, in special circumstances (e.g., a patient with suspected right ventricular infarction or congenital heart disease), additional leads are placed on the right side of the chest. For example, lead V3R is equivalent to lead V3, but the electrode is placed to the right of the sternum.
THE 12-LEAD ECG: FRONTAL AND HORIZONTAL PLANE LEADS
You may now be wondering why 12 leads are used in clinical electrocardiography. Why not 10 or 22 leads? The reason for exactly 12 leads is partly historical, a matter of the way the ECG has evolved over the years since Einthoven's original 3 limb leads. There is nothing sacred about the electrocardiographer's dozen. In some situations, for example, additional leads are recorded by placing the chest electrode at different positions on the chest wall. Multiple leads are used for good reasons. The heart, after all, is a three-dimensional structure, and its electrical currents spread out in all directions across the body. Recall that the ECG leads were described as being like video cameras by which the electrical activity of the heart can be viewed from different locations. To a certain extent, the more points that are recorded, the more accurate the representation of the heart's electrical activity.
The importance of multiple leads is illustrated in the diagnosis of myocardial infarction (MI). An MI typically affects one localized portion of either the anterior or inferior portion of the left ventricle. The ECG changes produced by an anterior MI are usually best shown by the chest leads, which are close to and face the injured anterior surface of the heart. The changes seen with an inferior MI usually appear only in leads such as II, III, and aVF, which face the injured inferior surface of the heart (see Chapters 8 and 9 ). The 12 leads therefore provide a three-dimensional view of the electrical activity of the heart.
Specifically, the six limb leads (I, II, III, aVR, aVL, aVF) record electrical voltages transmitted onto the frontal plane of the body ( Fig. 3-10 ). (In contrast, the six precordial leads record voltages transmitted onto the horizontal plane.) The frontal plane can be illustrated by the image of facing a large window: The window is parallel to the frontal plane of your body. Similarly, heart voltages directed upward and downward and to the right and left are recorded by the frontal plane leads.
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FIGURE 3-10 Spatial relationships of the six limb leads, which record electrical voltages transmitted onto the frontal plane of the body. |
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The six chest leads (V1 through V6) record heart voltages transmitted onto the horizontal plane of the body ( Fig. 3-11 ). The horizontal plane cuts your body into an upper and a lower half. Similarly, the chest leads record heart voltages directed anteriorly (front) and posteriorly (back), and to the right and left.
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FIGURE 3-11 Spatial relationships of the six chest leads, which record electrical voltages transmitted onto the horizontal plane. |
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The 12 ECG leads are therefore divided into two sets: the 6 extremity leads (3 unipolar and 3 bipolar), which record voltages on the frontal plane of the body; and the 6 chest (precordial) leads, which record voltages on the horizontal plane. Together these 12 leads provide a three-dimensional picture of atrial and ventricular depolarization and repolarization. This multilead display is analogous to having 12 video cameras continuously recording cardiac electrical activity from different angles.
CARDIAC MONITORS AND MONITOR LEADS
BEDSIDE CARDIAC MONITORS
Up to now, this chapter has considered only the standard 12-lead ECG. It is not always necessary or feasible to record a full 12-lead ECG, however. For example, many patients require continuous monitoring for a prolonged period. In such cases, special cardiac monitors are used to give a continuous beat-to-beat record of cardiac activity from one monitor lead. Such ECG monitors are ubiquitous in intensive care units, operating rooms, and postoperative care units, as well as in a variety of other inpatient settings.
Fig. 3-12 is a rhythm strip recorded from a monitor lead obtained by means of three disk electrodes on the chest wall. As shown in Figure 3-13 , one electrode (the positive one) is usually pasted in the V1position. The other two are placed near the right and left shoulders. One serves as the negative electrode and the other as the ground.
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FIGURE 3-12 Rhythm strips from a cardiac monitor taken moments apart but showing exactly opposite patterns. This is because the polarity of the electrodes was reversed in the lower strip (B). |
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FIGURE 3-13 Monitor lead. A chest electrode (+) is placed at the lead V1 position (between the fourth and fifth ribs on the right side of the sternum). The negative (–) electrode is placed near the right shoulder. A ground electrode (G) is placed near the left shoulder. This lead is therefore a modified V1. Another configuration is to place the negative electrode near the left shoulder and the ground electrode near the right shoulder. |
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When the location of the electrodes on the chest wall is varied, the resultant ECG patterns also vary. In addition, if the polarity of the electrodes changes (e.g., the negative electrode is connected to the V1position and the positive electrode to the right shoulder), the ECG shows a completely opposite pattern (see Fig. 3-12 ).
AMBULATORY ECG TECHNOLOGY: HOLTER MONITORS AND LOOP (PATIENT-ACTIVATED EVENT) RECORDERS
The cardiac monitors just described are useful for patients confined to a bed or chair. Sometimes, however, the ECG needs to be recorded in ambulatory patients over longer periods. A special portable system, designed in 1961 by N. J. Holter, records the continuous ECG of patients as they go about their daily activities.
The Holter monitors currently in use consist of electrodes placed on the chest wall that are connected to a special portable analog or digital ECG recorder. The patient can then be monitored over a long period (typically 24 hours). Two ECG leads are usually recorded. The ECG data are played back, and the P-QRS-T complexes are displayed on a special screen for analysis and annotation. Printouts of any portion of the ECG can be obtained for further study and permanent records.
Portable external loop or patient-activated ECG monitors are also available to record ECGs in individuals with more intermittent symptoms. These “event recorders” are designed with replaceable electrodes so that patients can be monitored for prolonged periods (typically up to 2 weeks) as they go about their usual activities. The ECG is continuously recorded and then automatically erased unless the patient presses an event button. When patients experience a symptom (e.g., light-headedness, palpitations, chest discomfort), they can push a button so that a portion of the ECG obtained during the symptom is stored. The saved ECG includes a continuous rhythm strip just before the button was pressed (e.g., 45 sec), as well as a recording after the event mark (e.g., 15 sec). The stored ECGs can be transmitted by phone to an analysis station for immediate diagnosis.
Event recorders can also be used to monitor the ECG for asymptomatic drug effects and potentially important toxicities (e.g., excessive prolongation of the QT interval with drugs such as sotalol, quinidine, or dofetilide) or to detect other potentially proarrhythmic effects (see Chapter 19 ) of drugs.
REVIEW
The electrical currents produced during atrial and ventricular depolarization and repolarization are detected by electrodes placed on the extremities and chest wall; 12 leads are usually recorded:
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The six limb (extremity) leads record voltages from the heart that are directed onto the frontal plane of the body. (This plane divides the body into front and back halves.) The extremity leads include three standard (“bipolar”) limb leads (I, II, and III) and three augmented (“unipolar”) limb leads (aVR, aVL, and aVF).
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The six chest (precordial) leads (V1 to V6) record voltages from the heart as directed onto the horizontal plane of the body (dividing the body into an upper and a lower half). They are taken with electrodes in specific anatomic locations (see Fig. 3-8 ). |
In addition to the 12 conventional leads, ECGs can be taken in special ways. Monitor leads, in which electrodes are placed on the chest, are generally used in cardiac and intensive care units (CCUs and ICUs). Continuous ECGs are often recorded with the Holter apparatus for a period of 24 or more hours in ambulatory patients who are suspected of having a transient or an unpredictable arrhythmia. Very sporadic symptoms can be correlated with ECG rhythm changes by using patient-activated event recorders for periods of up to 2 weeks or so.
QUESTIONS
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Leads I and II are shown below. Draw the P-QRS-T pattern in lead III.
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Leads I, II, and III are shown below. What is wrong with them?
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Draw the hexaxial lead diagram that shows the six frontal plane (limb) leads. |
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Why does the P-QRS-T pattern in lead aVR usually show a reverse of the pattern seen in lead II? |
