The dialyzer is a selective filter for removing toxic or unwanted solutes from the blood. The filtration process uses a semipermeable membrane between blood flowing on one side and dialysis fluid, called dialysate, flowing on the opposite side. The delivery system prepares dialysate of correct chemical composition, and then delivers it at proper temperature and other parameters to the dialyzer.
All dialyzers consist of a series of parallel flow paths designed to provide a large surface area between blood and membrane, and membrane and dialysate. There are two basic flow path geometries: (1) rectangular cross section, seen in parallel plate dialyzers; and (2) circular cross section, seen in hollow-fiber dialyzers. Virtually all hemodialyzers in clinical use today are the hollow-fiber type; however, for historical purposes, the parallel plate and coil dialyzers will be discussed.
Coil dialyzers
What are characteristics of coil dialyzers?
Coil dialyzers consist of a cellulose acetate membrane tightly wrapped around a plastic or metal coil and encased in a rigid plastic housing. The coil dialyzer was the first type of dialyzer sold commercially and mass produced, and is known as the dialyzer of the 1950s. The priming volume of the coil dialyzer was very large, ultrafiltration was unpredictable, and blood leaks were quite common.
Parallel plate dialyzers
What are characteristics of parallel plate dialyzers?
Parallel plate dialyzers are assembled in layers, like a sandwich. Sheets of membrane are placed between supporting plates, which have ridges, grooves, or crosshatches to support the membrane and allow the flow of dialysate along it. The blood flows through the sheets of the membrane. The contained volume of blood is small, and heparin requirements are also usually small. A disadvantage of parallel plate dialyzers is that they are compliant. This means that the volume of blood that they hold increases as the transmembrane pressure (TMP) increases. The main disadvantage of parallel plate dialyzers is that they are not well suited for reuse.
Hollow-fiber dialyzers
The hollow-fiber artificial kidney (HFAK) is by far the most commonly used dialyzer. HFAKs are available in a wide variety of sizes and membranes.
What serves as the semipermeable membrane in the hollow-fiber artificial kidney?
Tiny hollow fibers of about 150 to 250 μm in diameter are used. Blood flows through these tens of thousands of hollow fibers. They are formed from a variety of materials, both cellulosic and synthetic. Wall thickness may be as little as 7 μm, although some synthetics have walls of 50 μm or more (Fig. 6-1).
Figure 6-1 Hollow-fiber dialyzer.
What are advantages of hollow-fiber dialyzers?
The contained blood volume is very low in relation to the dialyzer’s surface area because of the dialyzer’s flow geometry. Resistance to blood flow is low because of the large number of blood passages. Hollow-fiber dialyzers are not compliant; therefore, they do not increase in shape or in the volume they hold under high TMP. Ultrafiltration can be precisely controlled. They are well adapted to reuse.
What are disadvantages of the hollow-fiber artificial kidney?
Meticulous deaeration of the fiber bundle is required before beginning a dialysis procedure. Otherwise, fibers may air lock and not admit blood. There may be uneven distribution of blood at the inflow header space, with reduced perfusion of some of the center fibers. Hollow-fiber dialyzers may be sterilized with ethylene oxide (ETO). Residual toxic products of ETO sterilization retained in the potting material of the headers can cause adverse patient reactions. Patient may require higher heparin doses to keep hollow fibers from clotting.
Are there other ways to sterilize hollow-fiber dialyzers?
Yes. Some producers use gamma irradiation. Other manufacturers employ steam sterilization. Both methods are effective. Electron beam, or e-beam, is a newer method of factory sterilization of hollow-fiber dialyzers. Electron beam sterilization involves the use of high-energy electrons to process dialyzers. The DNA chains of the microorganisms become disrupted and are inactivated, which produces a sterile dialyzer. The electron beam does not use chemicals or radioactive materials for its sterilization process and may be a good alternative for the patient who is sensitive to ETO.
Membranes for hemodialysis
Membranes used in hemodialysis are of two basic types: (1) organic cellulose derivatives and (2) synthetic membranes.
Willem Kolff used cellulose sausage casings for the first successful clinical dialysis. Cellulosic membranes continue to be basic for many dialyzers. Synthetic membranes were developed in the search for efficient, large-volume seawater desalinization by reverse osmosis. The development of volume-controlled ultrafiltration equipment for hemodialysis made the use of these high-hydraulic permeability membranes practical. They, in turn, have made high-flux hemodialysis, hemofiltration, and continuous renal replacement therapy (CRRT) viable options in renal treatment.
What is the nature of a cellulosic membrane?
Cellulose (C6H10O5) is a complex carbohydrate polymer that is the structural material of plants. Commercial cellulose is obtained from wood products and cotton. Treatment with heat and chemicals produces a liquid slurry, which is coagulated and formed into sheets or extruded through dies as hollow fibers. Different kinds of processing result in membranes of various thicknesses, water-absorptive qualities, and permeabilities.
What accounts for the permeability of cellulosic membranes?
Electron microscopy shows that the fibers of cellulosic membranes swell when wet, forming a tortuous maze. The “pores” are actually twisting, irregular tunnels that force water or a solute molecule to travel a distance several times the thickness of the membrane to get through it.
What are some cellulosic membranes now used in hemodialysis?
Cuprophan has been widely used. The cellulose is treated with ammonia and copper oxide during manufacture; cuprammonium rayon and Hemophan are modifications. Saponified cellulose ester, cellulose acetate, and triacetate are other widely used cellulose materials.
What are the advantages and disadvantages of cellulosic membranes?
One distinct advantage of cellulosic membranes is the fact that they have been used for many years, and therefore their transport characteristics are well known. They are also relatively inexpensive. However, all cellulosic membranes have some degree of bioincompatibility with blood. This poses a number of problems, which will be discussed later in this chapter.
What are features of synthetic membranes?
The synthetic membranes are thermoplastics. They have a thin, smooth luminal surface supported by a spongelike wall structure. Those used for hemodialysis include polyacrylonitrile (PAN), polysulfone (PS), polyamide, polymethyl methacrylate (PMMA), and others. Convective transfer accounts for their overall mass transport. You will see greater removal of middle and large molecules with these membranes. All have ultrafiltration coefficients of 20 to 70 mL/h/mm Hg or more. They are well adapted to reuse. Synthetic membranes have much fewer bioincompatibility problems than cellulosic membranes.
What are negative aspects of synthetic membranes?
Synthetic membranes have several negative aspects, including the following:
• They are expensive in comparison with cellulosic membranes.
• Automated ultrafiltration control is required because of the very high water permeability.
• Adsorption of proteins to the membrane surface can be a problem.
• The high permeability creates a risk of backfiltration from dialysate to blood.
Membrane biocompatibility
Each time blood comes in contact with a foreign surface, an inflammatory response is elicited. This response is used to gauge the biocompatibility of a hemodialysis membrane. When there is an intense reaction and a high level of inflammation, the membrane is said to be bioincompatible. When the response and inflammation are mild, the membrane is classified as biocompatible. The level of membrane biocompatibility may be associated with both short- and long-term consequences.
How does one measure the level of inflammatory response resulting from blood-membrane interaction during hemodialysis?
The intensity of the reaction is measured by the level of complement generation following initiation of hemodialysis. Markers used to evaluate complement activation are C3a, C5a, and the “membrane attack complex”—C5b through C9—in the patient’s blood.
What is complement activation?
The complement system is a series of plasma proteins that react sequentially to cause a variety of biologic events. This system works with the immune system to defend the body from substances that the body determines to be “nonself.” When blood encounters a hemodialysis membrane, the response elicited is similar to the one that occurs when the body’s immune system is challenged by bacteria.
What are some of the intradialytic manifestations of complement activation?
The first clinical manifestation to be associated with complement activation is leukopenia. Immediately after starting hemodialysis using cellulosic membrane, patients’ white blood cell (WBC) counts drop sharply. This begins to correct after about 15 minutes. By the end of a four-hour dialysis, the WBC count will have returned to the initial level or perhaps be slightly higher, due to a compensatory response by the bone marrow. This leukopenia is transient, but it may be important to patients with compromised cardiac or pulmonary systems. C5a, an end product of the complement cascade, activates WBCs. When WBCs are activated, they become “sticky.” These cells aggregate, or clump, and are sequestered in the first capillary bed they encounter, usually the lungs. The clumps of WBCs reduce pulmonary capillary perfusion and reduce the patient’s ability to efficiently exchange oxygen and carbon dioxide between blood and alveolar air. This may manifest as intradialytic hypoxemia. Other intradialytic problems likely associated with complement activation include chest pain, back pain, coagulation abnormalities, and, in severe cases, anaphylaxis. Activation of complement peaks at 15 minutes and can last as long as 90 minutes. The amount of complement generated relates to the type and surface area of the membrane being used.
Which membranes induce the highest levels of complement activation?
Cellulose and cellulose-based membranes induce more complement activation than do synthetic membranes (Table 6-1). The chemical composition of the cellulosic surface is similar to that of the cell wall of bacteria: both are chains of polysaccharide structures. The body responds to blood-cellulose contact in much the same way as it does to invasion by bacteria. Free hydroxyl groups on the membrane surface are likely the primary source of the intense complement activation. Chemical alterations to buffer the free hydroxyl groups are used to create “modified cellulosic membranes,” such as cellulose acetate and Hemophan. Membranes of cellulose acetate have some of the surface hydroxyl linked with acetyl groups. Hemophan membranes have amino groups attached to the reactive sites to buffer them. Both of these modifications reduce the amount of complement generated; however, these membranes are still less effective than synthetic membranes in minimizing complement production.
Table 6-1 Types of Dialyzer
Why do synthetic membranes induce less complement activation than cellulosic membranes?
Being synthetic, these membranes lack the reactive sites found on cellulose-based membranes; thus the amount of complement activation generated during hemodialysis is less than with cellulosic membranes.
What are some of the long-term considerations when selecting a membrane for hemodialysis?
Long-term use of bioincompatible membranes may be associated with an increased incidence of infection and malignancy and with impaired nutritional status. Patients dialyzed on cellulosic membranes have a higher incidence of β2-amyloid disease (β2AD) than those dialyzed on synthetic membranes. The increased risk of infection and malignancy is thought to be due to repeated attacks on the patient’s immune system. When patient blood is repeatedly exposed to bioincompatible surfaces, the body responds as though under attack. The immune system kicks in, complement is generated, and the inflammatory response is triggered. There can be tissue damage, and future stimuli may elicit only a limited response, thus predisposing the individual to infection and potential malignancy.
Malnutrition is a major contributor to morbidity and mortality of patients on hemodialysis. Even with adequate protein intake, malnutrition is a problem and seems to relate to an accelerated catabolic process, most evident on dialysis days. A catabolic effect associated with bioincompatible membrane is well documented. However, recent studies have demonstrated an increase in protein catabolism during hemodialysis with synthetic membranes as well (Ikizler et al., 2002). β2AD is important in long-term morbidity. Clinical manifestations include arthropathies, bone lesions and pathologic fractures, soft tissue swelling, and carpal tunnel syndrome. Patients being dialyzed with cellulosic bioincompatible membranes exhibit more pronounced clinical symptoms of amyloidosis (Schiffl et al., 2000). Possible reasons for the difference include the following:
• Cellulosic membranes do not have the capability to remove molecules as large as β2-microglobulin.
• Cellulosic membranes induce high levels of complement generation, and some products of complement activation may be responsible for the release of β2-microglobulin from monocytes.
How does membrane biocompatibility affect patients with some residual renal function?
In one study, patients dialyzed on cellulose acetate membranes appeared to lose residual renal function more rapidly than those dialyzed on polysulfone membranes.
Does biocompatibility of the membrane affect patients in acute renal failure?
This is not clear. There is a consensus that the more compatible membranes do contribute to better recovery and survival. Less complement activation, less WBC activation, and less inflammation associated with more biocompatible membranes are believed to be responsible.
Dialyzer reuse
Dialyzer reprocessing is the process of cleaning and sterilizing a dialyzer once used, to be used again on that same patient. “Reuse” refers to the clinical use of the reprocessed dialyzer. Dialyzer reuse has been done for many years in many dialysis clinics and is a safe and effective way to keep the cost of the dialysis treatment within reason. Dialyzer reuse peaked in the late 1990s, with 80% of dialysis centers participating in dialyzer reprocesing. Since then, dialyzer reuse has declined, with approximately only 40% of dialysis centers participating in a reuse program (Lacson & Lazarus, 2006). Strict standards must be followed to reprocess dialyzers; these are set by the Association for the Advancement of Medical Instrumentation (AAMI) Standards and Recommended Practices for reuse of hemodialyzers.
What are the advantages of reuse?
Fundamentally, with reuse the average cost per dialysis is substantially reduced. The “first-use syndrome,” an infrequent phenomenon of chest or back pain, nausea, and malaise occurring in the first half hour of a run with a new cellulose dialyzer, is absent or rare with reused dialyzers. A decrease in the generation of biomedical waste is also an advantage of dialyzer reuse.
What are disadvantages of reuse?
Processing, testing, identification, and storage of reused units require space and personnel time. Consumption of high-quality water is greatly increased. Sterilizing agents, particularly formaldehyde, are a hazard to personnel and to patients. Quality control of manual processing is difficult to ensure. Automated systems minimize these problems, but at high initial cost. Risk of bacterial contamination to the patient and the potential for transmission of infectious agents are other disadvantages of dialyzer reuse.
Are there guidelines for reuse procedures?
Guidelines for reuse procedures are defined by the AAMI and subsequently were given the force of law by the U.S. Food and Drug Administration (FDA). The Centers for Medicare & Medicaid Services (CMS) Conditions for Coverage for End-Stage Renal Disease require facilities that practice dialyzer reuse to meet the AAMI guidelines for dialyzer reprocessing. (See Chapter 9 for more on reuse of dialyzers.)
Delivery systems
The delivery system prepares and delivers dialysate to the dialyzer unit. Most systems provide dialysate for a single patient; others have the capacity to supply several dialyzer stations simultaneously.
What is the solution delivery system?
The Solution Delivery System (SDS) is a method of delivering the solutions used to make dialysate to the machine. Bicarbonate from a mixing tank and acid from a storage tank are transferred to an overhead holding tank called a “head” tank. The solutions are then gravity fed to a solution distribution system and then fed to the patient care area to be delivered through a series of pipes to the machines.
What are the functions of dialysate?
Dialysate carries away the waste materials and fluid removed from the blood by the dialysis procedure; prevents the removal of essential electrolytes while helping to normalize electrolyte levels; and averts excess water removal during the procedure. Dialysate also functions to correct the acid-base balance of the patient. These functions are achieved by making the chemical composition of the dialysate correspond as nearly as possible to that of normal plasma water.
What chemicals are used?
There are usually five compounds involved in dialysate: sodium chloride, sodium bicarbonate or sodium acetate, calcium chloride, potassium chloride, and magnesium chloride. Glucose may be included in some formulations.
How are the chemicals made into dialysate?
Manufacturers provide dialysis concentrate in containers of various sizes. The sodium chloride content is near saturation, and the other constituents are in proportion to their final concentration in the dialysate. There is some equipment for making concentrate on site from dry chemicals, which reduces transportation costs.
How is bicarbonate-based dialysate prepared?
Calcium and magnesium will not remain in a solution with bicarbonate because of the low hydrogen ion content. To solve this, two separate concentrates are used. The proportioning (delivery) system is more complex because it must mix and monitor three liquids instead of only two. The dialysate is prepared by combining treated water with an acid concentrate and a base concentrate.
What chemicals are in the bicarbonate concentrates?
The “A” (indicating acidified) concentrate contains most of the sodium; all of the calcium, magnesium, and potassium; chloride; and a small amount of acetic acid to maintain pH low enough to keep the calcium and magnesium in solution when mixed into dialysate.
The “B” (bicarbonate) concentrate contains the sodium bicarbonate. Some systems include part of the sodium chloride as well as the B concentrate; this raises the total conductivity, making it easier to monitor the concentrate. Table 6-2 shows the tabular formula used with one volume-volume type dilution.
Table 6-2 Tabular Formula for One Volume-Volume Type Dilution
In the proportioning system, the B concentrate is usually diluted partially with water; the A concentrate is then proportioned into the mixture just before it goes to the dialyzer. In the closed system, carbon dioxide cannot bubble off, the reaction between sodium bicarbonate and acetic acid cannot proceed to completion, and the hydrogen ion content keeps the calcium in solution.
What are the potential problems with bicarbonate dialysate?
Liquid B concentrate is not stable; some manufacturers add a small amount of special polymer as a stabilizer. Others provide dry NaHCO4 as the powder, to be mixed at the facility. The mixing process requires care so that much of the carbon dioxide formed during the procedure is not lost from solution; the concentrate must be used within 24 hours of mixing.
Bicarbonate concentrate is very susceptible to bacterial contamination and proliferation. The stabilized solution should be used within the manufacurer’s designated time frame. All containers for mixing, holding, or dispensing B concentrate must be scrupulously sanitized at regular intervals. Contamination must be avoided. One manufacturer uses a delivery system that accepts a closed container of dry bicarbonate on a special holder. Warm water passes through the column, producing a saturated solution of bicarbonate that is proportioned with water and then with the A concentrate by a conductivity-controlled feedback system.
There are many formulations of A concentrate to tailor the final dialysate sodium, potassium, calcium, and magnesium to the dialysis prescription. Each brand of delivery system has its unique proportioning and mixing ratios. Extreme care must be exercised to ensure that the concentrates selected are correct for the delivery system being used.
What happens if the bicarbonate concentrate is overmixed?
Care must be taken so as not to overmix the bicarbonate concentrate because vigorous mixing may result in a loss of carbon dioxide from the solution. This will increase the pH of the solution and potentiate the precipitation of calcium and magnesium carbonate in the fluid pathway. This can cause the patient to experience a drop in serum calcium as the calcium level in the dialysate is lowered. A timer should be used to prevent overmixing of the dialysate concentrate.
What kind of water is used to prepare dialysate?
The water used to prepare dialysate must meet AAMI standards for chemical content and for bacterial and pyrogen content. In most instances, this involves complex and expensive treatment of feed water. Chapter 8 discusses the various processes involved to achieve “dialysis-quality water.”
Current AAMI standards for product water used to prepare dialysate suggest that the microbial count be less than 200 colony forming units (CFU)/mL and the endotoxin concentration be less than 2 endotoxin units (EU)/mL, with respective action levels of 50 CFU/mL and 1 EU/mL.
What is the lal test?
The LAL test is used to detect and quantify bacterial endotoxins. LAL is the abbreviation for limulus amebocyte lysate. It is an assay for endotoxin that uses a protein extract from the Limulus, or horseshoe crab. It is reported in nanograms per milliliter or in endotoxin units (EU) (1 ng/mL = 5 EU/mL).
Why is dialysate verification and monitoring so important?
Serious patient reactions and deaths have resulted from dialysate preparation errors or equipment malfunction. Dialysate must be verified for each dialysis. Each delivery system should have a function check daily.
What methods are used to check the dialysate composition?
Two general methods, primary and secondary, are used to check the dialysate composition. A primary method specifically measures the concentration of one solute by a laboratory method of known reliability. Usually two solutes are determined, such as sodium by flame photometry and chloride by titration. This is particularly important for bicarbonate dialysis to ensure proper ratio of acid to bicarbonate, as well as the ratio of concentrate to water.
What secondary tests are used?
The most common test of dialysate is the total conductivity. This does not measure specific ions but the overall conductivity contributed by all ions (hence it is a secondary test). Conductivity meters must be calibrated carefully to the “normal” or “safe” range for each type of concentrate used. If two or more dialysate formulas are used, the safe range for each must be clearly identified because each has a different ionic concentration. Most manufacturers indicate on the label of the concentrate containers the conductivity of their dialysate when it is properly mixed.
An alternative secondary check of dialysate is the measurement of total osmolality by either freezing-point depression or vapor pressure. These measure total solute in the dialysate.
How do proportioning systems correctly mix dialysate?
Liquid concentrate is required to correctly mix dialysate. Several systems for mixing the correct proportions of concentrate and water have been used, but those in most widespread use employ microprocessor circuitry to control the speed of proportioning pumps, based on continuous conductivity and other parameter monitoring downstream from the mixing area (Fig. 6-2). The speed of the pumps and thus the volumes of the concentrates added are precisely controlled by the electronic feedback circuit to ensure that the dialysis fluid is properly mixed.
Figure 6-2 Prototype of electronic proportioning system for bicarbonate dialysate, with volumetric ultrafiltration control.
What are disadvantages of proportioning systems?
These very complex, highly sophisticated, microprocessor-controlled electronic and hydraulic devices are quite expensive. Many functions are preprogrammed and may not be readily changed. Sensors and monitoring devices must be fail-safe and redundant. Troubleshooting is often difficult, and factory-based service personnel may be needed for repairs.
How does a sorbent regenerative supply system work?
In a sorbent regenerative supply system a small volume of used dialysate is recirculated through a cartridge of adsorbent materials and chemically regenerated. Metabolic waste products transferred from the dialyzer to the dialysate are removed and the electrolyte content and pH are restored (Fig. 6-3).
Figure 6-3 Sorbent regenerative supply system.
How does a sorbent system function?
Three actions are involved in a sorbent system: (1) conversion of urea to ammonium carbonate, (2) adsorption of creatinine and other nonionized solutes, and (3) exchange of the ions sodium zirconium phosphate and zirconium oxide.
What is the process of urea conversion?
As dialysate enters the sorbent regenerative cartridge (Fig. 6-4) it first passes a carbon layer, which removes heavy metals and oxidants; it then contacts a bed containing urease. Urease is an enzyme that converts urea to ammonium and carbonate.
Figure 6-4 Sorbent regenerative cartridge.
How are ammonium ions handled?
The next segment of the sorbent regenerative cartridge contains sodium zirconium phosphate, which acts as an exchange resin, taking up the ammonium ions and releasing sodium and hydrogen ions at a ratio of about 1 Na+ to 9 H+. The carbonate ions equilibrate with hydrogen ions to yield bicarbonate ions (as NaHCO3) and carbon dioxide.
Are other ions exchanged in the column?
Calcium, magnesium, and potassium ions are also exchanged for sodium ions by the sodium zirconium phosphate. In addition, the third portion of the sorbent regenerative cartridge contains hydrated zirconium oxide, which removes phosphate ions and fluoride.
How are creatinine and nitrogenous materials removed?
The final layer of the sorbent column contains activated carbon (charcoal) that binds by adsorption of creatinine, uric acid, guanidines, and other organic metabolites.
Is it necessary to replace calcium, magnesium, or potassium?
Yes. An infusion of calcium and magnesium is made just before the dialysate returns to the dialyzer to keep the desired concentration of these ions. Potassium is added, or omitted, according to the physician’s desire.
What are advantages and disadvantages of the sorbent system?
Advantages of the sorbent system include its portability and the absence of need for a special water supply or drain connections; it can be used wherever electricity is available.
Disadvantages of the sorbent system include the somewhat expensive sorbent cartridge. There is a limit to the capacity to digest urea and to the absorption of ammonia. Ammonia may accumulate in the system and in the patient. Very large patients or those with very high serum urea values may require more than one cartridge per dialysis. Conversely, if you are dialyzing a patient with a low urea content or are dialyzing for a short length of time, a risk of hyponatremia or metabolic acidosis exists because of the small amount of urea available for the cartridge to generate bicarbonate and sodium.
Additional equipment and functions
Along with the dialysate mixing function, heaters, deaerators, and pH and concentration monitors, DDSs include a number of complementary functions essential to the hemodialysis procedure. These include the blood pump with an indicator for an estimate of blood flow rate, heparin infusion pump, air/foam detector, and inflow and outflow pressure sensors. On the dialysate side are temperature controls and monitors, conductivity monitors, flow rate controls, pressure sensors, and ultrafiltration volume controls. An online urea sensor in the effluent dialysate line has potential for truly accurate quantification of urea removal. These functions are controlled by several electronic microcontrollers and programmable microprocessors with appropriate parameters displayed on video screens.
How is dialysate temperature controlled?
The heater and/or heat exchanger is controlled by one or more sensors and a microcontroller circuit. Fluid temperature should hold within 0.5°C of the set point. There should be a separate sensor, independent of the heat control, for online monitoring with visual and audible alarms for any out-of-limits state. Accuracy should be checked regularly with a certified glass thermometer. Many patients with CKD have a body core temperature of 36°C to 36.5°C. Added heat in excess of replacement causes a vasodilatory response, which may be detrimental at a time when the normal vasoconstrictive response to reduced volemia from ultrafiltration is acting to minimize hypotension. Fluid temperature greater than 41°C causes hemolysis of red blood cells, which can continue for several hours.
Why are deaeration devices necessary?
Water contains considerable dissolved air and microbubbles. When it is warmed, the dissolved air comes out of solution as expanding microbubbles. These have a negative effect on temperature and conductivity sensors and on flowmeters. Bubbles can reduce dialysate-membrane contact in hollow-fiber dialyzers.
Most deaeration devices use warmers along with negative pressure to bring the dissolved gases out of solution. An air trap or coalescing filter then captures the gases and vents them to the outside.
What problems are associated with dialysate flowmeters?
Solute film tends to build up with time and reduce accuracy. Calibration of the flowmeter or flow controller should be part of the routine servicing of the machine. At the bedside, actual flow can be quickly determined by a timed, measured outflow collection from the drain hose. Ultrafiltration control should be set to zero temporarily during the measurement.
What is the importance of the dialysate pressure monitors?
If ultrafiltration is regulated by adjusting dialysate negative pressure, both inflow and outflow monitors must have high and low alarm set points, and must be accurate within ±20 mm Hg, or 10% of reading. Manufacturer’s directions must be followed carefully in adjusting and calibrating these monitors.
If ultrafiltration is controlled volumetrically or flowmetrically, the dialysate pressure monitors serve as a check on the ultrafiltration control and the TMP.
How is dialysate concentration controlled and monitored?
The most suitable apparatus to control and monitor dialysate concentration is the conductivity monitor (Fig. 6-5), which must be temperature compensated. Normal accuracy is ±1% to 3%. The conductivity sensor is basically an electrolytic cell, as described in Chapter 3. The electrodes of continually operating monitors are eroded by electrolytic action, with a gradual loss of sensitivity over time. Most delivery systems use at least dual conductivity sensors, the readings of which must match. A confirmation test with a handheld conductivity meter should be routine each day. Before any adjustment of the conductivity monitors, a primary test, such as the laboratory measurement of sodium or of chloride, should be done to verify the actual composition of the dialysate at the time.
Figure 6-5 Conductivity monitor.
Is dialysate ph monitored?
There should be some type of pH verification. The approved AAMI pH range is 6.9 to 7.6. There should be audible and visual alarms for any out-of-limits state. Sensors for pH drift with the passage of time and must be recalibrated by the manufacturer’s personnel.
How is blood flow rate measured?
Blood is moved through the dialyzer by peristalic rollers that work by progressively compressing special segments of blood tubing against the semicircular housing. Most blood pumps have speed indicators calibrated to show flow according to the speed of rotation. The internal diameter of the pumping segments of the tubing in use must match that for which the pump indicator was calibrated. Variations in tubing, pressure conditions in the blood circuit, and lack of linearity across the indicator scale cause discrepancies of ±10% to 15% between indicated and actual blood flow.
The calibration of each pump should be verified regularly, under standard conditions, with the same brand and lot of tubing used clinically. Water at 37°C should be pumped from a container through the tubing, which is partially clamped to approximate the negative pressure between the needle and the inflow side of the pump during dialysis. The outflow should be collected in a graduated cylinder for three to five minutes. Volume (milliliters) divided by time (minutes) gives the flow rate. A record of each calibration should be kept on the machine and in the central file.
How do blood leak detectors work?
Blood leak detectors are situated in the effluent dialysate line (Fig. 6-6). A beam of light is directed through a column of dialysate onto a photoelectric cell. A change in translucence and light scatter in dialysate reduces the light received by the photocell, stopping the blood pump and activating visible and audible alarms.
Figure 6-6 Blood leak detector.
(Adapted from Nosé Y: Manual on artificial organs, vol 1, The artificial kidney, St. Louis, 1969, Mosby.)
AAMI standards require detection of less than 0.45 mL/min of blood at hematocrit 45 over a range of dialysate flows. Particulate matter or air bubbles are frequent sources of false alarms. If a blood leak is not easily confirmed visually, the dialysate should be checked with a Hemastix. If standard maintenance procedures do not eliminate the problem, the manufacturer’s representative should correct or replace the unit.
How do air bubbles in blood detectors behave?
Whenever a pump is used to propel blood through the extracorporeal circuit, some degree of negative pressure is created at the intake side. Air may be sucked into the line at connections that are not absolutely tight (such as at the needle hub), through needle punctures or breaks in the tubing, or from empty fluid containers attached at the infusion sidearm. These potential air sources are especially important because pumping speed is increased in the efforts to achieve high blood flow rates. Air in blood can obstruct fibers of HFAKs and, if the quantity is sufficient, air may pass the venous bubble trap and go on to cause massive air embolism to the patient.
A commonly used detector employs an ultrasonic beam to identify air, foam, and microbubbles in blood (Fig. 6-7). Sound travels more quickly through fluid than through air, thus even minuscule bubbles slow the sonic beam and result in an alarm. Sonic detectors may be armed while the bloodlines contain only saline; they do not respond to light or light changes in the surrounding environment.
Figure 6-7 Sonic air/foam detector.
Most air/foam detectors have no external sensitivity adjustment. The dialysis delivery system can be operated with the detector disarmed for priming and rinsing. There should be a low-level alarm, clearly discernible from a distance, to indicate the disarmed status. No patient should be permitted to dialyze with the air/foam detector in the disarmed state.
How do ultrafiltration controls work?
These devices exactly match the outflow dialysate volume with the inflow volume, in addition to a precisely measured extra effluent volume representing the desired (programmed) ultrafiltrate. There are two basic types of ultrafiltration controllers: volumetric and flowmetric (Figs. 6-8 and 6-9).
Figure 6-8 Volumetric ultrafiltration control.
(Redrawn from Vlchek DL: Staying tuned in to the high-tech world. Part 2: Dialysis delivery systems, Dial Trans, Aug 18, 1989.)
Figure 6-9 Servo-feedback ultrafiltration control.
(Redrawn from Vlchek DL: Staying tuned in to the high-tech world. Part 2: Dialysis delivery systems, Dial Trans, Aug 18, 1989.)
How do volumetric ultrafiltration devices operate?
The most common volumetric system employs two diaphragm chambers to balance the inflow and outflow dialysate (Fig. 6-10). While one side of the first chamber is filling with fresh dialysate, its diaphragm is forcing out an equal volume of used fluid from the other side. Simultaneously in the second chamber, one side is filling with spent fluid from the dialyzer while the opposite side is ejecting an equal volume of fresh dialysate to the dialyzer. When the diaphragms have deflected across the width of the chambers, valves are reversed so that the side that was emptying now fills, and vice versa. The volume for ultrafiltrate is removed from the outflow dialysate channel by a metering pump before the outflow volume is matched to inflow, thus removing the desired amount of ultrafiltrate from the patient. Because the volumes in and out of the controller are precisely equalized, whatever pressure (negative or positive) is necessary will be created for the removal of the measured ultrafiltrate volume by the dialyzer.
Figure 6-10 Volumetric ultrafiltration. Matched double diaphragm chambers.
How does the flowmetric ultrafiltrate control system work?
In a flowmetric system there are one or two very accurate flowmeters in both the inflow and the outflow dialysate pathways to measure the flow of fluid passing through these pathways. The speed of the dialysate pump in the outflow path is varied by the electronic control module so that the volume through the outflow meter is exactly equal to the volume through the inflow meter, plus the programmed amount of ultrafiltrate.
High-efficiency and high-flux dialysis
What are equipment needs for high-efficiency dialysis?
High-efficiency dialysis has the following four requirements:
• A highly permeable cellulosic membrane (ultrathin cuprophan, Hemophan, cellulose acetate ester, etc.) with surface area 1.5 m2 or more
• Reliable blood flow of 350 mL/min or more; dialysate flow of 750 mL/min or more
• Bicarbonate dialysate delivery system
• Ultrafiltration control system
The combination of a large area of membrane of high mass transfer capability and high blood flow rate and dialysate flow rate produces increased small molecule transfer. Intermediate-size and large solute transfer rates are enhanced by the area and permeability increases.
What are the system requirements for high-flux dialysis?
As with high-efficiency dialysis, in the high-flux system it is important to maintain high blood flow rate, high dialysate flow rate, and precision control of ultrafiltration volume. High-flux dialyzers use synthetic membranes of very high permeability, with convective transfer providing a major share of solute transport (see page 84). These dialyzers have ultrafiltration coefficients of 20 to 70 mL/h/mm Hg or more. You will see a much greater volume of urea clearance with these dialyzers.
The ultrafiltration controller precisely manages net fluid removal, but in so doing generates a dialysate pressure profile that creates reverse filtration from dialysate to blood in the distal portion of the dialyzer (Fig. 6-11). A problem of contamination of blood by pyrogenic material and endotoxin fragments is created because high-flux membranes readily pass particles of 2000 to 10,000 Da. The LAL test (see Glossary and page 67) is used to monitor dialysate for endotoxins.
Figure 6-11 Backfiltration and pressure distribution.
(From Baurmeister U, et al.: Dialysate contamination and back filtration may limit the use of high-flux dialysis membranes, ASAIO Transact 35:21, 1989.)
How might the problem of reverse filtration be countered?
AAMI standards require less than 200 colony-forming units (CFU) of bacteria per milliliter in water for dialysate, and less than 200 CFU of bacteria or 2 EU by LAL test for endotoxins in dialysate leaving the delivery system. Bacterial multiplication continues as the dialysis fluid courses through the dialyzer; pyrogenic materials increase and cross the high-flux membrane during backfiltration.
Addition of a molecular filter or ultrafilter (see Chapter 8) to the dialysate path immediately ahead of the dialyzer may be necessary for high-flux dialysis. A 50,000- to 100,000-Da ultrafilter will reject intact endotoxins; a 1000- to 10,000-Da ultrafilter will be necessary if endotoxin fragments are the problem. Some manufacturers have a provision for ultrafilters in their delivery systems.