1. What are the components used during a hemodialysis procedure?

The components needed to conduct dialysis include a dialysis machine, a method to access the patient’s arterial and/or venous system, a supply of treated water in order to make dialysate, and the dialyzer. The dialysis machine consists of two major components: (1) a blood pump and associated safety equipment for the blood pump to monitor pressures in the system and to ensure that air does not enter the blood circuit; and (2) a dialysate pump, with associated safety devices to ensure that the dialysate is at the correct temperature, has the correct concentration of electrolytes, and has not been exposed to blood from a leak in the dialyzer membrane. The dialysis machine uses the blood pump to move blood through the dialysis circuit, from the patient, through the dialyzer, and back to the patient. The patient needs to have some type of access to allow for the rapid movement of blood through the dialysis circuit. Treated water is needed to make the dialysate used during the dialysis procedure.

2. What is a dialysis access?

A dialysis access allows blood to be removed from the patient, sent to the dialyzer, and then returned to the patient. The minimum blood flow rate that should be delivered is 300 mL/min; thus, peripheral intravenous (IV) lines, Hickman catheters, and peripherally inserted central catheter lines cannot be used for this purpose because none of these devices can deliver a high blood flow rate. There are three different types of vascular access that can be used for hemodialysis: an arteriovenous (AV) fistula, a synthetic graft, or a catheter. The preferred type of vascular access is an AV fistula.

3. What is an AV fistula?

The AV fistula is created by making a surgical anastomosis between an artery in the forearm or upper arm with an adjacent vein. Over a period of 4 to 8 weeks, the increased pressure that the vein is exposed to from the arterial bed causes the vein to dilate and develop a thicker vessel wall. Once the fistula has matured—defined as a blood flow rate of at least 600 mL/min and a diameter of at least 6 mm—the fistula can be cannulated with dialysis needles to allow for the removal and return of blood. Typically, 15-gauge needles are used as dialysis needles, although smaller needles may be used at first if the fistula is not fully mature.

4. What is a synthetic graft and why is it not the optimal type of dialysis access?

A synthetic graft (e.g., GORETEX graft) is created by the surgical interposition of a synthetic blood vessel between an artery and a vein. Both the AV fistula and the synthetic graft are below the skin. An AV fistula is preferred over a synthetic graft because the AV fistula has fewer complications and a longer primary patency rate (intervention free access survival) and secondary patency rate (access survival until abandonment). Specifically, there is a much higher rate of intimal hyperplasia at the vein anastomosis in grafts versus fistulas, resulting in stenosis and ultimately obstruction with thrombosis. Grafts also have higher infection rates due to the presence of a foreign body. Nonetheless, about 30% to 50% of fistulas that are placed are abandoned prior to use due to thrombosis, inadequate blood flow, or complications from access placement.

5. When is a catheter used for dialysis access?

A catheter is used for hemodialysis access when a patient requires dialysis and does not have a mature AV fistula or graft. This circumstance can occur if either the patient needs dialysis acutely and does not have a functional AV fistula or graft in place, or if the patient has no suitable site to place an AV fistula or graft and thus uses the catheter for permanent hemodialysis access. In observational studies, catheters are the least desirable, because patients with this access have a higher rate of morbidity and mortality than do patients with either AV grafts or AV fistulas. Permanent catheters are usually placed into the superior vena cava through the internal jugular vein using a subcutaneous tunnel to decrease the risk of infection. Temporary catheters can be placed without a subcutaneous tunnel into an internal jugular vein or into a femoral vein.

6. What is a dialyzer?

A dialyzer consists of a container that contains a semipermeable membrane that separates the dialysate from the blood that has been removed from the patient. A hollow fiber dialyzer, the type most commonly used today, consists of a cylinder that contains more than 10,000 hollow fibers that are made of a semipermeable material. To maximize the diffusion that takes place between the blood and dialysate, blood travels through the hollow fibers, and dialysate flows around the outside of these hollow fibers in a countercurrent direction from the blood. The size of the dialyzer is measured by the surface area of the semipermeable membrane and is expressed in square meters. Most adult hemodialysis membranes have a surface area between 1.5 and 2.5 m ² ; pediatric dialyzers are often less than 1 m ² . A number of different materials are used for the semipermeable membranes. These materials vary in the degree to which small and middle molecules can pass through the membrane, which, in turn, is determined by the number and size of the pores in the dialyzer membrane. In the United States, most membranes are composed of either semisynthetic or synthetic materials that allow for the removal of larger molecules to some degree. Typical dialyzer membranes consist of polysulfone, polyethersulfone, cellulose acetate, biacetate or triacetate, or polyamides.

7. What is dialysate?

Dialysate is a physiologic solution that consists of both inorganic ions found in the body and glucose. The dialysate concentration of sodium and chloride is usually physiologic, whereas the concentration of magnesium and phosphorus is usually less than physiologic to allow for the removal of these substances on dialysis. The bicarbonate concentration is usually higher than the physiologic concentration to allow for the treatment of metabolic acidosis, which is common in patients undergoing dialysis. Typically, several different potassium and calcium concentrations are available so that the rate of removal of these ions can be varied as clinical circumstances dictate. The dialysate flows through the dialyzer in a countercurrent direction, preferably at a rate that is 1.5 times the blood flow rate to maximize diffusion. The temperature of the dialysate is usually set at just below the patient’s body temperature, as this setting will allow for vasoconstriction and thus minimize the risk of hypotension with volume removal on dialysis. The dialysate temperature can be adjusted by 1°C to 2°C to assist with volume removal.

8. What occurs during the dialysis procedure?

The two processes that occur during a hemodialysis session are diffusion and ultrafiltration. Diffusion refers to a process by which small and middle molecules move, based on concentration gradients, between the blood and dialysate compartments of a dialyzer via a semipermeable membrane. Molecules can move through the semipermeable membrane by both diffusion and ultrafiltration. Small molecules move across the semipermeable membrane from an area of higher concentration (usually the blood) to an area of lower concentration (usually the dialysate). The overall effect is to remove small molecules that are likely to be toxins in high concentrations (such as potassium, phosphorus, and urea) while repleting those small molecules that are likely to be deficient (such as calcium or bicarbonate). Larger molecules do not move as readily across the membrane, and molecules that are bound to protein are unlikely to be removed by dialysis. In addition, fluid can be removed during the dialysis procedure via the process of ultrafiltration (see Question 10).

9. What determines the rate of toxin removal?

The rate of toxin removal is traditionally measured by the removal of urea. The removal of urea during the hemodialysis session is increased by any of these factors:

• •
  

  Higher blood flow rate and dialysis flow rate: The higher the blood flow rate, the more urea diffusion that will occur per unit time. The limiting factor is the dialysis access, with a fistula usually being able to deliver the highest flow rates (up to about 550 mL/min) and a catheter usually being able to deliver the lowest flow rates (300 to 350 mL/min).

  
  
• •
  

  Higher efficiency of the dialyzer: A higher-efficiency dialyzer typically has a large surface area, a thin membrane, and increased porosity. This efficiency is expressed as the dialyzer mass transfer area coefficient or KoA of the dialyzer.

  
  
• •
  

  Longer time on dialysis: The longer the time for a single dialysis treatment, the more urea diffusion will occur. However, the urea removal per hour diminishes with each additional hour of dialysis provided.

  
  
• •
  

  Frequency of dialysis: The standard form of in-center hemodialysis is three sessions per week, but increasing frequency increases the amount of urea and other uremic toxins that can be removed.

10. How does fluid removal occur during the dialysis procedure?

The removal of water during the dialysis treatment is referred to as ultrafiltration. During a hemodialysis treatment, a transmembrane hydrostatic pressure gradient develops between the blood and dialysate compartments. The total pressure difference between these two compartments determines the rate of ultrafiltration. The removal of fluid during the hemodialysis session is increased by any of these factors:

• •
  

  Higher transmembrane hydrostatic pressure: In most modern dialysis machines, the amount of fluid to be removed during the dialysis session is set on the dialysis machine, and the machine will automatically adjust the pressures to allow for the appropriate amount of fluid to be removed. A higher transmembrane pressure results in more fluid being removed per unit time.

  
  
• •
  

  Higher ultrafiltration coefficient (K _Uf ) of the dialysis membrane: The value of this coefficient is dependent on the dialyzer surface area, composition, thickness, and porosity.

  
  
• •
  

  Longer duration of a dialysis session. Ultrafiltration will vary based on K _uf and transmembrane pressure. This will result in a set amount of ultrafiltration per minute; by extending the number of minutes, one can get more ultrafiltration.

11. How does one determine the amount of fluid to remove during a hemodialysis session?

Typically, a patient will gain between 1% and 5% of their body weight from fluid accumulation between dialysis sessions. Patients receiving chronic hemodialysis are assigned a dry weight by their nephrologist. A common definition of dry weight is the weight below which patients become hypotensive on dialysis. A more precise physiologic definition of dry weight is the body weight at a physiologic extracellular volume state. Practically speaking, the dry weight is the weight at which the patient is euvolemic on a minimal number of blood pressure medications. The dry weight is set based on clinical findings and by patient response to removing additional fluid. The patient’s dry weight will vary over time as a result of changes in appetite, the presence of diarrhea, and the like; thus, the patient’s dry weight should be reassessed on a regular basis.

12. How does one prevent clotting of blood in the blood circuit system during hemodialysis?

Heparin is routinely given during the hemodialysis treatment to prevent thrombosis in the extracorporeal circuit. Heparin is usually given as a bolus at the initiation of dialysis, followed by a constant infusion. The initial dose of heparin required is weight based; however, over time, it is individualized for each patient receiving dialysis to prevent complications. The appropriate dose of heparin is generally the amount that prevents clotting in the extracorporeal circuit but, at the same time, does not lead to bleeding from the needle puncture sites for more than 10 minutes after the needles are removed at the end of the hemodialysis treatment. In patients who are at high risk for bleeding, such as in postoperative patients or those with gastrointestinal bleeding, no heparin is given with monitoring of the dialyzer to ensure that it does not clot.

13. What are the difference types of hemodialysis that can be performed?

The most common type of hemodialysis performed in the United States is in-center hemodialysis. Other types of hemodialysis include in-center nocturnal hemodialysis and home hemodialysis. In-center hemodialysis is typically performed three times per week, with a session duration of 3 to 4.5 hours. A fourth treatment per week is sometimes added if additional fluid needs to be removed due to excess weight gain between dialysis treatments.

14. What are the different types of home hemodialysis?

Home hemodialysis is performed by about 1% of patients receiving chronic hemodialysis in the United States. Patients performing home hemodialysis need a partner to assist with the dialysis procedure (or at a minimum to assist with emergencies) and will need about 4 to 8 weeks of training to learn the home hemodialysis procedure. Patients can perform dialysis with a catheter or by self-cannulation using a fistula. Several different hemodialysis modalities can be performed at home, including conventional three times per week dialysis, short (2 to 3 hours per session) daily (six times per week) hemodialysis, and overnight or nocturnal (6 to 8 hours per session) hemodialysis performed three to six times per week. For the latter method, several different types of alarms are used to assess for blood or dialysate leakage as patients typically sleep during this form of hemodialysis.

15. How is in-center nocturnal hemodialysis performed?

Nocturnal in-center hemodialysis is performed in an outpatient hemodialysis center overnight for 6 to 8 hours per treatment, and is typically performed three times per week. Patients typically perform dialysis in bed and sleep during the dialysis session.

16. How is the dose of dialysis determined?

The dose of dialysis is determined by measuring the urea concentration in the blood at the start and end of the dialysis procedure to determine the amount of urea that is removed during the dialysis session and concurrently determining the amount of fluid removed during a single hemodialysis session. Once these values are available, a variety of techniques can be used to determine the dose of dialysis provided to the patient.

17. What are the methods for measuring the dose of dialysis?

The dose of dialysis can be expressed as the urea reduction ratio (URR), the single pool Kt/V, the double pool or equilibrated Kt/V, or the weekly or standard Kt/V. The first three expressions can be used only for patients who receive hemodialysis three times per week, whereas the standard or weekly Kt/V can be used to estimate the dose of dialysis regardless of the number of times per week that the patient receives hemodialysis.

• 1.
  

  The URR is expressed as follows:

Related posts:

Urinalysis Obstructive uropathy Viral hepatitis-associated glomerulonephritis Kidney diseases in the elderly Pharmacologic treatment of hypertension Nephrology beginnings

Stay updated, free articles. Join our Telegram channel

Join