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Saturday, February 6, 2021



When kidney dysfunction is severe enough to cause homeostatic abnormalities that cannot be corrected with diet or medications, dialysis is performed to artificially replace the kidney’s major functions. The major goals of dialysis are to support the elimination of nitrogenous waste products, restore fluid and electrolyte homeostasis, and restore normal plasma pH. The major indications are listed in the plate.


Plate 10-9


Dialysis employs a semipermeable membrane to alter the composition of blood. Blood is located on one side of the membrane, whereas a wash solution, known as the dialysate, is on the opposite side. The objective is for the desirable electrolytes to move from the dialysate to the blood and for the undesirable electrolytes to move in the opposite direction. The movement of fluid and solutes across the membrane depends on two physical forces: diffusion and convection.

In diffusion, solute transport is directly dependent on the concentration gradient of the solute, diffusivity of the solute, permeability of the membrane, and surface area across the membrane. The smaller the molecule, the more rapidly it will diffuse. Molecules continue to move across the membrane until equilibrium is achieved.

In convection, solutes are dragged across a membrane in the solvent that contains them. An analogy would be an ocean wave (the solvent) pushing sea shells (the solute) onto the shore. The solvent carrying these solutes crosses the membrane in a process known as ultrafiltration, which depends on the pressure gradients across the membrane.

Diffusion is more efficient at clearing small molecular weight substances (less than 500 Da), such as electrolytes. In contrast, convection is more efficient at clearing medium molecular weight substances (500 to 5000 Da), such as vitamin B12 or drugs (e.g., vancomycin).

Hemodialysis. In hemodialysis, blood leaves the patient and flows through tubing into a dialyzer. The dialyzer contains numerous hollow fibers composed of semipermeable membranes. As blood flows through these fibers, dialysate flows around them in the opposite direction. Molecules are exchanged across the fiber walls. The blood then returns to the patient.

Because blood and dialysate flow in opposite directions, concentration gradients are maintained across the entire length of the dialyzer.  As a result, potassium, nitrogenous waste products, phosphorus, and other substances that have accumulated in the blood diffuse into the dialysate. Meanwhile, substances that are concentrated in the dialysate, such as bicarbonate and other electrolytes at specific concentrations, diffuse into the blood to restore desired levels. As diffusion occurs, the hydrostatic pressure in the dialyzer leads to ultrafiltration of fluid and convection of larger solutes.

The patient’s vasculature can be accessed using either a central venous catheter (CVC) or a connection between an artery and vein (fistula or graft). A CVC used for dialysis contains two lumens and is inserted into a large central vein. The rapid and substantial amount of flow (up to 400 to 500 mL/min) drawn from these veins allows blood to efficiently exit the vein through one lumen, enter the dialysis circuit, and return to the vein through another lumen. The high blood flow also prevents stasis, which could lead to clotting, and optimizes the exchange of solutes across the membrane. Heparin is often used at intervals to prevent clotting within the dialysis circuit.

The main disadvantage of CVCs is their infection risk. To decrease this risk, catheters are often tunneled, meaning they are passed through a subcutaneous tract before being inserted into the central vein. This process lengthens the distance that skin flora must travel before being able to cause a systemic infection.

In addition to the risk of infection, catheters can also clot and kink, and they can incite an inflammatory reaction that leads to venous stenosis. Thus even tunneled CVCs should be considered temporary access routes to be used only while awaiting creation of a more permanent solution, such as an arteriovenous fistula or graft. An arteriovenous fistula permits the high blood flow of the artery to be shunted into a neighboring vein. After a fistula is surgically created, the vein will dilate and thicken over the course of 6 to 8 weeks, after which dialysis needles can safely be inserted and removed as needed. Fistulas most often join the cephalic vein and radial artery in a side-to-side or end-to-side anastomosis, although many other configurations are possible.

If a patient has diseased peripheral vasculature that would not permit the creation of a fistula, usually because of complications from diabetes mellitus, an artificial graft can be implanted to join the artery and vein. These grafts, often made of polytetrafluoroethylene, can be used for hemodialysis within 1 to 2 weeks of implantation. Their disadvantages, however, are that they do not remain patent for as long as fistulas, and that they are more likely to become stenosed or thrombosed.

Once vascular access has been established, several different hemodialysis schedules can be used. A standard schedule consists of 3- to 4-hour sessions occurring three times per week. Nocturnal hemodialysis, also performed three times per week, consists of 8- to 10-hour sessions (at reduced blood and dialysate flow rates) performed while the patient sleeps. Short daily hemodialysis consists of 2- or 3-hour sessions occurring five to six times per week. At-home dialysis is becoming increasingly common and allows for more flexible schedules compared to in-center hemodialysis.


Peritoneal Dialysis. In peritoneal dialysis, dialysate is instilled into the intraperitoneal space. Blood in the peritoneal capillaries exchanges material with the dialysate using the peritoneal membrane as a natural semipermeable membrane. The dialysate dwells in the peritoneum for 2 to 12 hours and is then removed. Each sequence of instillation, dwelling, and draining is known as a cycle (or exchange).

The dialysate is a sterile solution that contains variable concentrations of glucose. While the dialysate is in the peritoneum, substances such as urea and potassium diffuse from the capillaries into the dialysate, whereas glucose and lactate diffuse in the opposite direction. Fluid ultrafiltration occurs because of the osmotic gradient established by the glucose, resulting in the convective clearance of larger molecules.

The dialysate is instilled into the abdominal cavity through a surgically tunneled catheter (Tenckhoff), which remains in place between sessions. Given the risk of peritonitis with an indwelling catheter, patients must be instructed to perform each exchange using sterile technique.

Several different schedules may be used. In continuous ambulatory peritoneal dialysis (CAPD), approximately four exchanges are performed per day. During each session, 1.5 to 3 L of dialysate dwell in the peritoneal cavity for 6 hours. The patient must manually instill and then drain or remove the dialysate. In automated peritoneal dialysis (APD), 4 to 5 exchanges occur overnight. During each session, 1.5 to 3 L of dialysate dwell in the peritoneal cavity for 2 hours. In this case, a machine performs the exchanges. Continuous cyclic PD (CCPD) is a regimen in which 3 to 4 exchanges are performed automatically overnight, while 1 to 2 long dwell exchanges are performed manually during the day.

Peritoneal dialysis is less efficient than hemodialysis; however, because it is performed daily, patients still attain adequate clearance and generally feel better and have fewer dietary restrictions than with in-center hemodialysis. In addition, the patient can perform peritoneal dialysis at home and with less equipment than required by hemodialysis.




Continuous renal replacement therapy (CRRT) is similar to hemodialysis in some respects; however, sessions are continuous, rather than discrete, and the fl ow rate is lower (100 to 300 mL/min). CRRT is performed when patients require dialysis but are hemodynamically unstable or have homeostatic abnormalities that cannot be addressed with an individual hemodialysis session. For example, if a patient in renal failure is expected to receive a large volume load (in the form of transfusions or antibiotics), it may be advantageous to receive continuous renal replacement therapy.

As in hemodialysis, a connection is established between the patient’s vasculature and an extracorporeal apparatus. Access is established using a central venous catheter. AV fistulae or grafts cannot be used because the constant presence of needles in these vessels (in contrast to the episodic presence associated with hemodialysis sessions) could lead to damage and infection, which would prevent future use.

CRRT uses the principles of hemodialysis (diffusive clearance) and hemofiltration (convective clearance) either alone or in combination. A variety of different configurations may be used. The most common include:

Slow continuous ultrafiltration (SCUF) In this modality, fluid is removed from the blood by hemofiltration alone. No dialysate is used. This modality is generally used for fluid-overloaded patients (e.g., congestive heart failure) who are not responsive to diuretics but who have preserved electrolyte balance.

Continuous venovenous hemofiltration (CVVH) In this modality, convective clearance is achieved by using hydrostatic pressure to ultrafilter plasma across a membrane, as with SCUF. In this case, however, a replacement fluid is added either before or after the blood enters the filter cartridge; it is similar in content to dialysate and, when mixed with blood, brings its electrolyte composition into a desirable range.

Continuous venovenous hemodialysis (CVVHD) CVVHD is similar to hemodialysis as described earlier but is continuous rather than episodic. This modality consists primarily of diffusive clearance of small molecules. Some convective clearance occurs, but to a lesser extent than diffusive clearance.

Continuous venovenous hemodiafiltration (CVVHDF) CVVHDF combines both CVVHD and CVVH. In this modality, a replacement solution infuses into the blood either prefiltration or postfiltration. At the same time, a dialysate solution runs countercurrent to the blood in the filter cartridge. This modality removes both small and medium-sized molecules from the blood.

Studies are currently being performed to compare the relative advantages of and indications for CVVH, CVVHD, and CVVHDF.

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