HEMODIALYSIS, PERITONEAL DIALYSIS, AND CONTINUOUS THERAPIES
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.
PRINCIPLES OF DIALYSIS
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.
|VASCULAR ACCESS FOR 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.