Nephrology nurses, in their roles as educators and patient advocates, are responsible for knowing about both hemodialysis and peritoneal dialysis so that they can provide accurate and complete information to their patients. This article will review the principles of dialysis, describe the commonalities between the two modalities, and present a general overview of PD.

Basic Concepts. Dialysis is the process of removing waste products and water from the bloodstream. Necessary elements include a semipermeable membrane, dialysate solution, and a surgically created access. Blood is separated from dialysate solution by a semipermeable membrane. Because the dialysate solution contains physiologic amounts of electrolytes and buffers, exposure to blood across the membrane allows diffusion to begin. Solutes dissolved in the blood, such as blood urea nitrogen (BUN) and creatinine, cross the membrane from an area of greater to an area of lesser concentration. Osmosis is the other process at work, whereby water moves across the membrane into the dialysate, which by virtue of its composition has a lesser concentration of water molecules. (Levy, Morgan, & Brown, 2001). Both HD and PD rely on these basic principles of osmosis and diffusion.

Membranes. Hemodialyzer membranes have characteristics that provide varying ultrafiltration (UF) and waste clearance properties. In PD, the peritoneal membrane also has unique characteristics, which promote clearance and UF. It measures from 1–2 square meters in adults, and many fine walled capillaries provide blood flow (White, Korthius, & Granger, 1994). Though the object of both PD and HD is clearance and UF, the process by which they occur differs.

Dialysate. Dialysate plays a role in UF in both PD and HD because it contains osmotic agents to allow the removal of excess water molecules from the bloodstream. The sodium content of dialysate is varied during an HD treatment to achieve fluid balance, while peritoneal dialysate employs various dextrose concentrations or glucose polymers. HD has the added advantage of helping fluid transfer through use of volumetric control machines, while the PD patient is taught to use the different dialysate solutions to regulate body water. Increasing the percentage of dextrose in the PD dialysate increases the UF. However, if PD solution is allowed to remain in the peritoneal cavity (dwell) for much longer than 8 hours, equilibrium occurs between the dialysate and the blood. Dextrose may actually be reabsorbed along with dialysate water. Therefore, it is crucial for a PD patient to maintain a regular schedule of treatments. A newer solution called icodextrin employs a long chain glucose polymer in a long dwell (greater than 8 hours). The premise is that the icodextrin molecule is too large to be reabsorbed and will maintain a concentration gradient and, thus, UF over time (Frampton & Plosker, 2003).

Adequate Dialysis. Adequate dialysis is achieved in terms of small solute clearance and is an equal challenge in HD and PD. In PD, the Kt/V and creatinine clearances are reported weekly rather than per treatment. When small solute clearance values fall below standard in both modalities, steps are taken to achieve adequate dialysis. In HD, dialyzer size and/or membrane, blood flow, and time on treatment can be manipulated to improve clearances. In PD, the volume of dialysate in the abdomen can be increased as well as the amount of time the fluid is in contact with the membrane before being replaced with fresh solution (an “exchange”). In addition, the number of exchanges is often increased to meet adequacy targets.

Access. A well functioning access is key to both HD and PD. While vascular accesses are created to move blood outside the body and deliver it to the dialyzer, a peritoneal access must deliver dialysate to the peritoneal membrane. PD catheters come in a variety of configurations. They may commonly be referred to as Tenckhoff catheters because Dr. Henry Tenckhoff developed the first successful catheter for chronic use in 1968 (Twardowski & Khanna, 1994). They may either be placed in the operating room, in an interventional radiology suite, or peritoneoscopically (Asif, Merrill, Brouwer, Roth, & Ash, 2004). In all cases, the catheter is first placed into the lower abdominal cavity. A skin tunnel is then created to provide a barrier against infection, much like tunneled vascular access catheters. The catheter comes through the skin at the exit site, and, at some point, a transfer set is attached, which allows inflow and outflow of dialysate. A brand new PD catheter should be dressed with a non-occlusive dressing that is changed only under sterile conditions by a PD nurse to prevent early bacterial colonization and exit site or tunnel infection (Gokal et al., 1998). The new PD catheter cannot be used for about 2 weeks to allow for healing and prevent subcutaneous dialysate leakage. Early care includes low volume, in and out flushes with peritoneal dialysate to ascertain patency and function. Potential problems initially include fibrin occlusion, migration, visceral damage, malfunction due to adhesion formation, infection, subcutaneous leak, pleural leak, and hematoma formation in the tunnel.

Choices. Several factors help decide if PD is the right choice for an individual’s lifestyle and physical condition. Patients who are ambulatory, work, want independence, live a distance from the nearest dialysis unit, like to travel, and like to participate in their care are apt to choose PD. Often, patients who are severely limited in mobility or who live in a nursing home will opt for PD because someone else is available to perform the therapy. Medical reasons to use PD include failed vascular accesses and severely compromised cardiac function. Because there are no fluctuations in blood volume, patients who experience intradialytic hypotension may fare better on PD. Studies have found that the incidence of delayed graft function and acute renal failure immediately posttransplant are lower in patients who had been treated with PD (Van Biesen, Vanholder, & Lameire, 2000). This may be a compelling reason for anyone desiring transplant to choose PD. In addition, residual renal function is generally maintained over a longer period of time compared to patients on HD, which is helpful in reaching fluid balance and small solute clearance goals (Lysaught, 1996). A particular advantage of PD is that because of its continuous nature, potassium is not nearly as restricted in the diet as it is on HD, nor is fluid. In fact, it is not uncommon to find hypokalemia and dehydration in certain PD patients. The only absolute contraindications to PD are adhesions and a scarred membrane, which would impede free flow of dialysate in and out of the peritoneal cavity and could also hamper clearance and UF. Relative contraindications are abdominal conditions such as colitis or diverticulitis, severe psychiatric disorders, a proven history of non-adherence, and lack of support systems or adequate housing.

PD is offered either as continuous ambulatory peritoneal dialysis (CAPD), which is done manually, or automated peritoneal dialysis (APD), which uses a cycler to do exchanges while the patient sleeps.

Nursing Responsibilities. The PD nurse is responsible for training and follow up for all aspects of the therapy. Phone contact may be as often as daily, as education is ongoing. The PD nurse may carry  from 20-25 primary patients who may visit weekly, monthly, or as needed for emergencies. PD nurses are on call when the clinic is closed and, as a result, may serve to prevent hospital admissions. Complications of PD include peritonitis, exit site infection, and catheter malfunction. The experienced PD nurse can manage these using standing orders, and most often they resolve without modality failure.

In HD, a nurse or a technician cares for the patient during treatment. The PD patient performs self-care, with the support and encouragement of the PD team. By knowing about the basics of PD and HD, nephrology nurses are in a good position to answer questions from patients who may have an interest in the modality. They can also understand more about PD and HD patients and have a better appreciation of the roles of PD and HD nurses.


References
Asif, A., Merrill, D., Brouwer, D., Roth, D., & Ash, S. (2004). Procedural nephrology: Changing the face of renal disease care. Dialysis & Transplantation, 33(5), 258-265.

Frampton, J.E., & Plosker, G.L. (2003). Icodextrin: A review of its use in peritoneal dialysis. Drugs, 63(19), 2079-2105.

Gokal, R., Alexander, S., Ash, S., Chen, T.W., Danielson, A., Holmes, C., Joffe, P., Moncrief, J., Nichols, K., Piraino, B., Prowant, B., Slingeneyer, A., Stegmayr, B., Twardowski, Z., & Vas, S. (1998). Peritoneal catheters and exit-site practices toward optimum peritoneal access: 1998 update. Peritoneal Dialysis International, 18(1), 11-33.

Levy, J., Morgan, J., & Brown, E. (2001). Oxford handbook of dialysis (p. 74). New York: Oxford University Press.

Lysaught, M. (1996). Preservation of residual renal function in maintenance dialysis patients. Peritoneal Dialysis International, 16, 126-127.

Twardowski, Z., & Khanna, R. (1994). Peritoneal dialysis access and exit site care. In R. Gokal & K. Nolph (Eds.), The textbook of peritoneal dialysis (pp. 271-314). Netherlands: Kluwer Academic Publishers.

Van Biesen, W., Vanholder, R., & Lameire, N. (2000). The role of peritoneal dialysis as the first-line renal replacement modality. Peritoneal Dialysis International, 20(4), 375-383.

White, R., Korthius, R, & Granger, D.N. (1994). The Peritoneal microcirculation in peritoneal dialysis. In R. Gokal & K. Nolph (Eds.), The textbook of peritoneal dialysis (pp. 45-68). Netherlands: Kluwer Academic Publishers.