540 UNIT VIII RENAL AND UROLOGICAL DISORDERS
There are two types of dialysis: hemodialysis and peritoneal dialysis. One functioning kidney can perform all functions and maintain homeostasis. When both kidneys are no longer functioning and the patient is in relatively good health, renal transplant may be considered. Peritoneal Dialysis In peritoneal dialysis (PD) , the patient’s peritoneum is filled with a dialysis solution that pulls wastes and extra fluid from the blood into the abdominal cavity. The dialysis solution, called the dialysate, contains certain electrolytes that cause diffusion of solutes and ultrafiltration of fluid from the blood to cross the peritoneal membrane. The process works based on the principle that diffusion of substances in water tends to move from an area of high concentration to an area of low concentration. After the fluid is instilled, it sits in the peritoneal cavity for a period, called a dwell time, of approximately 4 hours. After the dwell time, the solution is drained from the peritoneal cav- ity and discarded. PD is uncommon, but at times it is used as an alternative to hemodialysis. The process of draining and filling takes about 30 to 40 minutes. A typical schedule of PD requires approximately four exchanges a day, each with a dwell time of 4 to 6 hours (See Fig. 22-5).
Hemodialysis Hemodialysis is a treatment during which the patient’s blood is drawn out of the body at a rate of 200 to 400 mL/minute and passed through a device called a dialyzer. There are two sections in the dia- lyzer: the section for dialysate and the section for the blood. The two sections are divided by a semiperme- able membrane, which has microscopic perforations that allow only some substances to cross. Because it is semipermeable, the membrane allows water and waste to pass through, but does not allow blood cells to pass through. Commonly, a patient has an arteriove- nous fistula created in the arm that can facilitate this process. For example, a tubular connection between the brachiocephalic artery and cephalic vein is often surgically implanted. Blood is drained from the bra- chiocephalic artery and pumped into a dialyzer, which removes excess solutes and fluid from the blood. The blood is then returned to the body via the cephalic vein. The dialysis solution is a sterile solution of electrolytes. Urea and other waste products, such as potassium and phosphate, diffuse into the dialysis solution. During the treatment, the patient’s entire blood volume (about 5000 mL) circulates through the machine every 15 minutes. Electrolytes, serum albumin, BUN, and serum creatinine are normalized during the dialysis procedure. The procedure is usually required at least three times a week, and each session lasts 4 to 6 hours (See Fig. 22-6).
Dialysis solution
Heparin pump (to prevent clotting)
Dialyzer inflow pressure monitor
Peritoneal lining
Inflow
Venous pressure monitor
Air trap and detector
outflow
Dialyzer
Peritoneal cavity
Air detector clamp
Arterial pressure monitor
Blood removed for dialysis
Dialyzer blood return to body
Blood pump
FIGURE 22-5. Peritoneal Dialysis : In peritoneal dialysis, dial- ysate solution is instilled into and removed from the perito- neal cavity at regular intervals to achieve clearance of solutes. The dialysate solution has a high concentration of dextrose. Ultrafiltration of water is achieved by the creation of an osmotic gradient across the peritoneal membrane. It is a slow process that is sometimes better tolerated by hypotensive patients than hemodialysis.
FIGURE 22-6. Hemodialysis: In hemodialysis, blood is removed from the body and filtered through a man-made membrane called a dialyzer, and then the filtered blood is returned to the body.
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