Title 22. RENAL DIALYSIS.pdf ✅

PHARMD GURU Page 1  Dialysis is an artificial process in which the accumulation of drugs or waste metabolites is removed by diffusion from the body into the dialysis fluid.  Two common dialysis treatments are peritoneal dialysis and hemodialysis.  The principle underlying both processes is that as the uremic blood or fluid is equilibrated with the dialysis fluid across a dialysis membrane, waste metabolites from the patient’s blood or fluid diffuse into the dialysis fluid and are removed.  The dialysate is balanced with electrolytes and with respect to osmotic pressure.  The dialysate contains water, dextrose, electrolytes (potassium, sodium, chloride, bicarbonate, acetate, calcium, etc), and other elements similar to normal body fluids without the toxins.  Peritoneal dialysis uses the peritoneal membrane in the abdomen as the filter.  The peritoneum consists of visceral and parietal components.  The peritoneum membrane provides a large natural surface area for diffusion of approximately 1–2 m2 in adults; it is permeable to solutes of molecular weights ≤30,000 Da (Merck Manual, 1996–1997).  However, only a small portion of the total splanchnic blood flow (70 mL/min out of 1200 mL/min at rest) comes into contact with the peritoneum and gets dialyzed.  Placement of a peritoneal catheter is surgically simpler than hemodialysis and does not require vascular surgery and heparinization.  The dialysis fluid is pumped into the peritoneal cavity, where waste metabolites in the body fluid are discharged rapidly.  The dialysate is drained and fresh dialysate is reinstilled and then drained periodically. RENAL DIALYSIS DIALYSIS PERITONEAL DIALYSIS
PHARMD GURU Page 2  Peritoneal dialysis is also more amenable to self-treatment. However, slower drug clearance rates are obtained with peritoneal dialysis compared to hemodialysis, and thus longer dialysis time is required.  Continuous ambulatory peritoneal dialysis (CAPD) is the most common form of peritoneal dialysis.  Many diabetic patients become uremic as a result of lack of control of their disease.  About 2 L of dialysis fluid is instilled into the peritoneal cavity of the patient through a surgically placed resident catheter.  The objective is to remove accumulated urea and other metabolic waste in the body.  The catheter is sealed and the patient is able to continue in an ambulatory mode.  Every 4–6 hours, the fluid is emptied from the peritoneal cavity and replaced with fresh dialysis fluid.  The technique uses about 2 L of dialysis fluid; it does not require a dialysis machine and can be performed at home.  Hemodialysis uses a dialysis machine and filters blood through an artificial membrane.  Hemodialysis requires access to the blood vessels to allow the blood to flow to the dialysis machine and back to the body.  For temporary access, a shunt is created in the arm, with one tube inserted into an artery and another tube inserted into a vein.  The tubes are joined above the skin. For permanent access to the blood vessels, an arteriovenous fistula or graft is created by a surgical procedure to allow access to the artery and vein.  Patients who are on chronic hemodialysis treatment need to be aware of the need for infection control of the surgical site of the fistula. HEMODIALYSIS
PHARMD GURU Page 3  At the start of the hemodialysis procedure, an arterial needle allows the blood to flow to the dialysis machine, and blood is returned to the patient to the venous side.  Heparin is used to prevent blood clotting during the dialysis period.  During hemodialysis, the blood flows through the dialysis machine, where the waste material is removed from the blood by diffusion through an artificial membrane before the blood is returned to the body.  Hemodialysis is a much more effective method of drug removal and is preferred in situations when rapid removal of the drug from the body is important, as in overdose or poisoning.  In practice, hemodialysis is most often used for patients with end-stage renal failure.  Early dialysis is appropriate for patients with acute renal failure in whom resumption of renal function can be expected and in patients who are to be renally transplanted.  Other patients may be placed on dialysis according to clinical judgment concerning the patient’s quality of life and risk/ benefit ratio (Carpenter and Lazarus, 1994).  Dialysis may be required from once every 2 days to 3 times a week, with each treatment period lasting for 2–4 hours.  The time required for dialysis depends on the amount of residual renal function in the patient, any complicating illness (eg, diabetes mellitus), the size and weight of the patient, including muscle mass, and the efficiency of the dialysis process.  Dosing of drugs in patients receiving hemodialysis is affected greatly by the frequency and type of dialysis machine used and by the physicochemical and pharmacokinetic properties of the drug.  Factors that affect drug removal in hemodialysis are listed in Table 24-8. These factors are carefully considered before hemodialysis is used for drug removal.  In hemodialysis, blood is pumped to the dialyzer by a roller pump at a rate of 300–450 mL/min.  The drug and metabolites diffuse from the blood through the semipermeable membrane.
PHARMD GURU Page 4  In addition, hydrostatic pressure also forces the drug molecules into the dialysate by ultrafiltration.  The composition of the dialysate is similar to plasma but may be altered according to the needs of the patient.  Many dialysis machines use a hollow fiber or capillary dialyzer in which the semipermeable membrane is made into fine capillaries, of which thousands are packed into bundles with blood flowing through the capillaries and the dialysate circulating outside the capillaries.  The permeability characteristics of the membrane and the membrane surface area are determinants of drug diffusion and ultrafiltration.  The efficacy of hemodialysis membranes for the removal of vancomycin by hemodialysis has been reviewed by De Hart (1996).  Vancomycin is an antibiotic effective against most Gram-positive organisms such as Staphylococcus aureus, which may be responsible for vascular access infections in patients undergoing dialysis.  In De Hart’s study, vancomycin hemodialysis in patients was compared using a cuprophan membrane or a cellulose acetate and polyacrylonitrile membrane.  The cellulose acetate and polyacrylonitrile membrane is considered a “high-flux” filter.  Serum vancomycin concentrations decreased only 6.3% after dialysis when using the cuprophan membrane, whereas the serum drug concentration decreased 13.6%–19.4% after dialysis with the cellulose acetate and polyacrylonitrile membrane.  In dialysis involving uremic patients receiving drugs for therapy, the rate at which a given drug is removed depends on the flow rate of blood to the dialysis machine and the performance of the dialysis machine.  The term dialysance is used to describe the process of drug removal from the dialysis machine.  Dialysance is a clearance term similar in meaning to renal clearance, and it describes the amount of blood completely cleared of drugs (in mL/min).  Dialysance is defined by the equation: