Question

12. Discuss the mechanisms of peritoneal dialysis (PD) and hemodialysis (HD) as renal replacement therapies.

Answer 1

Mechanism of peritoneal dialysis:

Peritoneal access is obtained by inserting a catheter through the anterior abdominal wall. The catheter is connected to a sterile tubing system and secured to the abdomen with tape. The catheter is irrigated immediately with heparinised dialysate to clear the blood and fibrin from it. Prophylactic antibiotics may also be instilled. The irrigation may continue for 12-24 hours using small volumes of dialysate.

The 3 phases of the PD cycle are inflow(fill), dwell(equilibration), and drain. The three phases are called an exchange. During inflow, a prescribed amount of solution, usually 2L is infused through an established catheter over about 10 minutes. After the solution has infused, the inflow clamp is closed before air enters the tubing.

The next part of the cycle is the dwell phase, during which diffusion and osmosis occur between the patient's blood and the peritoneal cavity. The dwell time can last 20-30minutes to 8 or more hours depending on the method of PD.

Drain time takes 15-30 minutes and may be facilitated by gently massaging the abdomen or changing position. The cycle starts again with the infusion of another 2L of solution.

Mechanism of hemodialysis:

To initiate dialysis in a patient with an arteriovenous graft or arteriovenous fistula, two needles are placed in the fistula or graft. The needle close to the fistula or the red catheter lumen is used to pull blood from the patient and send it to the dialyzer with the assistance of a blood pump. The dialyzer and blood lines are usually primed with up to 1000mL of saline solution to eliminate air from the system. Heparin is added to the blood as it flows in to the dialyzer because anytime blood contacts a foreign substance, it had a tendency to clot. When the blood enters the extracorporeal circuit, it is propelled through the top of the dialyzer by a blood pump at a flow rate of 200-500mL/min, while the dialysate circulates in the opposite direction at a rate of 300-900mL/min.Blood is returned from the dialyzer to the patient through the second needle or blue catheter.

Dialysis is terminated by flushing the dialyzer with the saline solution to return all blood through the access. Needles are then removed from the patient and firm pressure is applied to the venipuncture site until the bleeding stops.