Peritoneal Dialysis I: Introduction and Procedure

Peritoneal dialysis (PD) is a procedure that facilitates the exchange of solutes, waste products, electrolytes, and excess fluid between the blood in the peritoneal capillaries and a dialysis solution introduced into the peritoneal cavity.

Principles of Peritoneal Dialysis (PD)

• Diffusion: Waste products such as urea and electrolytes move from high concentrations in the blood to low concentrations in the dialysate across the peritoneal membrane. This mechanism is driven by the concentration gradient and the membrane's permeability.
• Osmosis: Water moves from the blood into the hyperosmotic dialysate due to the osmotic gradient generated by the high glucose concentration in the dialysate, which assists in removing excess fluid from the body.
• Ultrafiltration: Fluid removal is enhanced by an osmotic pressure gradient created by glucose in the dialysate. This gradient draws water across the peritoneal membrane into the peritoneal cavity, aiding in the removal of excess fluid.
• Convection: As water moves across the peritoneal membrane into the dialysate, it carries solutes in a process known as convection or solvent drag. This process assists in removing both fluid and waste products from the body.

Other important factors that influence the efficiency of PD include:

• Membrane permeability: The peritoneal membrane's ability to transfer solutes and water varies among patients, affecting PD's effectiveness.
• Dwell time: The length of time the dialysate remains in the peritoneal cavity impacts both waste and fluid removal.
• Dialysate composition: The dialysate composition, including glucose and electrolyte concentrations, can be adjusted to balance fluid and electrolyte levels based on individual patient needs.

Catheter Placement

A catheter is inserted through the anterior abdominal wall to access the peritoneal cavity. The catheter is approximately 24 inches (60 cm) long and has one or two Dacron cuffs that anchor it in place and prevent microorganisms from entering the peritoneum. Over several weeks, fibrous tissue grows into the Dacron cuff, securing the catheter and preventing bacterial penetration. The catheter's tip, which rests in the peritoneal cavity, has multiple perforations near the distal end to facilitate fluid exchange.

Dialysis Cycle

PD involves instilling a dialysis solution into the peritoneal cavity. The PD cycle has three phases: inflow (fill), dwell (equilibration), and drain, collectively known as an exchange. An exchange in manual PD typically takes about 30 to 50 minutes. During the inflow phase, a prescribed volume of solution, usually 2 liters, is infused through the catheter over about 10 minutes. If the patient feels discomfort, the flow rate can be adjusted to a slower pace. Once the solution is infused, the inflow clamp is closed.

Dialysis Solutions

PD solutions vary based on the patient's needs, with the exchange volume primarily determined by the size of the peritoneal cavity. An average-sized person usually uses a 2-liter exchange, while larger individuals may require 3 liters. Smaller volumes are used for patients with smaller bodies, pulmonary compromise, or inguinal hernias.

Ultrafiltration in PD, the process of removing excess fluid, relies on osmotic forces. Dextrose is the most commonly used osmotic agent due to its safety and low cost. However, high rates of peritoneal glucose absorption can lead to complications like hypertriglyceridemia, hyperglycemia, and long-term peritoneal membrane dysfunction. Alternatives to dextrose solutions include icodextrin and amino acid solutions. Icodextrin, an iso-osmolar preparation, induces ultrafiltration through its oncotic effect, while amino acid PD solutions can provide nutritional support for patients needing supplementation.