What is the mechanism of peritoneal dialysis (PD)?

Mechanism of Peritoneal Dialysis

Peritoneal dialysis uses the peritoneal membrane as a natural semipermeable barrier, where dialysate solution instilled into the peritoneal cavity removes uremic toxins through diffusion and convection, while ultrafiltration is achieved by creating an osmotic gradient using hypertonic glucose or icodextrin solutions. 1

Core Physiological Principles

The Peritoneal Membrane as Dialysis Interface

• The peritoneum serves as the natural semipermeable membrane, with blood flow provided by the peritoneal microcirculation rather than an artificial extracorporeal circuit 1
• The peritoneal cavity is filled with sterile dialysis solution through a permanent catheter, allowing the solution to contact peritoneal capillaries for solute and water exchange 2

Solute Removal Mechanisms

Diffusion is the primary mechanism for small molecule clearance:

• Solutes move from areas of high concentration (blood) to low concentration (dialysate) across the peritoneal membrane 1
• Movement continues until equilibrium is reached between blood and dialysate 1
• This process effectively removes uremic toxins like creatinine, urea, and electrolytes 1

Convection provides additional solute clearance:

• Water (solvent) carries dissolved solutes across the membrane through "solvent drag" 1
• Solutes are removed together with fluid during ultrafiltration 1
• Both diffusion and convection proceed simultaneously during peritoneal dialysis 1

Fluid Removal (Ultrafiltration)

Osmotic gradient creation is essential for fluid removal:

• Hypertonic glucose or icodextrin solutions are instilled into the peritoneal cavity 1
• These osmotic agents create a concentration gradient versus peritoneal capillaries 1
• Water moves from blood into the dialysate along this osmotic gradient, removing excess fluid 2

Dialysis Modalities

Continuous Ambulatory Peritoneal Dialysis (CAPD)

• The patient manually performs 4-5 dialysate solution exchanges throughout the day 3
• Dialysis is continuous, providing stability of the internal environment that most closely resembles normal kidney function 3

Automated Peritoneal Dialysis (APD)

• A cycling machine performs dialysate exchanges automatically, typically overnight while the patient sleeps 3, 2
• This allows the patient to engage in other activities during the day 3

Important Limitations

Peritoneal dialysis achieves only 10-20% of normal kidney clearance for index substances like urea and creatinine, with even lower clearance for higher molecular weight solutes 1

Critical functions NOT replaced by peritoneal dialysis:

• Tubular secretive and reabsorptive function 1
• Endocrine function of the kidney 1

Nutrient losses occur during treatment:

• Low molecular weight macronutrients and micronutrients (amino acids, water-soluble vitamins) are easily removed during dialysis 1
• Continuous protein loss occurs with peritoneal dialysis, which can contribute to malnutrition 1, 3

Membrane Transport Characteristics

Middle molecule clearance is maximized by continuous 24-hour peritoneal dialysis without dry periods, as it depends more on total dialysis time than dialysate flow rate 1

Long-term complications include inflammation, neoangiogenesis, and peritoneal fibrosis, which can lead to ultrafiltration failure due to increased effective surface area and reduced glucose-driven osmotic pressure 4