How does peritoneal dialysis work and what are the differences in dialysate compared to hemodialysis in patients with end-stage renal disease (ESRD)?

How Peritoneal Dialysis Works

Core Mechanism

Peritoneal dialysis uses the peritoneum as a natural semipermeable membrane, 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

Blood Supply and Membrane Function

• The peritoneal microcirculation provides blood flow rather than an artificial extracorporeal circuit, allowing solute and water exchange across the peritoneal membrane 1, 2
• The peritoneal membrane is dominated by small pores that allow transport of water and small-molecular-size solutes including electrolytes through both diffusion and convection 3
• Specialized water channels (aquaporin 1) present in peritoneal capillaries allow water transport without solute (free water) in response to osmotic force induced by glucose, and these channels are upregulated by glucose 3

Solute Removal Mechanisms

• Diffusion is the primary mechanism for small molecule clearance (creatinine, urea, electrolytes), where solutes move from high concentration in blood to low concentration in dialysate across the peritoneal membrane 1, 3
• Convection provides additional clearance where water carries dissolved solutes across the membrane through solvent drag during ultrafiltration 1, 3
• Middle molecule clearance is maximized by continuous 24-hour dialysis without dry periods, as it depends more on total dialysis time than dialysate flow rate 1

Ultrafiltration Process

• Glucose in the dialysis solution generates osmotic force to drive convection and water removal 3
• Net loss or gain of electrolytes and base is determined by both their gradient between capillary blood and dialysis solution and the net ultrafiltration volume 3

Critical Limitations of Peritoneal Dialysis

Peritoneal dialysis achieves only 10-20% of normal kidney clearance for urea and creatinine, with even lower clearance for higher molecular weight solutes. 4, 1, 2

Functions NOT Replaced

• Tubular secretive and reabsorptive function is not replaced 4, 1, 2
• Endocrine function of the kidney is not replaced 4, 1, 2
• Continuous protein loss occurs (averaging 5-15 g/24 hours) and can contribute to malnutrition 5, 1, 2
• Peritoneal amino acid losses average about 3 g/day 5

Dialysate Differences: Peritoneal Dialysis vs Hemodialysis

Composition and Osmotic Agents

• PD dialysate contains glucose at supraphysiologic concentrations to generate osmotic force for ultrafiltration, whereas hemodialysis does not rely on glucose for fluid removal 3
• PD solutions contain physiologic concentrations of electrolytes and base, similar to hemodialysis dialysate 3
• Icodextrin can be used as an alternative osmotic agent in PD for longer dwells 1

Biocompatibility Issues

• Conventional PD solutions are hyperosmolar, acidic, contain lactate buffer, and have high concentrations of glucose and glucose degradation products (GDPs) 6
• Chronic exposure to conventional PD fluids provokes activation of inflammatory, fibrogenic, and angiogenic cytokines, leading to progressive peritoneal fibrosis, vasculopathy, and neoangiogenesis 6
• Newer, more biocompatible PD solutions have been developed to preserve peritoneal membrane function and improve outcomes 6

Clearance Efficiency Differences

• In hemodialysis, uric acid clearance is approximately 70-100 mL/min with plasma uric acid falling by about 50% with each 6-hour treatment 5
• PD has lower efficiency in removing solute and fluid compared to intermittent hemodialysis and continuous renal replacement therapies, limiting its usefulness when significant rapid solute removal (uric acid, urea, potassium, phosphate) is required 5

Protein and Nutrient Losses

• Protein losses into peritoneal dialysate (5-15 g/24 hours) are almost invariably higher than protein losses into hemodialysate 5
• During peritonitis episodes, dialysate protein losses can be considerably higher 5
• This necessitates higher dietary protein requirements for PD patients (1.2-1.3 g/kg/day) compared to hemodialysis patients (1.2 g/kg/day) 5

Hemodynamic Considerations

• PD offers smaller hemodynamic shifts compared to hemodialysis, making it advantageous for hemodynamically unstable patients 5, 2
• Hemodialysis requires high ultrafiltration rates administered 3-4 times weekly, creating greater hemodynamic disturbances 5
• PD provides more prolonged, continuous therapy with better hemodynamic stability and slower solute shifts 2

Common Pitfalls to Avoid

• Do not perform formal peritoneal equilibration testing during the first 4 weeks of therapy, as initial dialysate instillation causes changes in local cytokine production, peritoneal vascularity, and blood flow that affect transport measurements 4
• Recognize that approximately 85% of patients on automated peritoneal dialysis will need one or more daytime dwells added to their prescription 4
• Anticipate that instilled volumes will almost certainly need to increase over time as residual kidney function declines 4
• When increasing dietary protein intake in PD patients to compensate for peritoneal losses, adjust therapy accordingly (dialysis dose, phosphate binders, bicarbonate supplementation, cholesterol management) since protein-containing foods are major sources of phosphorus, hydrogen ions, and fats 5