What is the process of performing peritoneal dialysis?

How Peritoneal Dialysis is Performed

Basic Mechanism

Peritoneal dialysis uses the peritoneum as a natural semipermeable membrane, where sterile dialysate solution is infused into the peritoneal cavity through a surgically placed catheter, allowing uremic toxins to be removed through diffusion and water to be removed through osmotic ultrafiltration. 1, 2

The peritoneal microcirculation provides blood flow to the membrane, eliminating the need for an extracorporeal circuit as required in hemodialysis. 1 The dialysate solution contains physiologic concentrations of electrolytes and base, plus glucose (often at supraphysiologic concentrations) to create the osmotic gradient necessary for fluid removal. 3

Catheter Placement and Initiation

Pre-Dialysis Preparation

• Wait 10 days to 2 weeks after catheter placement before starting peritoneal dialysis whenever possible. 4
• If dialysis must be started in less than 10 days following catheter placement, perform low-volume, supine dialysis only. 4
• Obtain baseline 24-hour urine collection for urea and creatinine clearance at initiation. 4

Initial Prescription Guidelines

For patients with estimated GFR < 2 mL/min choosing CAPD (Continuous Ambulatory Peritoneal Dialysis): 4

• Body surface area (BSA) ≤1.7 m²: Four 2.0 L exchanges per day
• BSA 1.7 to 2.0 m²: Four 2.5 L exchanges per day
• BSA >2.0 m²: Four 3.0 L exchanges per day

For patients choosing CCPD (Continuous Cycling Peritoneal Dialysis): 4

• BSA ≤1.7 m²: 2.0 L exchanges for 9 hours/night plus 2.0 L daytime dwell
• BSA 1.7 to 2.0 m²: 2.5 L exchanges for 9 hours/night plus 2.0 L daytime dwell
• BSA >2.0 m²: 3.0 L exchanges for 9 hours/night plus 3.0 L daytime dwell

Two Main Techniques

Manual CAPD (Continuous Ambulatory Peritoneal Dialysis)

• The patient manually performs 4-5 solution exchanges throughout the day. 2, 5
• Each exchange involves draining the used dialysate and infusing fresh solution into the peritoneal cavity. 2
• Dialysate remains in the peritoneal cavity continuously between exchanges. 5

Automated APD (Automated Peritoneal Dialysis)

• A cycling machine performs dialysis solution exchanges automatically, typically overnight while the patient sleeps. 2, 5
• This allows the patient to engage in other activities during the day without manual exchanges. 5
• The machine controls the timing, volume, and number of exchanges. 2

Physiological Transport Mechanisms

Diffusion (Primary Mechanism for Small Solutes)

• Solutes move from high concentration areas to low concentration across the peritoneal membrane. 1, 3
• This effectively removes uremic toxins including creatinine, urea, and electrolytes. 1
• The peritoneal membrane is dominated by small pores that allow transport of water and small-molecular-size solutes. 3

Convection (Solvent Drag)

• Water carries dissolved solutes across the membrane during ultrafiltration. 1, 3
• Solutes are removed together with fluid through small pores in response to osmotic force. 3

Aquaporin-1 Water Channels

• Specialized water channels in peritoneal capillaries allow water transport without solute (free water). 3
• These channels are upregulated by glucose and respond to osmotic force induced by glucose in the dialysate. 3

Membrane Transport Assessment

Baseline peritoneal membrane transport characteristics should be established 4 to 8 weeks after initiating daily peritoneal dialysis therapy. 4

The standard Peritoneal Equilibration Test (PET) is the most commonly used method: 4

• Involves a timed 4-hour dwell with 2.5% dextrose solution
• Measures dialysate-to-plasma (D/P) ratios for creatinine and other solutes
• Classifies patients as low, low-average, high-average, or high transporters
• Guides prescription adjustments to optimize clearance and ultrafiltration

The International Society for Peritoneal Dialysis recommends a modified PET using 3.86%/4.25% dextrose to optimally evaluate patients with ultrafiltration failure. 4

Important Limitations and Considerations

Clearance Limitations

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

Functions Not Replaced

• Tubular secretive and reabsorptive function is not replaced. 1
• Endocrine function of the kidney is not replaced. 1
• Continuous protein loss occurs and can contribute to malnutrition. 1

Monitoring Requirements

• Peritoneal membrane transport testing should be repeated when clinically indicated, as transport characteristics can change over time. 4
• Approximately 85% of patients on automated peritoneal dialysis will need one or more daytime dwells added to their prescription. 4
• Instilled volumes almost certainly will need to increase over time as residual kidney function declines. 4

Common Pitfalls to Avoid

• Do not perform formal peritoneal equilibration testing during the first 4 weeks of therapy, as initial instillation of dialysate causes mild changes in local cytokine production, peritoneal vascularity, and blood flow that affect transport measurements. 4
• Do not assume the initial prescription will remain adequate long-term; total solute clearance must be monitored and prescriptions adjusted as residual kidney function or peritoneal transport changes. 4
• Contraindications include extensive intra-abdominal surgery, presence of a stoma, and surgically irreparable diaphragmatic hernia. 6, 5