3.2 Peritoneal Dialysis Modalities, Adequacy and Care

Peritoneal Dialysis Modalities and Principles

Peritoneal dialysis (PD) is a home-based renal replacement therapy using the patient's peritoneum—the vascular membrane lining the abdominal cavity—as a semipermeable membrane. Dialysis fluid (dialysate), containing glucose as an osmotic agent, is infused into the peritoneal cavity via a permanent Tenckhoff catheter. Solute clearance occurs via diffusion and convection, while ultrafiltration is driven by the osmotic gradient. The two primary modalities are:

1. Continuous Ambulatory Peritoneal Dialysis (CAPD): A manual, continuous modality where the patient performs 4 to 5 exchanges daily. Dialysate dwells for 4 to 6 hours before draining, with an overnight dwell of 8 to 10 hours. CAPD requires no machine, giving patients independence, but multiple daily aseptic connections increase contamination risks. Because it provides continuous steady-state clearance, CAPD is gentle for patients with unstable cardiovascular systems who cannot tolerate rapid fluid shifts.
2. Automated Peritoneal Dialysis (APD): Uses an automated machine called a cycler to perform 3 to 5 short exchanges overnight (8 to 10 hours) while the patient sleeps. A daytime dwell may be used for continuous clearance. APD frees patients from daytime exchanges and reduces manual connections, lowering infection risk. APD is particularly indicated for patients who need high-volume clearance or who are active during the day.

The Peritoneal Equilibration Test and Membrane Transport

Membrane transport rates vary among patients. A peritoneal equilibration test (PET), performed 4 to 12 weeks after PD initiation, measures solute transport (creatinine and urea) and glucose absorption. The test compares dialysate creatinine at 4 hours to plasma creatinine (the dialysate-to-plasma (D/P) ratio), and dialysate glucose at 4 hours to initial glucose at 0 hours ($D/D_0$ ratio). Based on these values, patients are grouped into four categories:

• High Transporters: Characterized by a 4-hour D/P creatinine ratio of 0.81 to 1.03. They rapidly equilibrate creatinine but absorb glucose quickly, losing the osmotic gradient and ultrafiltration (UF) capacity during long dwells. Suited for APD with rapid overnight cycles.
• High-Average Transporters: Characterized by a 4-hour D/P creatinine ratio of 0.65 to 0.80. They show moderately rapid solute transport and average glucose absorption. Good clearance and UF make them suitable for both CAPD and APD.
• Low-Average Transporters: Characterized by a 4-hour D/P creatinine ratio of 0.50 to 0.64. They demonstrate moderately slow solute transport and slow glucose absorption. They maintain the osmotic gradient well for excellent UF and suit both modalities.
• Low Transporters: Characterized by a 4-hour D/P creatinine ratio of 0.26 to 0.49. They transport solutes slowly but maintain a strong osmotic gradient. Superb UF is achieved, but long dwell times are required for clearance, making them best suited for CAPD.

Targets for PD Adequacy

To ensure sufficient toxin removal, KDOQI guidelines establish adequacy targets. The primary marker is weekly Kt/V, where K is urea clearance, t is time, and V is urea distribution volume (total body water), which is calculated using the Watson formula based on the patient's sex, height, weight, and age.

• The weekly target is a total Kt/V of at least 1.7.
• It is measured using a 24-hour collection of both urine and dialysate effluent.
• Preserving residual renal function (RRF)—the remaining natural kidney function—is vital for survival and fluid balance. Staff must help preserve RRF by avoiding nephrotoxic drugs (such as NSAIDs or aminoglycosides) and avoiding dehydration.

Peritonitis Diagnostic Criteria and Care

Peritonitis is the most common infectious complication and can cause membrane scarring and catheter failure. Diagnosis requires meeting at least two of the following:

1. Clinical features: Abdominal pain, rebound tenderness, and/or cloudy dialysate effluent.
2. Effluent cell count: White blood cell (WBC) count of 100/$\mu$L or higher with at least 50% polymorphonuclear neutrophils (PMNs) after a dwell of $\ge$ 2 hours.
3. Microbiological: A positive culture showing bacteria or fungi.

Effluent samples for cell count, differential, and culture must be collected before administering antibiotics. Empiric therapy must target Gram-positive (Staphylococcus species) and Gram-negative (Pseudomonas or Escherichia coli) organisms. Intraperitoneal (IP) antibiotics are preferred because they deliver medication directly to the infection site. Heparin (500 to 1000 units/L) is often added to prevent fibrin clots from obstructing the catheter.

A related infectious issue is exit site infection (defined by purulent drainage at the catheter-epidermal interface) and tunnel infection (tenderness or erythema over the subcutaneous catheter pathway). These can lead to peritonitis if untreated.

Non-Infectious and Mechanical Complications

Mechanical complications arise from catheter presence and elevated intra-abdominal pressure:

• Dialysate Leaks: Fluid leaking around the catheter or into subcutaneous tissue causes localized swelling or decreased UF. A leak can be confirmed by testing the leaking fluid for glucose; dialysate has extremely high glucose compared to serum. Risk is minimized by delaying PD for 2 to 3 weeks post-insertion to allow surgical healing, or by using low-volume, supine dwells if early dialysis is required.
• Hernias: High intra-abdominal pressure from dialysate pre-disposes patients to inguinal, umbilical, incisional, or ventral hernias, which require surgical repair. During recovery, patients may require temporary hemodialysis or low-volume supine PD.
• Catheter Displacement: The catheter can migrate out of the pelvis, causing inflow or outflow obstruction. Constipation is a major cause of catheter displacement due to altered bowel peristalsis; regular bowel function must be maintained with stool softeners.