Peritoneal Dialysis: Physiology, Adequacy & Complications
Peritoneal Membrane Anatomy & Function
The peritoneal membrane: - Large surface area (~2 m²; roughly size of adult skin) - Highly vascularized (rich blood supply) - Semipermeable — allows water, electrolytes, small uremic toxins to cross - Acts as dialyzer when dialysate is instilled into peritoneal cavity
Key Anatomical Points
- Visceral peritoneum: Covers organs (bowel, liver, spleen)
- Parietal peritoneum: Covers abdominal wall and diaphragm
- Peritoneal cavity: Normally <50 mL serous fluid; expands to accommodate ~2 L dialysate
Transport Mechanisms: How PD Works
1. Diffusion
Concentration gradient: Solutes move from high (blood) to low (dialysate) concentration
Molecular weight dependency: - Small solutes: Urea, creatinine, K (cross easily in early dwell; rapid diffusion) - Medium solutes: β2-microglobulin, phosphate (slower clearance) - Large solutes: Albumin, proteins (cross poorly; protective for nutrition)
Rate determined by: - Capillary pore density - Diffusion distance - Dwell time (longer dwell = more diffusion)
2. Ultrafiltration (Convection)
Osmotic gradient: Dextrose or icodextrin in dialysate draws water from blood
Mechanism: - Dextrose (glucose) in dialysate → osmotic gradient → water follows (osmosis) - Water carries small solutes with it (solvent drag)
Dextrose concentrations: 1.5%, 2.5%, 4.25% - 1.5% dextrose: Mild UF; ~400–600 mL fluid removal per exchange - 2.5% dextrose: Moderate UF; ~600–900 mL per exchange - 4.25% dextrose: Aggressive UF; ~1–2 L per exchange (rarely used; hyperglycemia risk)
Clinical pearl: Dextrose absorbed systemically → hyperglycemia risk. ~50–80% of dextrose absorbed over 4-hour dwell. Diabetics may need insulin adjustment.
3. Lymphatic Absorption
- Peritoneal lymphatics also absorb some fluid + solutes
- Reabsorption of UF: Even with hypertonic solution, some UF reabsorbed by lymphatics
- Mechanism not fully understood; limits maximum UF achievable
PET Test: Peritoneal Equilibration Test
What it measures: How quickly solutes equilibrate across peritoneal membrane
How it’s done: 1. Install 2 L of standard PD solution (1.5% dextrose) 2. Dwell for 4 hours 3. Draw samples of dialysate at 2h and 4h (also baseline serum creatinine) 4. Calculate: D/P ratio = dialysate Cr / plasma Cr
PET Classification
| Category | D/P Ratio @ 4h | Membrane Characteristics | Clinical Implications |
|---|---|---|---|
| High | >0.81 | Fast equilibration; rapid solute transfer | Fast peritoneal clearance; rapid UF loss (solute reabs.) |
| High-Average | 0.61–0.81 | Moderately fast | Good balance of solute + UF |
| Low-Average | 0.41–0.60 | Slow equilibration | Requires longer dwells; good UF retention |
| Low | <0.41 | Very slow; small pores | Excellent UF; slow solute removal |
Clinical Decisions Based on PET
High transporters: - Short dwells (2–3 hours) - May struggle to maintain UF (solutes reabsorbed quickly) - Nocturnal APD (longer total dwell time; smaller dwells to maximize clearance)
Low transporters: - Longer dwells (4–6 hours) to maximize diffusion - Excellent UF retention (can use lower dextrose) - May be suitable for CAPD
High-average or low-average: Most flexible; suitable for CAPD or APD
Peritoneal transport can change over time — repeat PET every 1–2 years if significant change in adequacy.
CAPD vs APD: Modality Comparison
CAPD (Continuous Ambulatory Peritoneal Dialysis)
Prescription: - 4–5 exchanges per day - ~2 L per exchange - Dwell times: 4–6 hours during day, overnight dwell 8–12 hours
Workflow: 1. Drain previous exchange 2. Install fresh dialysate (manual infusion; gravity-assisted) 3. Disconnect; carry with catheter capped during dwell 4. Repeat 4–5×/day
Advantages: - Patient control; flexible timing - No machine required - Gentle ultrafiltration (less glucose load) - Preserves residual renal function well
Disadvantages: - High technique failure (patient must perform correctly; contamination risk) - Peritonitis risk higher (more exchange connections) - Body image (visible abdominal catheter; bulge from fluid) - Workload (5×/day exchanges = 45+ min/day minimum) - Clearance lower than APD if low transporter
APD (Automated Peritoneal Dialysis)
Prescription: - Machine cycles 8–10 exchanges overnight (8–10 hours) - Daytime optional exchange (1–2 L, 4–6 hour dwell) - Cycles usually every 2–3 hours at night
Workflow: 1. Connect cycler tubing to catheter before bed 2. Cycler automatically fills, dwells, drains 3. Disconnect in morning; resume daily activities 4. Optional daytime exchange
Advantages: - Less frequent exchanges (more flexibility) - Higher total clearance (longer total PD hours) - Better solute removal for high transporters - Reduced peritonitis risk (fewer connections; cycler is automated) - Daytime peritoneal-free time (comfort, social, work flexibility) - Better suited for working patients
Disadvantages: - Machine cost: ~$3,000–5,000 (expensive; insurance coverage variable) - Nightly connection required (less flexibility for travel) - Potential sleep disruption (machine noise, tube movement) - Increased dialysate use (cost ~$8,000–12,000/year vs $5,000–8,000 for CAPD)
PD Solutions & Composition
Standard Solutions (Dextrose-Based)
| Component | Concentration | Function |
|---|---|---|
| Dextrose | 1.5%, 2.5%, 4.25% | Osmotic agent (UF) |
| Sodium | 132 mEq/L | Electrolyte balance |
| Potassium | 0 mEq/L | Allows K removal |
| Calcium | 3.5 mEq/L | Maintains Ca balance |
| Magnesium | 0.5 mEq/L | Electrolyte |
| Chloride | 96–101 mEq/L | Electrolyte; prevents acidosis |
| Bicarbonate/Lactate | 35–40 mEq/L | Acid-base balance |
| Glucose | As osmotic agent | — |
Alternative Solutions
Icodextrin (glucose polymer): - Osmotic agent; slow absorption - Better for long nocturnal dwells (preserves UF throughout dwell) - Less hyperglycemia than dextrose - Cost: Higher; insurance may require prior authorization
Amino acid-based PD: - Contains amino acids instead of dextrose - Improves protein nutrition (protein loss in dialysate major issue) - Limited evidence; not routine
PD Adequacy: How to Know If Patient Is “Getting Enough”
Kt/V Calculation
Kt/V = dialysis adequacy metric: - K = peritoneal clearance (mL/min) - t = dialysis time (minutes) - V = body water volume (L)
Target: Kt/V ≥1.7/week (KDIGO recommendation)
How to assess: 1. Peritoneal clearance: Measured from 24-hour spent dialysate collection + urine 2. Residual renal clearance: Measured from 24-hour urine 3. Total Kt/V = peritoneal Kt/V + renal Kt/V
Urea Clearance (Ccr)
Target: ≥60 L/week/1.73 m² (peritoneal + renal combined)
Measured: 24-hour urine + dialysate collections; normalize to BSA
Clinical Assessment of Adequacy
Signs of inadequacy: - Persistent uremia (nausea, itching, fatigue) - Hypertension difficult to control - Anemia worsening (despite ESA) - Hyperkalemia (despite K-free solution) - Hyperphosphatemia (despite binders)
If inadequate: 1. Increase exchanges (add daytime exchange if APD) 2. Increase dwell volumes (from 2 L to 2.5 L, if tolerated) 3. Switch to APD if on CAPD 4. Consider transition to HD
Complications of PD
Peritonitis (~0.4 episodes/patient-year)
Risk factors: - Contamination during exchanges (major cause) - Catheter biofilm - Poor hand hygiene - Tunnel infections
Clinical presentation: - Cloudy dialysate (key finding; turbid/milky appearance) - Abdominal pain, tenderness - Fever (may be absent) - Nausea, vomiting
Diagnosis: - WBC >100/μL in dialysate (normally <50) - Positive culture (identify organism) - Gram stain
Treatment: 1. Antibiotics in dialysate (intraperitoneal) - Empiric: Vancomycin + ceftazidime (covers Staph, gram-negatives) - Duration: 10–14 days minimum - Continue during PD
- Monitor response:
- Dialysate should clear within 5–7 days
- Repeat culture if not improving
- Failure to improve = catheter removal + transition to HD
Prevention: Teach impeccable technique (hand hygiene, mask during exchanges, sterile connections). Antibiotic caps/discs on connectors reduce infection.
Catheter Complications
| Complication | Mechanism | Management |
|---|---|---|
| Catheter migration | Tip moves out of pelvis | Reposition; if failed, replace |
| Catheter obstruction | Fibrin, omentum wrapping | Try flushing; ultrasound; possible surgery |
| Drainage problems | Obstruction during drain phase | Reassess position; reposition if needed |
| Tunnel/cuff infection | Bacterial colonization; abscess | Antibiotics; if failed, catheter removal |
| Leaks (early) | Defect at catheter insertion | Conservative: reduce volume, APD; surgical repair if large |
Abdominal Wall Hernias
Incidence: ~40% at 5 years
Mechanism: Chronically elevated intra-abdominal pressure from dialysate + weakened fascia
Types: - Incisional hernia (at catheter insertion site) - Inguinal/umbilical hernia (more common with PD)
Management: - Small, asymptomatic: Observe; no intervention - Symptomatic or enlarging: Surgical repair (possible while continuing PD)
Encapsulating Peritoneal Sclerosis (EPS)
Definition: Fibrosis, inflammation, scarring of peritoneal membrane → eventual loss of function
Incidence: ~0.5–3% at 4 years; ↑ with longer PD duration
Risk factors: - Prolonged PD (>5 years) - Multiple peritonitis episodes - Eosinophilic peritonitis - Chlorhexidine exposure (old tubing)
Clinical presentation: - Gradual inadequacy (Kt/V ↓) - Abdominal pain, distension - Adhesions, bowel obstruction (rare but serious)
Management: - Transition to HD (if possible) - Consider immunosuppression (experimental; limited evidence) - Monitor closely; possible surgical intervention if bowel obstruction
Ultrafiltration Failure
Definition: Unable to achieve adequate fluid removal despite maximum osmotic strength (4.25% dextrose)
Causes: - High transporter (rapid solute reabsorption) - Aquaporin-1 dysfunction (water-only loss impaired) - EPS (rare) - Lymphatic reabsorption excessive - Sodium sieving (Na reabsorbs disproportionately; solute-free water follows)
Management: 1. Switch to APD (shorter dwells; preserve UF) 2. Use icodextrin (alternative osmotic agent) 3. Transition to HD if refractory
Protein Loss & Nutrition Management
Peritoneal protein loss: 1–2 g/day (more with higher dwell volumes, peritonitis)
vs Hemodialysis: 5–10 g/week (much less protein loss in HD)
Nutritional consequences: - Malnutrition risk - Visceral protein depletion (prealbumin ↓) - Muscle wasting
Management: 1. Adequate dietary protein: 1.2–1.3 g/kg/day (higher than HD) 2. Adequate caloric intake: 30–35 kcal/kg/day 3. Monitor prealbumin, albumin (yearly) 4. Consider amino acid-based PD (experimental; not routine) 5. Avoid glucose overload (reduce dextrose exposure; use icodextrin)
Self-Test Questions
- PET result: D/P ratio 0.85 (HIGH transporter)
- Recommendation: Short dwells (2–3h); consider APD for higher clearance
- Why: Fast equilibration means prolonged dwells lose UF as solutes reabsorb
- PD patient with cloudy dialysate, 400 WBC/μL, gram-positive cocci
- Diagnosis: Peritonitis (likely Staph aureus)
- Treatment: Vancomycin + ceftazidime in dialysate × 14 days
- Monitor: Dialysate should clear; repeat culture at day 3–5
- Adequate CAPD with 4×2L exchanges daily; Kt/V 1.8; residual GFR 3 mL/min
- Assess: Adequacy is borderline good (goal >1.7)
- Residual function: 3 mL/min helps; expect Kt/V to ↓ when RRF declines further
- Action: Monitor closely; may need increase to 5 exchanges/day or APD if RRF lost
Version 1.0 | PA/Medical student level | Updated 2026-02-28
References: ISPD PD Adequacy & Transport Guidelines. KDIGO 2024 Dialysis Initiation, Adequacy & Complications. Peritoneal Dialysis Essentials: Physiology & Clinical Practice.