# 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

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## 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

**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

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## 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

Clinical Warning

**Peritoneal transport can change over time** — repeat PET every 1–2 years if significant change in adequacy.

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## 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

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### **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)

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## 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

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## 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

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## 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

1.  **Monitor response:**
    - Dialysate should clear within 5–7 days
    - Repeat culture if not improving
2.  **Failure to improve = catheter removal + transition to HD**

Clinical Pearl

**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)

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### **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

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### **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

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## 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)

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## Related Resources

- Comprehensive PD Physiology & Management
- RRT Initiation & Modality Selection
- CRRT & Acute Dialysis

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## Self-Test Questions

1.  **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
2.  **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
3.  **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

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*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.


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