How does peritoneal dialysis work and what are the different types of cycles and their solutes?

How Peritoneal Dialysis Works and Its Different Cycles/Solutes

Peritoneal dialysis (PD) is an intracorporeal kidney replacement therapy that uses the patient's own peritoneal membrane as a natural dialyzing surface to remove waste products, correct electrolyte imbalances, and remove excess fluid through diffusion and convection processes. 1

Basic Principles of Peritoneal Dialysis

Peritoneal dialysis works through two fundamental physiological processes:

1. Diffusion: Movement of solutes (like urea, creatinine, and electrolytes) from an area of high concentration to low concentration across the peritoneal membrane until equilibrium is reached 1

2. Convection: Movement of water (solvent) carrying solutes across the membrane through a process called ultrafiltration, driven by osmotic gradients 1

The peritoneal membrane serves as the dialyzing surface, with blood flow provided by the peritoneal microcirculation. Unlike hemodialysis which uses an artificial membrane in an extracorporeal circuit, PD uses the patient's natural peritoneal membrane 1.

Components of the PD System

• Access: A permanent catheter inserted into the peritoneal cavity 2
• Dialysate: Sterile solution containing physiologic electrolytes, buffer, and an osmotic agent (typically glucose) 2
• Peritoneal Membrane: Acts as the exchange surface between blood and dialysate 2

Types of Peritoneal Dialysis Cycles

1. Continuous Ambulatory Peritoneal Dialysis (CAPD):

   • Manual exchanges performed by the patient 4-5 times daily 3
   • Each exchange involves:
     • Draining used dialysate (outflow)
     • Infusing fresh dialysate (inflow)
     • Allowing a dwell time (typically 4-6 hours during day, 8-10 hours overnight)
   • Patient carries dialysate in abdomen continuously

2. Automated Peritoneal Dialysis (APD):

   • Uses a cycling machine to perform exchanges automatically 2
   • Typically performed overnight while the patient sleeps
   • Subtypes include:
     • Continuous Cyclic Peritoneal Dialysis (CCPD): Nighttime cycling with one or more daytime dwells 1
     • Nightly Intermittent Peritoneal Dialysis (NIPD): Exchanges only at night with "dry day" (empty peritoneal cavity during day) 1

Dialysate Composition and Solutes

The standard PD solution contains:

• Electrolytes: Sodium, calcium, magnesium, chloride at physiologic concentrations 4
• Buffer: Lactate or bicarbonate to correct metabolic acidosis 4
• Osmotic Agents:

  1. Glucose (dextrose): Most commonly used in concentrations of:

     • 1.5% (low strength)
     • 2.5% (medium strength)
     • 4.25% (high strength)
     • Higher concentrations create stronger osmotic gradients for greater ultrafiltration 4

  2. Icodextrin: A glucose polymer used for long dwells (typically overnight in CAPD or daytime in APD) that provides sustained ultrafiltration 1

Peritoneal Transport Characteristics

Patients are categorized based on their peritoneal membrane transport properties using standardized tests like the Peritoneal Equilibration Test (PET) 1:

• High Transporters: Rapid solute transport but poor ultrafiltration due to quick dissipation of osmotic gradient
• Average-High Transporters: Moderate-fast solute transport with moderate ultrafiltration
• Average-Low Transporters: Moderate-slow solute transport with good ultrafiltration
• Low Transporters: Slow solute transport but excellent ultrafiltration

These transport characteristics help determine the optimal PD prescription for each patient 1.

Clearance Mechanisms in PD

1. Small Solute Clearance:

   • Small molecules (urea, creatinine, electrolytes) are primarily removed by diffusion
   • Measured by Kt/V urea and creatinine clearance 1

2. Middle Molecule Clearance:

   • Larger uremic toxins are removed by both diffusion and convection
   • PD provides good middle molecule clearance due to continuous nature of therapy

3. Water Removal (Ultrafiltration):

   • Driven by osmotic gradient between hypertonic dialysate and plasma
   • Water moves through both small pores (with solutes) and specialized water channels (aquaporin-1) 4

Clinical Considerations

• PD preserves residual kidney function longer than hemodialysis 3
• Provides more stable internal environment due to continuous nature 3
• Solute clearance in PD is only about 10-20% of normal kidney function, even with optimal therapy 1
• Some beneficial substances like amino acids and water-soluble vitamins can be lost during PD 1

Pitfalls and Caveats

• Peritoneal membrane function may change over time, requiring periodic reassessment 1
• High transporters may experience poor ultrafiltration with long dwells due to rapid glucose absorption 1
• Peritoneal damage can occur with long-term PD use, limiting its longevity as a treatment option 3
• Protein loss through the peritoneal membrane can contribute to malnutrition if not compensated by adequate dietary protein intake 1

Understanding the patient's peritoneal transport characteristics is essential for optimizing the PD prescription to achieve adequate solute clearance and ultrafiltration while minimizing complications.