PD Modalities: CAPD and APD

PD Modalities: CAPD and APD

Quick Answer: Peritoneal dialysis (PD) uses the peritoneal membrane as a natural filter. Continuous ambulatory peritoneal dialysis (CAPD) relies on manual bag exchanges during waking hours (typically 3–5 exchanges with 4–6 hour dwells). Automated peritoneal dialysis (APD) uses a cycler machine for exchanges—usually overnight—with optional dry-day schedules. Both achieve clearance through diffusion and convection, and both are high-yield on the NNCC CDN exam (15–17% of the blueprint).

How PD Works: Diffusion, Convection, and Dwell Time

Unlike hemodialysis, which depends on a blood circuit and artificial membrane, PD circulates sterile dialysate into the peritoneal cavity through a permanent catheter. Waste solutes (urea, creatinine, potassium, phosphorus) move down concentration gradients from blood capillaries across the peritoneal membrane into dialysate—this is diffusion. Water removal (ultrafiltration) occurs when an osmotic gradient pulls fluid into dialysate; in standard solutions, dextrose (glucose) is the primary osmotic agent.

Convection adds clearance when ultrafiltrated fluid drags solutes with it. CDN items frequently test whether candidates recognize that PD is not diffusion-only; the correct pairing is diffusion plus convection.

Dwell time is the interval dialysate remains in the abdomen. Adequate solute clearance in CAPD generally requires dwells of at least 4 hours; shorter dwells reduce efficiency because equilibrium has not been reached. After the dwell, the patient drains effluent and infuses fresh solution—one complete exchange.

CAPD: Continuous Ambulatory Peritoneal Dialysis

CAPD is the original home PD modality. A typical prescription includes 3–5 exchanges per day, each with 4–6 hours of dwell. Standard CAPD schedules do not rely on an automated cycler for overnight exchanges—the patient performs each connection manually.

Exchange technique uses sterile dialysate bags, a transfer set, and strict aseptic technique. CAPD advantages include no machine dependency, flexible scheduling, and continuous clearance that may better preserve residual renal function compared with thrice-weekly in-center hemodialysis. Disadvantages include the burden of daytime exchanges, risk of touch contamination, and the need for adequate peritoneal cavity space.

CAPD Prescription Example (Exam Scenario)

A CDN-style scenario might describe a patient performing four exchanges daily: 2.0 L fill volume, dwells of approximately 5 hours, using 1.5% dextrose for three exchanges and 2.5% for one exchange to support ultrafiltration. The nurse's teaching priority is confirming the patient understands which bag strength to use when, how to record drain volumes and weights, and when cloudy effluent requires immediate culture.

APD: Automated Peritoneal Dialysis (Including CCPD)

APD uses a cycler machine to perform multiple fill-drain cycles, most often overnight while the patient sleeps. Continuous cycling peritoneal dialysis (CCPD) is an APD variant in which the cycler runs several short cycles overnight, sometimes leaving a long daytime dwell or a dry day (no fluid in the abdomen during waking hours). CDN questions may use APD and CCPD interchangeably when asking which modality uses a cycler—both are correct.

APD advantages include fewer manual connections (lower peritonitis risk from touch contamination), freedom during the day, and programmable ultrafiltration targets. Disadvantages include machine cost, alarm management, and dependence on electricity. Patients must still learn emergency manual exchanges if the cycler fails.

Dialysate Solutions and Osmotic Agents

Standard PD solutions contain electrolytes approximating physiologic concentrations, a buffer (usually lactate, absorbed and converted to bicarbonate), and dextrose at 1.5%, 2.5%, or 4.25% concentrations. Higher dextrose creates a steeper osmotic gradient and greater ultrafiltration but exposes the peritoneum to more glucose over years of therapy.

Icodextrin (a glucose polymer) is used for long dwells (often 8–16 hours) because it provides sustained ultrafiltration without additional glucose load—highly testable when a vignette describes poor long-dwell ultrafiltration with dextrose alone.

Lactate-based solutions are stored at low pH (approximately 5.0–5.5) for stability, which can cause infusion pain. Bicarbonate-based solutions (physiologic pH ~7.0–7.4) offer better biocompatibility and less pain.

Choosing a Modality: Nursing Assessment

Modality selection is shared decision-making among the patient, nephrologist, and dialysis nurse. CAPD suits patients who want daytime autonomy without technology. APD suits those who prefer sleeping during treatment. Relative contraindications to PD include prior extensive abdominal adhesions, uncorrected hernias, and severe COPD with respiratory compromise—increased intra-abdominal volume can impede diaphragmatic excursion.

CDN Exam Traps

1. CAPD vs APD timing: CAPD = manual daytime exchanges; APD = cycler overnight.
2. Mechanism: PD clearance = diffusion and convection, not diffusion alone.
3. Osmotic agent: Standard UF uses dextrose; long dwells may use icodextrin.
4. CCPD is APD: Both use a cycler; do not insist on only one acronym.