Modalities and Prescription

The contract between this chapter and the reader.

1. 1. Distinguish the PD modalities — CAPD, APD (CCPD/NIPD), incremental, and tidal — and their indications.
2. 2. Identify the levers of a prescription: fill volume, exchanges, dwell time, glucose strength, and icodextrin.
3. 3. Match a prescription to the patient's transport status and residual kidney function.
4. 4. Construct a first-pass prescription for a new patient.
5. 5. Adjust the prescription for volume overload or underdialysis.
6. 6. Explain why residual kidney function matters and how incremental PD leverages it.
7. 7. Recognise modality choice as a preference-sensitive decision when the membrane permits either.
8. 8. Conduct a shared-decision conversation about CAPD versus APD.
9. 9. Evaluate when assisted PD is preferable to abandoning the therapy.

A sixty-second reading. Each bullet stands alone.

• CAPD uses manual daytime exchanges with fluid always present; APD uses an overnight cycler, freeing the day.
• Incremental PD starts below full dose while residual kidney function is substantial, then steps up as that function declines.
• The prescription levers are fill volume, number of exchanges, dwell time, glucose strength, and icodextrin.
• Set dwell length from transport status: fast transporters need short dwells, slow transporters long ones.
• Fast transporters suit APD with short cycles plus icodextrin for the long dwell; slow transporters suit longer dwells and larger volumes.
• Larger fill volumes raise clearance and ultrafiltration surface but also intra-abdominal pressure, risking hernia and leak.
• Glucose drives ultrafiltration but adds calories and injures the membrane over time, so keep the strength no higher than needed.
• Residual kidney function predicts survival and contributes clearance and fluid removal; preserve it and let incremental dosing exploit it.
• The prescription targets euvolemia, adequate clearance, and symptom control — not a single number; a total Kt/V urea around 1.7 per week is a guide.
• When the membrane permits either modality, CAPD versus APD is a values-driven choice — work, autonomy, cost, home setup, and dexterity all count.
• For limited self-care capacity, assisted PD is often better than abandoning the therapy.
• For volume overload, raise ultrafiltration and cut salt and water rather than endlessly adding cycles.

The medical core. An expert should agree prescription-building is fully covered here.

Why it matters at the bedside

Aprescription is where membrane physiology meets the patient's life. The same transport data can become a regimen that controls fluid and fits a job — or one that overloads the patient and ends the therapy. Build it from the membrane and the residual kidney, then shape it around how the person actually lives.

The modalities

• CAPD. Three to four manual exchanges a day with dialysate continuously present. No machine or electricity, portable, and gentle on residual function; it occupies parts of the day and needs manual dexterity.
• APD (CCPD / NIPD). A cycler runs exchanges overnight, with or without a day dwell. It frees the daytime for work or school but depends on a machine and power and costs more.
• Incremental PD. A reduced starting dose while residual function is meaningful, increased as that function falls. It lowers glucose load, cost, and burden and helps preserve the kidney.
• Tidal PD. Leaves a reservoir volume between cycles to ease inflow or outflow pain and flow problems.

The prescription levers

• Fill volume. Larger fills raise clearance and the ultrafiltration surface but raise intra-abdominal pressure; titrate to comfort and to hernia and leak risk.
• Exchanges or cycles. More exchanges add clearance and ultrafiltration opportunities at the cost of burden, expense, and glucose exposure.
• Dwell time. Set by transport status — the single most important link to physiology.
• Glucose strength. The main ultrafiltration dial; higher strengths remove more water but add calories and membrane injury.
• Icodextrin. Sustains ultrafiltration across the one long dwell by colloid osmosis; used once daily.

Matching the prescription to the membrane

• Fast transporters absorb glucose early, so a long glucose dwell loses fluid; give them short, frequent dwells on APD and icodextrin for the long dwell.
• Slow transporters clear solute slowly but ultrafilter well; give them long dwells and larger volumes, and confirm clearance is adequate.
• Average transporters tolerate either modality, which is where the patient's values legitimately enter the decision.

Residual kidney function and incremental PD

• Residual function adds solute clearance and fluid removal and independently predicts survival, so it is protected: avoid nephrotoxins and contrast, use renin-angiotensin blockade where appropriate, and avoid the hypovolaemia of over-ultrafiltration.
• When residual function is substantial, an incremental prescription delivers enough therapy with fewer exchanges, less glucose, and lower cost; it is reassessed every few months and increased as the kidney declines.

What the prescription must achieve

• The goals are clinical euvolemia, adequate solute clearance, and symptom control, at a burden the patient can sustain. A total (peritoneal plus residual) Kt/V urea near 1.7 per week is a guide, but euvolemia and wellbeing outrank any single number.

Choosing a modality — where values lead

• When the membrane permits either CAPD or APD, the choice turns on the patient's life: employment and schooling, the wish for daytime freedom, autonomy, cost and reimbursement, electricity and home space, manual dexterity, and body image.
• For patients who cannot manage exchanges alone, assisted PD — a trained carer or nurse performing or supervising exchanges — often keeps them on the therapy they prefer rather than defaulting to haemodialysis.

Monitoring and adjusting

• Review fluid status, clearance, residual function, and tolerance at intervals. For overload, raise ultrafiltration and restrict salt and water; for underdialysis, add fill volume, cycles, or dwell time guided by transport status; for mechanical strain, reduce fill or change to a supine APD pattern.

Five fully-built tables.

Table A — The modalities

Table B — Prescription levers

Table C — Matching prescription to transport

Table D — What the prescription must achieve

Table E — Adjusting for problems

Visualise & Map

Causal chains, each ending in a named action.

Chain 1 — Transport sets the dwell

Fast transport → glucose absorbed early → long glucose dwell loses its gradient → fluid is reabsorbed → ACTION: prescribe short APD dwells and icodextrin for the long dwell.

Chain 2 — Fill volume and pressure

Larger fill → more clearance and UF surface → higher intra-abdominal pressure → hernia, leak, discomfort → ACTION: titrate fill to tolerance, not to a target number.

Chain 3 — Residual function as a resource

Residual kidney function → adds clearance and ultrafiltration → less PD dose needed → incremental prescription works → ACTION: start incremental and protect the kidney.

Chain 4 — Glucose load

Higher glucose strength → more ultrafiltration but more calories and membrane injury → long-term transport rise → ACTION: use the lowest glucose strength that achieves euvolemia, and icodextrin for the long dwell.

Chain 5 — Modality and the day

Overnight cycling → daytime is free → fits work and school → but needs a machine and power → ACTION: let lifestyle and resources guide CAPD vs APD when the membrane permits either.

Numbered rules. These numbers are the cross-reference handle for the cases and flowcharts.

Clinical Reasoning

Five cases. Each stops you at a decision before it answers it.

Every mechanism from Level 3 earns a bedside consequence and an action.

Exhaustive. Every threshold and rule in the chapter is here.

Safety & Evidence

Panel A — Red flags

Panel B — NEVER DO

Anti-patterns clinicians fall into. Each becomes a Level 22 distractor.

The grade reflects strength of evidence, not importance.

Patient Decisions

Outcomes as natural frequencies. Figures are representative of contemporary cohorts; ranges vary by programme.

Decisions where the right answer depends on the patient's values, not the evidence.

Effective-care decisions (not preference-sensitive)

• Matching dwell length to transport status — the physiology dictates it.
• Using icodextrin for the long dwell — evidence favours it.
• Starting incremental when residual function is substantial — the kidney decides it.
• Titrating fill volume to intra-abdominal-pressure tolerance — safety dictates it.

The conversation rehearsed as a skill. Numbers trace to Level 14.

Apply & Test

Copy-paste chart notes that map to the real decisions in this chapter.

Template 1 — New PD prescription note

• Transport status: D/P creatinine ___ (category ___); residual urine ___ mL/day.
• Modality: CAPD / APD (CCPD / NIPD); rationale ___.
• Dose: incremental / full; fill volume ___ L; exchanges or cycles ___; dwell ___.
• Osmotic agents: glucose strength ___; icodextrin for long dwell yes/no.
• Goals: euvolemia; target total Kt/V urea ___; symptom plan ___.
• Shared decision recorded (if membrane permits either): patient values ___.

Template 2 — Prescription review / adjustment note

• Fluid status: euvolemic / overloaded / depleted.
• Clearance and residual function: total Kt/V ___; residual urine ___.
• Problem identified: overload / underdialysis / mechanical / poor UF.
• Change made: ___ (lever adjusted and why).
• Next review and what would trigger escalation: ___.

Pre-rounds compression. Numbers and rules only.

Active recall. At least one card per objective.

Micro-teaching for rounds. Two scenarios, five steps each.

Metacognition anchored to this chapter's tensions. No answers provided.

1. 1. When the membrane permits either modality, how do you keep your own convenience from quietly deciding for the patient?
2. 2. Where is the line between a Kt/V you can defend on paper and the euvolemia and wellbeing the patient actually feels?
3. 3. Incremental PD asks you to do less at the start — what makes that harder to prescribe than full dose, and is the difficulty yours or the patient's?
4. 4. How would you decide when assisted PD has become a burden on a family rather than a way to honour a patient's wish to stay home?
5. 5. What would make you revisit a modality choice months later — and how do you keep that door visibly open for the patient?

Nine items, each anchored in this chapter. At least one per objective.