Building the Hemodialysis

The contract between this chapter and the reader.

1. 1. List the components of a haemodialysis prescription.
2. 2. Explain how clearance depends on blood flow, dialysate flow, and the dialyzer.
3. 3. Distinguish high-flux from low-flux dialyzers.
4. 4. Build a prescription to achieve a target dose.
5. 5. Use treatment time as a primary lever, not just a number.
6. 6. Set dialysate composition — sodium, potassium, calcium, bicarbonate, temperature — safely.
7. 7. Choose a frequency and schedule, weighing frequent and nocturnal HD.
8. 8. Explain why a higher dose beyond target does not improve survival.
9. 9. Set an ultrafiltration goal within a safe ultrafiltration rate.

A sixty-second reading. Each bullet stands alone.

• The prescription sets treatment time, frequency, blood and dialysate flows, the dialyzer, dialysate composition, the ultrafiltration goal, and anticoagulation.
• Standard care is three sessions a week of about three and a half to four hours.
• Small-solute clearance rises with blood flow, dialysate flow, dialyzer surface area, and membrane KoA — with diminishing returns.
• High-flux membranes have larger pores and clear middle molecules and water better than low-flux.
• Build the prescription to a target dose by choosing time, blood flow, dialyzer, and frequency together.
• Treatment time is a powerful lever: longer, gentler sessions improve volume tolerance and clear middle molecules and phosphate.
• Dialysate sodium is kept near plasma (about 135–140); high dialysate sodium worsens thirst and interdialytic weight gain.
• Dialysate potassium is usually 2–3 mmol/L; very low baths risk arrhythmia and are individualized.
• Dialysate calcium and bicarbonate are set for mineral-bone and acid-base goals.
• Cooler dialysate reduces intradialytic hypotension.
• Conventional thrice-weekly HD is standard; frequent or nocturnal HD improves volume, blood pressure, and phosphate control at the cost of burden and access.
• Randomised evidence shows that a higher dose and high-flux membranes do not improve survival overall — meet the target, do not chase it.
• Keep the ultrafiltration rate within a safe range; very high rates are linked to hypotension and mortality.
• Match the prescription to the patient — residual function, size, tolerance, and goals.

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

Why it matters at the bedside

Ahaemodialysis prescription is a set of dials — time, flows, the dialyzer, the bath, the fluid to remove — and how you turn them decides whether a patient leaves the unit cleared and euvolemic or cramping and overloaded. The physics tells you what each dial does; the evidence tells you which ones actually change outcomes.

The components of the prescription

• Every prescription specifies treatment time per session, frequency, blood-flow rate, dialysate-flow rate, the dialyzer (membrane, surface area, KoA, flux), dialysate composition (sodium, potassium, calcium, bicarbonate, temperature), the ultrafiltration goal to a target weight, and anticoagulation. Standard care is three sessions a week of about three and a half to four hours.

How clearance works

• Small solutes are cleared mainly by diffusion across the membrane, down the concentration gradient maintained by countercurrent dialysate flow. Clearance rises with the blood-flow rate, the dialysate-flow rate, the membrane's mass-transfer coefficient (KoA), and surface area — but with diminishing returns, so doubling a flow does not double clearance.

Flux: high versus low

• Flux describes membrane permeability. High-flux membranes have larger pores that clear middle molecules and water far better than low-flux membranes; low-flux membranes clear small solutes adequately but leave middle molecules behind. The choice matters most when middle-molecule clearance is the goal.

Building to a target dose

• Set the target dose first (the adequacy chapter details the Kt/V target), then assemble the dials to reach it: lengthen time, raise blood or dialysate flow, choose a larger or higher-KoA dialyzer, or add sessions. Several combinations reach the same dose, so the build is shaped by tolerance, access, and the patient's life.

Time as a lever

• Treatment time is the most undervalued dial. Longer sessions ultrafiltrate more gently (better blood-pressure tolerance), give more hours for middle-molecule and phosphate clearance, and allow lower ultrafiltration rates. When small-solute targets are met but the patient stays overloaded, hyperphosphataemic, or hypotensive, more time — not a bigger blood flow — is usually the answer.

Dialysate composition

• Sodium is kept near plasma (about 135–140); high-sodium dialysate eases cramps in the short term but drives thirst, interdialytic weight gain, and a worse blood-pressure and volume cycle. Potassium is usually 2–3 mmol/L and individualized; very low baths risk arrhythmia and rebound. Calcium and bicarbonate are set for mineral-bone and acid-base goals, and a cooler dialysate (around 35.5–36 °C) markedly reduces intradialytic hypotension.

Frequency and schedule

• Conventional thrice-weekly HD is the default. Frequent schedules — short daily, or long nocturnal — improve volume and blood-pressure control and phosphate clearance, and reduce left-ventricular hypertrophy, but cost the patient burden and the access more cannulations. Longer or nocturnal sessions are the gentlest and best for middle molecules and phosphate.

Why more dose is not better

• It is intuitive that a higher Kt/V or a high-flux membrane should help everyone, but the large randomised dose-and-flux trial found no overall survival benefit from either above the standard target. The lesson is to deliver the target reliably rather than chase a bigger number; the gains that do exist come from time, volume control, and frequency, not from pushing small-solute dose.

Ultrafiltration and the UF rate

• The ultrafiltration goal removes the interdialytic fluid gain to reach the target weight, but how fast it is removed matters: high ultrafiltration rates (beyond roughly 10–13 mL/kg/h) are linked to intradialytic hypotension, myocardial stunning, and mortality. When the rate is too high, the fix is more time or more frequent sessions — and less interdialytic gain — not simply pulling harder.

Evidence base

• The futility of dose and flux escalation above target rests on a large randomised trial; the benefits of frequent and nocturnal schedules on volume, blood pressure, and hypertrophy rest on randomised and observational data with uncertain mortality effect; and the harm of high ultrafiltration rates rests on consistent observational data.

Five fully-built tables.

Table A — Prescription components

Table B — Levers to raise clearance / dose

Table C — Dialysate composition

Table D — Schedules

Table E — High-flux versus low-flux

Visualise & Map

Causal chains, each ending in a named action.

Chain 1 — Clearance physics

Countercurrent dialysate maintains a diffusion gradient → small solutes cross the membrane → clearance rises with flows, KoA, and surface area (diminishing returns) → ACTION: raise the most effective dial, and use time and the dialyzer, not flow alone.

Chain 2 — Flux and middle molecules

Larger high-flux pores → middle molecules and water cross → better middle-molecule clearance → ACTION: choose high-flux when middle molecules matter.

Chain 3 — Time as the gentle lever

Longer time → lower ultrafiltration rate + more dialysis hours → better volume tolerance, phosphate, and middle-molecule clearance → ACTION: lengthen time when targets are met but the patient is overloaded or hyperphosphataemic.

Chain 4 — The sodium trap

High dialysate sodium → diffusive sodium gain → thirst and interdialytic weight gain → worse volume and blood pressure → ACTION: keep dialysate sodium near plasma.

Chain 5 — Pulling too hard

High ultrafiltration rate → intravascular underfilling and myocardial stunning → intradialytic hypotension and worse outcomes → ACTION: lengthen or add sessions and cut interdialytic gain rather than raising the rate.

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; the direction of effect is given where precise numbers are uncertain.

Apply & Test

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

Template 1 — HD prescription note

• Schedule: frequency ___; time per session ___ h.
• Flows: blood (Qb) ___ mL/min; dialysate (Qd) ___ mL/min.
• Dialyzer: type ___; surface area ___; flux (low/high).
• Dialysate: Na ___; K ___; Ca ___; bicarbonate ___; temperature ___.
• Ultrafiltration: target weight ___; UF goal ___; UF rate within safe range? yes/no.
• Anticoagulation: ___; target dose and how achieved: ___.

Template 2 — Prescription adjustment note

• Problem: dose below target / overload / phosphate / intradialytic hypotension / high UF rate.
• Lever changed: time / flows / dialyzer / frequency / dialysate / temperature.
• Rationale (and what was NOT done, e.g., not chasing Kt/V): ___.
• Expected effect and review date: ___.

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. A bigger Kt/V is easy to measure and feels like better care; how do you resist optimising the number the trials say doesn't matter?
2. 2. More time is the lever that helps most and the one patients resist most; how do you make the case for it without dismissing their lives outside the unit?
3. 3. A high-sodium bath buys a quiet session and a worse week; how often do short-term comfort and long-term control pull against each other in your prescriptions?
4. 4. When the ultrafiltration rate is too high, the real problem is often interdialytic gain you cannot control between sessions; where does the prescription end and the patient's week begin?
5. 5. Frequent and nocturnal HD improve the things you can measure but burden the patient daily; how would you weigh a better surrogate against a heavier life?

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