Chapter 2 — Modalities & Circuit Anatomy

Every modality is a different way of arranging the same three physical mechanisms around one filter. Learn the circuit, and the modalities become obvious rather than memorised.

2.1 Circuit Anatomy — The Common Backbone

Every CRRT circuit, regardless of modality, contains:

💡 Access is destiny. The most common cause of poor CRRT delivery is not modality choice or dose — it is a catheter that cannot sustain the prescribed blood flow. Preferred sites by KDIGO are right internal jugular first, then femoral, then left internal jugular, with subclavian last (because of stenosis risk in patients who may need permanent access). Use an uncuffed, non-tunnelled catheter for AKI.

2.2 The Modalities

SCUF — Slow Continuous Ultrafiltration

• Mechanism: ultrafiltration only. No dialysate, no replacement fluid.
• Achieves: plasma water removal.
• Does not achieve: meaningful solute clearance.
• Use case: diuretic-resistant volume overload with adequate solute control — classically cardiorenal syndrome.

⚠️ Declared gap: ultrafiltration for acute decompensated heart failure has been tested against pharmacological therapy with inconsistent results, and the trials studied selected cardiology populations rather than general ICU CRRT. Do not extrapolate a survival claim to the critically ill.

CVVH — Continuous Veno-Venous Haemofiltration

• Mechanism: convection only. Large ultrafiltration volume; replacement fluid returned pre- and/or post-filter.
• Strength: middle-molecule clearance.
• Cost: higher filtration fraction → shorter circuit life. Requires large replacement fluid volumes.

CVVHD — Continuous Veno-Venous Haemodialysis

• Mechanism: diffusion only. Countercurrent dialysate; no replacement fluid; ultrafiltration limited to net fluid removal.
• Strength: excellent small-solute clearance at low filtration fraction → longest circuit life for a given dose.
• Cost: poor middle-molecule clearance.

CVVHDF — Continuous Veno-Venous Haemodiafiltration

• Mechanism: diffusion + convection. Dialysate and replacement fluid.
• Strength: broadest solute spectrum; the most widely used modality in modern ICUs and the modality used in the RENAL trial.
• Cost: most complex prescription; two fluid streams to balance.

SLED / PIRRT — Sustained Low-Efficiency Dialysis / Prolonged Intermittent RRT

• Mechanism: predominantly diffusive, using a standard HD machine at reduced Qb and Qd over an extended session (commonly 6–12 hours).
• Strength: hybrid — better haemodynamic tolerance than conventional IHD, lower cost and less anticoagulation exposure than 24-hour CRRT, and it frees the patient for procedures and mobilisation.
• Nomenclature note: PIRRT is the ADQI umbrella term; SLED, SLEDD and EDD are variants distinguished mainly by session length and whether convection is added.

✅ Evidence status: meta-analyses comparing PIRRT/SLED with CRRT have not demonstrated a mortality difference. Choice between them is reasonably driven by haemodynamics, resources, anticoagulation constraints and logistics rather than by an outcome argument.

2.3 Comparison Table

2.4 Choosing a Modality — The Decision Logic

Work through four questions in order:

1. Is the problem fluid, solute, or both? Fluid alone → SCUF. Both → anything else.
2. What is the size of the target solute? Small only (K⁺, urea, acid–base) → diffusion suffices; CVVHD gives the best clearance-per-unit-filtration-fraction. Middle molecules genuinely targeted → add convection.
3. How fragile is the circuit? Marginal access, coagulopathy, or no anticoagulation → favour CVVHD or heavy pre-dilution to protect filter life.
4. How haemodynamically fragile is the patient? Vasopressor-dependent → continuous therapy (KDIGO 2B). Stabilising → PIRRT becomes reasonable.

⚠️ Declared gap — the honest headline of this chapter. No randomised trial has shown that any one CRRT modality improves survival over another. Convection versus diffusion is a solute-kinetics argument, not a mortality argument. Modality selection should be justified by the solute and fluid goal, circuit logistics and haemodynamics — never by an implied outcome benefit.

2.5 Guideline Anchoring

KDIGO 2012 AKI:

• Use continuous and intermittent RRT as complementary therapies in AKI patients. (Not graded)

• Use CRRT rather than standard intermittent RRT for haemodynamically unstable patients. (2B)

• Use CRRT rather than intermittent RRT in AKI with acute brain injury or other causes of increased intracranial pressure or generalised cerebral oedema. (2B)

• Initiate RRT in AKI via an uncuffed non-tunnelled dialysis catheter, preferring the right internal jugular vein, then femoral, then left internal jugular, with the subclavian vein of the dominant side last. (Not graded)

2.6 Chapter Summary

1. One circuit, five arrangements — modality is a configuration, not a different therapy.
2. CVVHD maximises small-solute clearance per unit of filtration fraction; CVVH maximises middle-molecule clearance at the cost of circuit life; CVVHDF is the general-purpose compromise.
3. PIRRT/SLED is a legitimate alternative, not a lesser therapy, once haemodynamics allow.
4. No modality has proven a survival advantage. Choose on kinetics and logistics.