How does CRRT differ from intermittent haemodialysis?

CRRT removes solute and fluid slowly and continuously over many hours, whereas intermittent haemodialysis does so rapidly over a few hours; the gradual approach is generally better tolerated by haemodynamically unstable patients.

How does CRRT remove waste and fluid?

It clears solutes by diffusion across a membrane (dialysis) and/or by convection (haemofiltration), and removes excess water by ultrafiltration, all within an extracorporeal circuit.

Continuous Renal Replacement Therapy (CRRT)

Continuous renal replacement therapy (CRRT) is a slow, continuous form of extracorporeal blood purification used to support critically ill patients with severe acute kidney injury, particularly those who are haemodynamically unstable. By removing solute and water gradually over hours to days rather than over a short intermittent session, CRRT aims to provide steadier fluid and solute control with less abrupt circulatory stress.

Definition

Continuous renal replacement therapy is an extracorporeal blood-purification technique delivered continuously (typically 24 hours per day) to replace kidney excretory function in critically ill patients, removing solutes by diffusion and/or convection and water by ultrafiltration.

Scope

This topic covers the rationale for CRRT, the main modalities and the mechanisms of diffusion and convection on which they rely, the questions of dose and timing that large trials have addressed, and how CRRT relates to intermittent haemodialysis. It is a reference-educational overview of the modality; it does not give prescriptions, circuit settings, anticoagulation regimens, or patient-specific instructions.

Key concepts

Diffusion (dialysis) and convection (haemofiltration)
Ultrafiltration and fluid removal
CVVH, CVVHD, and CVVHDF modalities
Effluent dose (mL/kg/hour)
Timing of initiation
Haemodynamic tolerability versus intermittent haemodialysis
Circuit anticoagulation

Mechanisms

CRRT circulates blood through an extracorporeal circuit and a semipermeable membrane. Solutes are cleared by diffusion when dialysate runs counter-current to blood (haemodialysis principle) and by convection when a pressure gradient drags solute-laden plasma water across the membrane (haemofiltration), with the removed volume replaced by substitution fluid. Net fluid removal is achieved by ultrafiltration. Because exchange occurs slowly and continuously, osmolar and volume shifts are gentler than in intermittent dialysis, which is the rationale for preferring CRRT in haemodynamically unstable patients. The delivered dose is expressed as effluent flow normalised to body weight, and an extracorporeal circuit generally requires anticoagulation to remain patent.

Clinical relevance

CRRT is a core organ-support modality in critical care nephrology, and understanding its principles, dose, and timing is part of interpreting the trial evidence on renal support. This entry describes the modality and the evidence around it for reference and education; choices about whether, when, and how to deliver renal replacement for a given patient belong to the treating team.

Epidemiology

A substantial minority of patients with severe acute kidney injury in the intensive care unit receive some form of renal replacement therapy, and CRRT is a commonly used modality for those who are haemodynamically unstable. Randomised trials have informed practice on the intensity of therapy and on the timing of initiation.

History

Continuous arteriovenous haemofiltration was introduced in the late twentieth century to support unstable patients who tolerated intermittent dialysis poorly, and pumped veno-venous techniques later superseded the original arteriovenous approach. The Ronco trial (2000) brought attention to the question of delivered dose, the RENAT trial and related studies examined intensity of therapy, and trials such as AKIKI (Gaudry, 2016) and STARRT-AKI (2020) addressed the timing of initiation, collectively shaping how renal replacement is prescribed in the intensive care unit.

Debates

When should renal replacement therapy be started in AKI?: Whether an earlier, pre-emptive start improves outcomes compared with waiting for conventional indications has been tested in several large trials with differing results, and the balance of evidence does not support routine early initiation in the absence of urgent indications.
Does a higher delivered dose improve outcomes?: After early signals suggested benefit from higher-intensity therapy, larger trials did not confirm an outcome advantage for higher effluent doses, refocusing attention on delivering an adequate rather than maximal dose.

Key figures

Claudio Ronco
Rinaldo Bellomo
Stephane Gaudry
Sean Bagshaw

Seminal works

ronco-2000
gaudry-2016
starrt-aki-2020

Frequently asked questions

How does CRRT differ from intermittent haemodialysis?: CRRT removes solute and fluid slowly and continuously over many hours, whereas intermittent haemodialysis does so rapidly over a few hours; the gradual approach is generally better tolerated by haemodynamically unstable patients.
How does CRRT remove waste and fluid?: It clears solutes by diffusion across a membrane (dialysis) and/or by convection (haemofiltration), and removes excess water by ultrafiltration, all within an extracorporeal circuit.