Renal Replacement Therapy in Critical Care

Renal replacement therapy (RRT) is the use of extracorporeal techniques to perform the kidney's excretory and homeostatic functions when acute kidney injury becomes severe. In the intensive care unit it can be delivered continuously or intermittently, and its modality, timing, and dose have been the subject of major randomized trials.

Definition

Renal replacement therapy in critical care is the extracorporeal support — by continuous renal replacement therapy, intermittent hemodialysis, or hybrid techniques — used to manage solute, fluid, and acid-base homeostasis in critically ill patients with severe acute kidney injury.

Scope

The entry describes the principles and modalities of RRT for AKI in critically ill patients (continuous, intermittent, and hybrid techniques), the conventional indications, and the trial evidence on dose and timing. It is a reference on concepts and evidence, not a protocol for prescribing dialysis to an individual.

Core questions

• What functions does renal replacement therapy substitute for in severe AKI?
• How do continuous and intermittent modalities differ?
• What are the conventional indications to initiate RRT?
• What does randomized evidence show about the dose and timing of RRT?

Key concepts

• Continuous renal replacement therapy (CRRT)
• Intermittent hemodialysis
• Hybrid / prolonged intermittent therapies
• Diffusion and convection (clearance principles)
• RRT dose and intensity
• Timing of initiation (early vs delayed)
• Conventional indications for RRT

Mechanisms

RRT removes solutes and fluid across a semipermeable membrane chiefly by diffusion (hemodialysis) and convection (hemofiltration), with continuous modalities running slowly over 24 hours and intermittent modalities running faster over a few hours; hybrid techniques fall in between. The choice among modalities in critical care balances hemodynamic tolerance, solute and fluid control, and logistics. Beyond modality, the prescribed dose (effluent or clearance) and the timing of initiation are the principal levers that trials have tested.

Clinical relevance

RRT is a cornerstone of critical care nephrology for severe AKI, and understanding its modalities, indications, and supporting evidence is essential to interpreting how kidney support is used in the intensive care unit. This entry conveys the concepts and trial evidence and is not a basis for individual prescribing decisions.

Epidemiology

A minority of patients with AKI in the intensive care unit progress to require RRT, but this subgroup has high illness severity and mortality; conventional indications for initiation include refractory hyperkalemia, acidosis, fluid overload, and uremic complications, while timing in the absence of such emergencies has been studied in trials.

Evidence & guidelines

The 2012 KDIGO guideline outlines indications, modality, and dosing principles for RRT in AKI. The VA/NIH ATN Study (Palevsky and colleagues, 2008) found no benefit of more-intensive over less-intensive RRT dose. Trials of initiation timing have diverged: AKIKI (Gaudry and colleagues, 2016) and IDEAL-ICU (Barbar and colleagues, 2018) found no overall survival benefit from earlier initiation, and the large STARRT-AKI trial (2020) likewise found no benefit and signaled potential harms of an accelerated strategy, shaping a generally watchful approach to timing.

History

Extracorporeal kidney support moved from intermittent hemodialysis toward continuous techniques developed in the 1970s and 1980s to suit hemodynamically unstable critically ill patients, helping define critical care nephrology as a field. From the 2000s onward, randomized trials of RRT dose (ATN, RENAL) and of initiation timing (AKIKI, IDEAL-ICU, STARRT-AKI) progressively refined practice and tempered earlier enthusiasm for high-intensity or early strategies.

Debates

When should renal replacement therapy be started in critically ill AKI?

Multiple randomized trials comparing earlier versus delayed initiation in the absence of emergency indications have generally found no survival benefit from starting early, and some signal harm, so the optimal timing — and how to individualize it — remains contested.

Does a higher RRT dose improve outcomes?

Large trials including the VA/NIH ATN study found no benefit of more-intensive renal support over a standard dose, establishing a dose target rather than escalation as the norm.

Key figures

• Claudio Ronco
• Rinaldo Bellomo
• Paul M. Palevsky
• Stephane Gaudry
• Sean M. Bagshaw

Seminal works

• palevsky-2008
• gaudry-2016
• starrt-aki-2020
• khwaja-2012

Frequently asked questions

What is the difference between continuous and intermittent renal replacement therapy?

Continuous renal replacement therapy removes solutes and fluid slowly over about 24 hours and is often used for hemodynamically unstable patients, whereas intermittent hemodialysis delivers faster clearance over a few hours; hybrid techniques combine features of both.

Does starting dialysis earlier in critically ill AKI improve survival?

Several large randomized trials comparing earlier versus delayed initiation, in patients without emergency indications, have generally found no survival benefit from starting earlier, which supports a watchful rather than reflexively early approach.