Renal Replacement Therapy

Renal replacement therapy substitutes for the excretory function of failing kidneys, removing waste products and excess fluid and helping to correct electrolyte and acid-base disturbances. In the intensive care unit it is most often delivered continuously, and its operation, monitoring, and circuit management are an important part of critical care nursing.

Definition

Renal replacement therapy is the use of an extracorporeal circuit and semipermeable membrane to perform the excretory functions of the kidneys — clearing solutes and removing fluid — in patients with kidney failure, delivered as continuous, intermittent, or hybrid modalities.

Scope

This entry introduces the principles of renal replacement therapy in the critically ill — the underlying processes of diffusion and convection, the main modalities including continuous and intermittent forms, and the broad evidence on when therapy is started. It presents these as reference essentials and does not specify prescriptions, settings, or anticoagulation regimens for an individual patient.

Core questions

• What physical processes allow an extracorporeal circuit to clear solutes and remove fluid?
• How do continuous and intermittent renal replacement modalities differ, and when is each used?
• What does the evidence show about the timing of starting renal replacement therapy in acute kidney injury?

Key concepts

• Diffusion and convection
• Haemodialysis, haemofiltration, and haemodiafiltration
• Continuous renal replacement therapy (CRRT)
• Intermittent haemodialysis
• Ultrafiltration and fluid removal
• Extracorporeal circuit and anticoagulation
• Timing of initiation
• Solute clearance and dose

Mechanisms

Blood is circulated through an extracorporeal circuit and across a semipermeable membrane, where solutes are cleared by diffusion (movement down a concentration gradient, as in dialysis) or by convection (solvent drag during ultrafiltration, as in haemofiltration), and excess fluid is removed by ultrafiltration. Continuous renal replacement therapy delivers this slowly over 24 hours, which can suit haemodynamically unstable patients, whereas intermittent haemodialysis achieves rapid clearance over shorter sessions; the circuit requires careful management and usually some form of anticoagulation (Tolwani, 2012). By controlling fluid balance and solute concentrations, the therapy also helps correct the electrolyte and acid-base disturbances of kidney failure.

Clinical relevance

Renal replacement therapy is a common organ-support modality in intensive care, and operating the circuit, monitoring fluid removal, and recognizing complications are defined critical care nursing responsibilities. This entry describes the principles, modalities, and evidence for reference and education; it is not a protocol for prescribing or adjusting therapy for any specific patient.

Evidence & guidelines

Reviews describe the principles and practice of continuous renal replacement therapy for acute kidney injury (Tolwani, 2012). The large STARRT-AKI randomized trial examined whether accelerated versus standard initiation of renal replacement therapy improves outcomes in critically ill patients with acute kidney injury (STARRT-AKI Investigators, 2020), informing the ongoing discussion about timing.

Debates

Timing of initiation in acute kidney injury

Whether to start renal replacement therapy early (before classic indications develop) or to wait has been tested in randomized trials, with major trials not showing a benefit from a routinely accelerated strategy; the optimal timing remains an active question.

Seminal works

• tolwani-2012
• starrt-aki-2020

Frequently asked questions

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

Continuous renal replacement therapy clears solutes and removes fluid slowly over about 24 hours and is often used for haemodynamically unstable patients, whereas intermittent haemodialysis achieves more rapid clearance in shorter sessions.

When is renal replacement therapy started in acute kidney injury?

It is used to support patients whose kidney failure threatens fluid, electrolyte, or acid-base balance. Randomized trials have examined whether starting earlier improves outcomes, and a routinely accelerated start has not been shown to be beneficial, so timing remains individualized and debated.