Renal replacement therapy in the ICU: when and which

Concept

When the kidneys fail in a very sick patient, a machine can clean the blood and remove extra fluid until the kidneys recover.

Updated Nov 13, 2026

What it is

Renal replacement therapy (RRT) takes over the work of failing kidneys: it removes potassium, acid, urea, toxins and excess water from the blood through a semipermeable membrane outside the body. In the ICU it is used mostly for acute kidney injury (AKI), which is staged by the KDIGO criteria (a rise in serum creatinine over hours to days, or a fall in urine output) and is common in sepsis, shock, major surgery and rhabdomyolysis. The decision has two parts. When: RRT is started for complications that are life-threatening and do not respond to medical treatment (refractory hyperkalaemia, severe metabolic acidosis, fluid overload with respiratory failure, uraemic complications such as pericarditis or encephalopathy, and some dialysable poisonings); trials have not shown a benefit from starting earlier in every patient with severe AKI who has none of these. Which: intermittent haemodialysis (IHD) clears solutes fast over a few hours, while continuous RRT (CRRT) works slowly over the whole day and removes fluid more gently, so it is chosen for patients who are haemodynamically unstable or who have raised intracranial pressure; prolonged intermittent techniques sit in between. Solutes cross the membrane by diffusion (dialysis, best for small molecules), convection (haemofiltration, which drags larger molecules with water) or both. Every technique needs a large-bore dialysis catheter and, usually, anticoagulation of the circuit; regional citrate is preferred for CRRT when it can be used safely.

Key points

KDIGO stages AKI by the rise in creatinine from baseline and by urine output; oliguria often appears before the creatinine rises.
Treat the cause first: restore perfusion, stop nephrotoxins, relieve obstruction and review every drug for renal adjustment.
Loop diuretics may help fluid balance in a patient who still responds, but they do not treat AKI or delay the need for RRT when it is indicated.
The dialysis catheter goes preferably in the right internal jugular vein, then the femoral vein; the subclavian vein is avoided because stenosis threatens a future fistula.
Drug clearance changes on RRT, especially for antibiotics: the pharmacist adjusts the regimen to the technique and its intensity.
RRT is stopped when urine output returns and the metabolic picture stays stable; recovery is checked by trial off therapy.

Components

Vascular access: a non-tunnelled, double-lumen dialysis catheter that draws and returns blood.
Extracorporeal circuit: a blood pump, the filter (haemofilter or dialyser) and pressure monitors that detect clotting or access problems.
Dialysate and replacement fluid: sterile buffered solutions that set the gradient for diffusion or replace the water removed by convection.
Anticoagulation: regional citrate with calcium replacement, unfractionated heparin, or none when bleeding risk is high.

How it works

Diffusion: solutes move from blood to dialysate down a concentration gradient across the membrane; efficient for small molecules.
Convection: pressure pushes plasma water through the membrane and carries dissolved solutes with it (solvent drag), including middle molecules.
Ultrafiltration: net removal of water set by the team to reach the fluid-balance goal.
Modes: CVVH (convection), CVVHD (diffusion), CVVHDF (both) and SCUF (fluid removal only); SLED and other prolonged intermittent modes run for part of the day.

Choosing the technique

Technique	How it works	Best for	Main drawback
Intermittent haemodialysis	Fast diffusion over a few hours	Stable patients; rapid removal of potassium or a toxin	Hypotension and fluid shifts; rebound of solutes
Continuous RRT	Slow convection, diffusion or both over the whole day	Shock, raised intracranial pressure, large fluid needs	Needs constant anticoagulation, staff and immobility
Prolonged intermittent RRT	Diffusion at a lower rate for part of the day	Patients stabilising; units with limited CRRT	Middle ground in both clearance and tolerance
Peritoneal dialysis	The peritoneum acts as the membrane	Children, settings without extracorporeal machines	Less control of clearance; needs an intact abdomen

In practice

Hypotension at the start of RRT is common: check volume, the fluid-removal goal and vasopressor needs before blaming the machine.
Frequent filter clotting usually means access trouble or inadequate anticoagulation; look at the catheter and the circuit pressures.
CRRT removes phosphate, potassium, magnesium, amino acids and water-soluble vitamins: monitor and replace them.
The extracorporeal circuit cools the blood and can hide a fever; a normal temperature does not exclude infection.
Catheter-related bloodstream infection is a risk of every dialysis line; it is managed like any central line.

Next step

Write down the indication, the technique, the anticoagulation and the daily fluid-balance goal, and review them at every round.
Call the nephrologist or the intensivist at once for hyperkalaemia with ECG changes, pulmonary oedema or rising acidosis despite treatment.
Check electrolytes, ionised calcium (with citrate), phosphate and acid-base at the intervals the protocol sets.
Ask the pharmacist to adjust antibiotics and other renally cleared drugs to the RRT technique.
After the ICU, arrange renal follow-up: survivors of AKI are at risk of chronic kidney disease.

Test yourself

Think of the answer, then tap the question to see it.

What are the urgent indications for renal replacement therapy in AKI?

Refractory acidosis, refractory hyperkalaemia, a dialysable intoxication, fluid overload not controlled by diuretics, and uraemic complications such as pericarditis or encephalopathy.

Why is continuous RRT chosen for a patient in shock?

It removes fluid and solutes slowly over the whole day, causing fewer swings in blood pressure and fluid than intermittent haemodialysis.

What is the difference between diffusion and convection in RRT?

Diffusion moves solutes down a concentration gradient and clears small molecules; convection drags solutes with water under pressure and also clears middle molecules.

Which vein is avoided for a dialysis catheter and why?

The subclavian vein, because it can cause central venous stenosis that would compromise a future arteriovenous fistula in the same arm.

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Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.

Sources: Continuous Renal Replacement Therapy (StatPearls (NCBI Bookshelf), 2024); Acute Kidney Injury (StatPearls (NCBI Bookshelf), 2026).

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