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Continuous renal replacement therapy: the circuit in general terms, fluid removal, anticoagulation checks, alarms and electrolytes

  • Safe practice

In continuous renal replacement therapy the nurse runs the circuit around the clock: pressures, alarms, fluid removal, anticoagulation checks and electrolytes.

Updated

What it is

Continuous renal replacement therapy (CRRT) cleans the blood and removes fluid slowly, 24 hours a day, in critically ill patients whose kidneys have failed. Blood leaves through a dialysis catheter, passes a filter in an external circuit and returns to the patient. In most ICUs the nurse is the main operator: starting and stopping the circuit, recording pressures and fluid hourly, responding to alarms, checking anticoagulation and electrolytes, and watching the patient for low blood pressure and bleeding. The machine settings come from the medical order and the unit's protocol. Acute kidney injury is explained on its condition page.

What it requires

  • A medical order for the mode, the fluid removal target and the anticoagulation, and a unit protocol for alarms and laboratory checks.
  • Trained nurses with 24-hour access to ionised calcium and electrolyte testing.
  • A dialysis catheter that is well positioned and handled with central line precautions.
  • Correctly labelled bags: citrate or replacement solutions must never be confused with other fluids.

Components

  • The access line takes blood from the patient to the pump and the filter.
  • In the filter, solutes and water cross a membrane; replacement or dialysis fluids are added as prescribed.
  • The return line brings the blood back to the patient.
  • Anticoagulation keeps the circuit from clotting: regional citrate (binds calcium in the circuit) or another method, as ordered.

How it is applied (and why)

  1. Check the order, the catheter and the circuit set-up as the protocol says — errors in fluids or settings act for hours.
  2. Record vital signs, weight and balance at the start and watch the patient closely — it shows how the therapy is tolerated.
  3. Record pre-filter, transmembrane and return pressures hourly and look for a rising trend — it warns of clotting early.
  4. For a catheter alarm, check the patient's position and the lines, then lower blood flow as the protocol allows — kinks and position cause most access alarms.
  5. For filter alarms, look at the filter for clotting and report — return the blood before the circuit clots if the protocol allows.
  6. Record fluid removed and given each hour and compare with the target — the net balance is the goal of therapy.
  7. With citrate, draw post-filter and patient ionised calcium as scheduled and report values outside the range — they guide citrate and calcium.
  8. Check electrolytes, magnesium, sodium, acid–base and other tests as ordered — the circuit removes and adds solutes all the time.
  9. Watch the catheter site, bleeding and blood pressure — anticoagulation and fluid removal carry these risks.

In nursing practice

  • Unplanned stops are most often from high circuit pressure or filter clotting, and blood in a clotted filter may be lost.
  • Report low blood pressure during therapy, with the current fluid removal rate.
  • Citrate overdose can cause severe low calcium, low blood pressure and cardiac arrest; calcium reverses it quickly, as the protocol says.
  • Citrate can also cause metabolic alkalosis, high sodium and low magnesium.
  • Keep the family informed about the machine and its alarms; patient movement and positioning are planned with the circuit in mind.

Common mistakes

  • Silencing pressure alarms without looking at the catheter, the patient's position and the filter.
  • Not recording pressures hourly, so the warning trend is missed.
  • Mixing up citrate or replacement bags with other fluids.
  • Leaving the circuit stopped for a long time without telling the team that fluid removal is behind.
  • Drawing ionised calcium late or from the wrong port.

Test yourself

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

Which pressures are recorded hourly during CRRT, and why?

Pre-filter, transmembrane and return pressures; a rising trend, often first in the pre-filter pressure, warns of filter clotting.

What usually causes access and return alarms?

Problems with the catheter or the patient's position, such as kinks or a catheter against the vessel wall.

What does a rising total-to-ionised calcium ratio suggest during citrate anticoagulation?

Citrate accumulation in the patient, which is reported so that the team can adjust the citrate and calcium.

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Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order.

Sources: Factors Influencing Unplanned Discontinuation of Continuous Renal Replacement Therapy: A Descriptive Time-Series Analysis (Jia W et al., Nurs Crit Care 2026;31(5):e70637) (Nursing in Critical Care, open access in PubMed Central, 2026); Bench-to-bedside review: Citrate for continuous renal replacement therapy, from science to practice (Oudemans-van Straaten HM, Ostermann M, Crit Care 2012;16(6):249) (Critical Care, open access in PubMed Central, 2012).

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Representative model, for educational purposes only.