Continuous Renal Replacement Therapy (CRRT)

Overview of the Condition

What is acute kidney injury?

• CRRT, or continuous renal replacement therapy, is a form of dialysis used in critically ill patients whose kidneys are not functioning properly
• Acute kidney injury (AKI) is when the kidneys suddenly stop working as well as they should, developing over hours to days rather than gradually over months or years
• AKI usually develops as a result of another serious illness, and it is especially common in patients in the intensive care unit (ICU)

Why is it important?

Studies show that up to half of ICU patients may develop some degree of acute kidney injury. In its most severe form, AKI can lead to dangerous build-up of waste products, fluid, and abnormal electrolyte levels, which can be life-threatening. CRRT essentially does the work of the kidneys for patients who are too medically unstable for standard dialysis, making it a critical, often life-sustaining treatment.

Who is commonly affected?

• Critically ill patients in the intensive care unit, particularly those with multiple organ problems
• People with hemodynamic instability, meaning unstable blood pressure or heart rate, who cannot safely tolerate standard intermittent dialysis
• People with severe sepsis, major surgery, or trauma, all of which can trigger acute kidney injury
• People with significant fluid overload who also require ongoing medications, blood products, or nutrition support that adds to their fluid intake

Causes / Why It Happens

Acute kidney injury, and the resulting need for CRRT, generally falls into three categories:

• Reduced blood flow to the kidneys (prerenal): caused by conditions such as low blood pressure, heart failure, severe dehydration, or significant blood loss
• Direct kidney damage (intrarenal): caused by conditions affecting the kidney tissue itself, including certain infections, inflammation, or the effects of some medications
• Blockage of urine flow (postrenal): caused by an obstruction in the urinary tract, such as kidney stones, an enlarged prostate, or a tumour

Critically ill patients often have more than one contributing factor at once, such as sepsis combined with low blood pressure and the effects of certain medications, which is part of why AKI is so common in the ICU setting.

Signs and Symptoms

Common symptoms of acute kidney injury

• Passing little or no urine
• Fatigue and weakness
• Swelling in the legs, ankles, or around the eyes, from fluid retention
• Nausea or vomiting
• Confusion or a change in mental alertness
• Shortness of breath, particularly if fluid builds up around the lungs

In milder cases, AKI may cause no noticeable symptoms at all and is instead found through routine blood tests during a hospital stay. In critically ill patients, AKI is often identified this way, through monitoring of kidney function tests, rather than through symptoms the person reports themselves.

Procedure Overview

What is CRRT, and why is it used?

• CRRT is a blood purification technique that removes excess fluid and waste products from the blood slowly and continuously, typically over 24 hours a day, closely mimicking the way healthy kidneys work
• This is different from standard (intermittent) haemodialysis, which is typically done in shorter sessions a few times a week
• CRRT is generally used for patients who are too haemodynamically unstable, meaning their blood pressure or heart rhythm is too unstable, to safely tolerate the more rapid fluid and solute shifts of standard dialysis

Who needs CRRT?

• Critically ill patients with severe acute kidney injury who cannot tolerate standard intermittent dialysis due to unstable blood pressure
• Patients with significant fluid overload, especially those needing ongoing fluids, medications, or nutrition support
• Patients with severe electrolyte or acid-base abnormalities that need steady, gradual correction rather than rapid change
• Some patients receiving ECMO (a form of life support for the heart and lungs) may also require CRRT at the same time

Before CRRT Begins

Key considerations

• CRRT is generally started in the ICU once the care team determines that a patient's kidney function and overall stability require this level of support
• Blood tests are used to assess kidney function, electrolyte levels, and acid-base balance before and throughout treatment
• The care team discusses the treatment plan, expected duration, and goals of CRRT with the patient's family, since patients receiving CRRT are typically very unwell and often unable to participate directly in these discussions

Vascular access

• CRRT requires a large intravenous catheter, called a central venous catheter, placed in a large vein, usually in the neck, chest, or groin, to allow blood to be continuously removed and returned
• This catheter is placed using a sterile procedure, generally at the bedside in the ICU
• Anticoagulation (blood-thinning) is generally used during CRRT, often with a technique called regional citrate anticoagulation, to help prevent the blood from clotting as it passes through the external circuit

Procedure Overview

During the procedure

• Blood is continuously removed from the body through the catheter, passed through a filter (the CRRT machine) that removes waste products and excess fluid, and then returned to the body
• The process is slow, gentle, and continuous, generally running 24 hours a day, rather than the more concentrated few-hours-at-a-time sessions used in standard dialysis
• A specialised ICU nursing and medical team closely monitors the patient and the CRRT machine throughout treatment, adjusting settings as needed

Types of CRRT

• Continuous venovenous haemofiltration (CVVH): removes waste and fluid mainly through filtration, passing a large volume of fluid across a membrane
• Continuous venovenous haemodialysis (CVVHD): removes waste products mainly through diffusion, passing blood alongside a dialysis fluid across a filter, with replacement fluid given to restore what is removed
• Continuous venovenous haemodiafiltration (CVVHDF): combines both filtration and diffusion to remove solutes and toxins from the blood

The choice between these techniques is made by the ICU and nephrology team based on the patient's specific clinical needs.

Duration

• CRRT typically continues 24 hours a day, for as long as needed, until the patient's condition improves enough to tolerate standard dialysis or their kidney function recovers sufficiently that dialysis is no longer needed
• The exact length of treatment varies widely between patients and depends on the severity of illness and how quickly the kidneys, and overall health, improve

Risks, Benefits and Monitoring

Risks and Complications

CRRT is generally a safe, well-established ICU treatment, but as with any invasive procedure in critically ill patients, some risks exist:

• Bleeding: related to the anticoagulation used to keep the CRRT circuit from clotting
• Clotting of the circuit: the external filter and tubing can clot, sometimes requiring the circuit to be replaced, which can interrupt treatment
• Infection: as with any central venous catheter, there is a risk of infection at the catheter site or in the bloodstream
• Blood pressure changes: although CRRT is gentler than standard dialysis, blood pressure changes can still occur during treatment
• Electrolyte imbalances: closely monitored and corrected throughout treatment, since both the illness and the CRRT process itself can affect electrolyte levels

Benefits and Expected Outcome

• Removes waste products, excess fluid, and helps correct electrolyte and acid-base imbalances gradually and gently
• Offers more stable blood pressure control during treatment compared with standard intermittent dialysis, making it more suitable for critically ill, unstable patients
• Allows continued administration of necessary fluids, medications, blood products, or nutrition support without worsening fluid overload
• Supports patients through the acute illness while giving the kidneys time to recover, when recovery is possible

Where CRRT Takes Place

• CRRT is performed exclusively in the intensive care unit (ICU), where patients can be closely monitored around the clock
• It requires specialised equipment and a trained ICU nursing and medical team throughout treatment

Recovery and What Happens Next

Monitoring during CRRT

• Vital signs, fluid balance, and blood tests are checked frequently throughout treatment to guide adjustments to the CRRT prescription
• The care team monitors the CRRT circuit closely to detect early signs of clotting or other problems that could interrupt treatment
• Family members are typically kept informed of the patient's progress and any changes to the treatment plan, since patients on CRRT are often unable to participate directly in these updates themselves

Transitioning off CRRT

• CRRT continues until the patient's condition stabilises enough to tolerate standard intermittent dialysis, or until kidney function recovers enough that dialysis is no longer needed
• Some patients recover kidney function fully after an episode of acute kidney injury; others may need ongoing dialysis if kidney function does not recover
• The care team reassesses regularly to decide when a transition away from CRRT is appropriate

Supporting Family Members

• Patients receiving CRRT are critically ill, and it can be a stressful, uncertain time for families
• The ICU team is available to explain what CRRT involves, answer questions, and provide updates on progress
• Asking the care team directly about the goals of treatment and what to expect can help families feel more informed during this time

Follow-up

• Kidney function is monitored with blood tests during recovery from acute kidney injury, even after CRRT is stopped
• Some patients who have had a severe episode of AKI have an increased risk of developing chronic kidney disease later on and may need longer-term follow-up with a kidney specialist

References

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