26.4: Continuous Renal Replacement Therapy

Continuous Renal Replacement Therapy, also known as CRRT, is a procedural treatment for acute kidney injury (AKI) that gradually removes uremic toxins and fluids while maintaining acid-base balance and stabilizing electrolytes. It is particularly useful for hemodynamically unstable patients. Unlike intermittent hemodialysis, which is faster, CRRT provides a gentler approach over 24 hours, closely mimicking the function of natural kidneys. However, CRRT is not ideal for patients with life-threatening conditions like severe hyperkalemia or pericarditis, where rapid removal of toxins is required.

Types of CRRT

There are different modalities of CRRT tailored to specific needs:

• Continuous Venovenous Hemofiltration (CVVH): This modality primarily removes fluid through ultrafiltration, helping manage fluid overload in patients with AKI. Blood is withdrawn through a double-lumen venous catheter, passes through a hemofilter, and is then returned to the patient. The hemofilter continuously removes fluid and gradually reduces excess fluid, making it suitable for patients with unstable blood pressure.
• Continuous Venovenous Hemodialysis (CVVHD): Similar to CVVH, CVVHD incorporates a dialysate solution that enhances the removal of uremic toxins through diffusion. As blood flows through the hemofilter, it interacts with the dialysate, allowing solutes to move down their concentration gradient from the blood into the dialysate. This method combines ultrafiltration and diffusion for more comprehensive solute removal.

Vascular Access and Filtration Process

CRRT requires a double-lumen catheter, often placed in the jugular or femoral vein. A blood pump moves blood through the circuit and into the hemofilter, which contains hollow fibers. The hemofilter extracts plasma water and solutes using hydrostatic and osmotic pressure, generating an ultrafiltrate. The ultrafiltrate drains into a collection device, and the filtered blood is reinfused to the patient.

The ultrafiltration rate (UFR) is typically set between 0 and 500 mL per hour, adjusted according to fluid balance needs. Replacement fluids can be infused before (pre-filter) or after (post-filter) the hemofilter. Pre-filter infusion reduces the risk of filter clotting, while post-filter infusion helps dilute remaining solutes like urea and creatinine in the bloodstream.

Fluid Replacement and Anticoagulation

Replacement fluids help maintain the patient's fluid and electrolyte balance, supplying sodium, chloride, and bicarbonate ions. The infusion rate is adjusted based on the patient's electrolyte needs. Anticoagulants are often administered either as a bolus at the start or continuously during therapy to prevent clotting within the circuit.

Duration and Monitoring

CRRT may continue for weeks, depending on the patient's condition and the recovery of kidney function. The hemofilter needs to be replaced every 24 to 48 hours to ensure efficient filtration and reduce the risk of clotting. A clear yellow ultrafiltrate indicates proper function, while a blood-tinged ultrafiltrate suggests a possible filter membrane rupture, requiring immediate cessation of therapy to prevent blood loss.

Nursing care during CRRT involves daily monitoring of the patient's weight, fluid balance, and lab values to ensure proper management. Hourly assessments of intake, output, and hemodynamic stability are crucial, focusing on maintaining the CRRT system and catheter patency to prevent complications. Significant changes in vital signs should be addressed promptly.

Once the patient's AKI resolves or CRRT is no longer needed, the therapy is discontinued, and the double-lumen catheter is removed.