13.24: Extracorporeal Removal of Drugs: Continuous Renal Replacement Therapy

Continuous Renal Replacement Therapy (CRRT) is an essential intervention for patients experiencing severe kidney dysfunction. This therapy offers a continuous mechanism for removing fluids and toxins from the bloodstream, leveraging the patient’s blood pressure to facilitate filtration through a specialized filter. This method contrasts with intermittent dialysis, providing a gentler and more consistent removal of waste products and excess fluid, which is particularly beneficial in critically ill patients.

CRRT operates primarily through continuous venovenous hemofiltration (CVVH) and continuous arteriovenous hemofiltration (CAVH). In CVVH, filtration involves positioning a filter between two veins, such as the femoral or jugular veins. This setup often includes a pump to achieve higher filtration rates, thereby enhancing the removal of solutes and fluid. The continuous nature of CVVH allows for a stable balance of fluid and electrolytes, which is crucial for patients with unstable hemodynamics.

Conversely, CAVH uses a filter placed between a femoral artery and a femoral vein. Unlike CVVH, CAVH does not typically require an external pump, as the patient’s arterial pressure drives the filtration process.

A critical parameter in CAVH is the sieving coefficient (S), which quantifies a solute's ability to pass through the membrane. When S equals 1, the solute passes freely through the membrane, whereas an S value of 0 indicates complete solute retention within the plasma. This coefficient is pivotal in determining the efficacy of drug removal during therapy. Additionally, the amount of drug removed can be quantified by integrating the product of clearance and the solute concentration over time.

Both CRRT modalities necessitate vigilant monitoring to ensure optimal performance and patient safety. This includes regular assessment of filter function, fluid balance, and electrolyte levels. Anticoagulation, often with heparin, is commonly employed to prevent clotting within the extracorporeal circuit, maintaining the continuity and efficiency of the therapy.

Continuous renal replacement therapy, or CRRT, is a vital treatment for patients with severe kidney dysfunction.

Unlike intermittent methods, CRRT continuously removes fluid and toxins from the bloodstream, using the patient's blood pressure or a pump to drive filtration through a specialized filter.

There are two main types of CRRT : continuous veno-venous hemofiltration or CVVH and continuous arteriovenous hemofiltration or CAVH.

CVVH involves placing a filter between veins such as the femoral or jugular, with a pump used to achieve higher filtration rates.

CAVH places a filter between the femoral artery and vein, and drug removal in this method depends on the sieving coefficient, S. When S equals one, the drug passes freely through the membrane. When S is less than one, the drug shows partial clearance, with some quantity retained in the plasma.

Clearance equals the sieving coefficient times the ultrafiltration rate. The amount of drug removed per unit time equals plasma concentration times the unbound fraction times the ultrafiltration rate.

Both methods require careful monitoring and include heparin to prevent clotting.