What is Continuous Renal Replacement Therapy (CRRT)?

What is Continuous Renal Replacement Therapy (CRRT)?

CRRT is an extracorporeal blood purification therapy that substitutes for impaired renal function continuously over 24 hours per day, primarily used in critically ill patients with acute kidney injury who are hemodynamically unstable. 1, 2

Core Definition and Mechanism

CRRT provides gradual, continuous purification of blood over extended periods rather than intermittent treatments, distinguishing it fundamentally from conventional hemodialysis which typically runs for 12 hours or less. 1, 2 The therapy operates continuously for or aims to operate for 24 hours daily, making it particularly suited for patients who cannot tolerate the rapid fluid and solute shifts associated with intermittent dialysis. 1, 3

Primary Clinical Modalities

CRRT encompasses several distinct techniques that differ in their mechanism of solute clearance:

Convective Modalities

• Continuous Venovenous Hemofiltration (CVVH) uses primarily convective clearance through a highly permeable membrane, where ultrafiltrate is produced and replaced with replacement solution. 1, 2
• Continuous High-Volume Hemofiltration is a CVVH variant using higher surface area filters with ultrafiltration volumes exceeding 35 mL/hour/kg. 1, 2

Diffusive Modalities

• Continuous Venovenous Hemodialysis (CVVHD) uses primarily diffusive clearance with dialysate solution delivered countercurrent to blood flow at rates typically 1-2 L/hour. 1, 2
• Continuous Venovenous High-Flux Dialysis (CVVHFD) uses highly permeable dialyzers with countercurrent blood and dialysate flow, balancing filtration and back-filtration without requiring replacement fluid. 1, 2

Combined Modalities

• Continuous Venovenous Hemodiafiltration (CVVHDF) combines both convective and diffusive clearance methods, optimizing ultrafiltration volumes while using dialysate to enhance solute removal. 1, 2

Clinical Indications

CRRT is specifically indicated for:

• Hemodynamically unstable patients with acute kidney injury who cannot tolerate the rapid fluid shifts of intermittent hemodialysis. 2, 3
• Patients with or at risk for cerebral edema, where gradual solute and fluid removal prevents dangerous intracranial pressure fluctuations. 2
• Continuous fluid removal requirements in volume-overloaded patients, particularly those with acute respiratory distress syndrome or acute lung injury. 2
• Severe acid-base disturbances requiring continuous correction rather than intermittent adjustment. 2
• Septic shock, ARDS, or burns where continuous removal of inflammatory mediators may provide benefit. 2

Technical Prescription Standards

Vascular Access

The right internal jugular vein is the first-choice access site, followed by femoral vein, then left internal jugular vein. 2 The subclavian vein must be avoided due to thrombosis and stenosis risk. 2 Ultrasound guidance is mandatory for catheter insertion, and chest radiograph is required after internal jugular or subclavian placement before first use. 2

Dosing Parameters

The recommended effluent volume is 20-25 mL/kg/hour. 2, 4 This dosing recommendation is based on level 1 evidence from the ATN and RENAL trials, which demonstrated that effluent flow rates exceeding 25 mL/kg/hour do not improve patient outcomes. 4

Anticoagulation Strategy

Regional citrate anticoagulation is the first-line choice for patients without increased bleeding risk. 2 For patients with contraindications to citrate, unfractionated or low-molecular-weight heparin can be used. 2 Patients with heparin-induced thrombocytopenia require direct thrombin inhibitors or Factor Xa inhibitors. 2

Fluid Composition

Bicarbonate is preferred over lactate as the buffer in dialysate and replacement fluid, especially for patients with circulatory shock, liver failure, or lactic acidemia. 2, 5 All dialysis and replacement fluids must comply with AAMI standards for bacterial and endotoxin contamination. 2

Key Advantages Over Intermittent Hemodialysis

CRRT provides greater hemodynamic stability in critically ill patients compared to intermittent hemodialysis. 2, 3 The continuous nature allows for better management of fluid balance and more effective treatment for patients with cerebral edema or increased intracranial pressure. 2 The therapy offers lower bleeding risk and a more physiologic approach to kidney support. 6

Critical Pitfalls to Avoid

Do not rely solely on BUN and creatinine thresholds for initiating renal replacement therapy; consider the broader clinical context including oliguria, volume overload, and metabolic derangements. 2, 7 Avoid using lactate-buffered solutions in patients with liver failure or lactic acidosis due to impaired lactate metabolism. 2, 5 Never use adapted intravenous infusion pumps for CRRT as they risk significant fluid balance errors; only use integrated fluid balancing systems specifically designed for CRRT. 1, 2 The actual delivered dose must be frequently assessed and the prescription adjusted accordingly, as prescribed dose often differs from delivered dose. 2

Current Utilization

Approximately one-fourth of all patients with acute renal failure in the United States are treated with CRRT, and utilization is increasing worldwide. 1, 2