What are the components of Continuous Renal Replacement Therapy (CRRT)?

Components of Continuous Renal Replacement Therapy (CRRT)

CRRT consists of five essential components: vascular access, blood pump and circuit, hemofilter/dialyzer, replacement/dialysate fluids, and anticoagulation system. 1, 2, 3

1. Vascular Access

Use a non-tunneled, double-lumen venous catheter inserted under ultrasound guidance. 1, 3

• Preferred site hierarchy: Right internal jugular vein (first choice), femoral vein (second), left internal jugular vein (third), and subclavian vein only as last resort 1, 3
• Avoid subclavian veins due to high risk of thrombosis and late stenosis that can compromise future dialysis access 4, 1
• The catheter provides both blood withdrawal and return through separate lumens 3

2. Blood Pump and Extracorporeal Circuit

The blood pump drives blood through the circuit at controlled rates, typically 100-200 mL/min. 2, 3

• Venovenous systems with external pumps are strongly preferred over arteriovenous systems because they provide higher solute clearance rates and lower complication risk 4, 1, 2
• The circuit includes blood lines connecting the vascular access to the filter and back to the patient 5, 6
• Integrated fluid balancing systems should be used rather than adapted IV pumps to prevent significant fluid balance errors 4, 1

3. Hemofilter/Dialyzer

Use a biocompatible membrane with surface area of 0.9-1.5 m² for most adults. 3

• High permeability membranes allow water and solute transport across the semipermeable membrane 4, 5
• Filter selection depends on the CRRT modality: high water permeability filters for high-volume hemofiltration, large surface area dialyzers for extended therapies 4
• Monitor filter performance regularly and replace when efficiency decreases 4, 3

4. Replacement and Dialysate Fluids

Use bicarbonate-buffered solutions containing physiologic electrolyte concentrations. 4, 1, 3

• Bicarbonate is mandatory as the buffer (not lactate), especially in patients with lactic acidosis, liver failure, or circulatory shock 4, 1, 3
• Avoid supraphysiologic glucose concentrations that cause hyperglycemia 4, 1
• Dialysate flow rate is typically 1-2 L/hour for diffusive therapies 2, 3
• Replacement fluid can be administered pre-dilution (before filter) or post-dilution (after filter), with pre-dilution enhancing ultrafiltration rates and reducing filter clotting 4
• Sterile fluid is imperative for replacement solutions 4

5. Anticoagulation System

Regional citrate anticoagulation is the first-line choice for patients without contraindications. 1, 3, 7

• Citrate provides regional anticoagulation in the circuit while avoiding systemic anticoagulation, prolonging filter life without increasing bleeding risk 7, 8
• Monitor post-filter and serum-ionized calcium frequently to titrate citrate dose and calcium replacement 4
• Alternative options include unfractionated heparin or low molecular weight heparin with monitoring of ACT/PTT or anti-factor Xa activity 4, 1
• CRRT can run without anticoagulation in high bleeding-risk patients, though circuit life may be less than 24 hours 4, 1

CRRT Modalities Based on Component Configuration

The combination of these components creates three main modalities:

• CVVH (Continuous Venovenous Hemofiltration): Uses convection as primary mechanism, producing ultrafiltrate that is replaced with replacement solution 1, 2
• CVVHD (Continuous Venovenous Hemodialysis): Uses diffusion as primary mechanism, with dialysate flowing countercurrent to blood 2, 3
• CVVHDF (Continuous Venovenous Hemodiafiltration): Combines both diffusion and convection for solute removal 1, 2

Critical Operational Parameters

Prescribe effluent dose of 25-30 mL/kg/h to achieve delivered dose of 20-25 mL/kg/h. 1, 2

• The prescribed dose must be higher than target delivered dose to account for treatment interruptions and decreased filter efficiency 1
• Frequently assess actual delivered effluent volume to avoid underdosing 2
• Net ultrafiltration rate should be adjusted based on volume status and hemodynamic tolerance, typically targeting 100 mL/hour 3

Common Pitfalls to Avoid

• Never use lactate-buffered solutions in patients with liver failure or lactic acidosis 4, 1, 3
• Never use adapted IV infusion pumps for CRRT due to significant fluid balance error risk 4, 1
• Never place subclavian catheters unless all other sites are exhausted 4, 1, 3
• Avoid volume overload, particularly in patients with acute lung injury, as it worsens outcomes 4, 1
• Do not assume prescribed dose equals delivered dose—monitor actual effluent volume 1, 2