Prefilter Fluid Administration to Reduce CRRT Circuit Clotting
Prefilter fluid administration (pre-dilution) should be used in patients experiencing frequent filter clotting during CRRT, as it dilutes blood before entering the hemofilter and enhances ultrafiltration rates, thereby reducing clotting risk. 1
Mechanism and Clinical Application
Pre-dilution works by diluting the blood before it enters the hemofilter, which reduces hemoconcentration and the risk of filter clotting 1, 2. This approach is particularly valuable in:
- High-volume CVVH where filter clotting is more common 2
- Patients with frequent filter clotting despite other interventions 1
- Situations where blood flow is limited and can be combined with post-dilution to optimize extracorporeal clearance 1, 2
Fluid Composition for Prefilter Administration
Use bicarbonate-based replacement fluid rather than lactate-based solutions for prefilter administration in most CRRT patients 1. The evidence strongly supports this recommendation:
- Bicarbonate is preferred over lactate (Grade 2C) for general AKI patients 1
- Bicarbonate is strongly recommended (Grade 1B) for patients with circulatory shock 1
- Bicarbonate is recommended (Grade 2B) for patients with liver failure or lactic acidemia 1
- Avoid supra-physiologic glucose concentrations in replacement fluids to prevent hyperglycemia 2
The rationale is that lactate metabolism may be impaired in critically ill patients, particularly those with shock or liver dysfunction, potentially worsening acidosis 1.
Anticoagulation Strategies to Complement Prefilter Fluid
While prefilter fluid helps reduce clotting, anticoagulation remains essential for most patients unless contraindicated 1:
For Patients Without Bleeding Risk:
- Regional citrate anticoagulation is preferred over heparin (Grade 2B) when no contraindications exist 1
- If citrate is contraindicated or unavailable, use unfractionated or low-molecular-weight heparin (Grade 2C) 1
For Patients With Increased Bleeding Risk:
- Regional citrate anticoagulation is still suggested over no anticoagulation (Grade 2C) if citrate is not contraindicated 1
- Avoid regional heparinization in patients with increased bleeding risk (Grade 2C) 1
Special Circumstances:
- In heparin-induced thrombocytopenia (HIT), immediately stop all heparin and use direct thrombin inhibitors like argatroban or Factor Xa inhibitors (Grade 1A) 1
Important Caveats and Pitfalls
Pre-dilution reduces solute clearance efficiency compared to post-dilution because the blood is diluted before filtration 1. To compensate:
- Higher ultrafiltration rates may be needed to achieve target solute clearance 1
- The prescribed effluent volume should be 20-25 mL/kg/h to ensure adequate delivered dose 1
- Monitor actual delivered dose frequently as RRT delivery often falls short of prescription 1
Saline flush alone is ineffective: One study demonstrated that saline flushing through the prefilter site without anticoagulation provided no benefit in preventing circuit clotting (mean circuit survival 21.2 hours vs 20.4 hours, p=0.8) 3. This emphasizes that proper replacement fluid composition and anticoagulation are essential, not just volume administration.
Monitoring Requirements
Monitor prefilter pressure trends rather than relying solely on transmembrane pressure (TMP) for early detection of circuit clotting 4. Prefilter pressure rises progressively before TMP increases, allowing earlier intervention and blood return to the patient 4.
Ensure replacement fluids meet AAMI standards for bacterial and endotoxin contamination (Grade 1B) 1.