Post-Replacement Fluid in CRRT
Replacement fluid should be administered post-filter (post-dilution) as the standard approach, with pre-dilution reserved for specific situations including frequent filter clotting, high-volume hemofiltration when ultrafiltration rates are limited, or when blood flow rates restrict adequate clearance 1.
Fluid Composition Requirements
Electrolyte Composition
- Use physiologic concentrations of electrolytes in replacement fluids, except when patients have extreme imbalances 1
- Avoid supraphysiologic glucose concentrations, as these cause excessive glucose intake and hyperglycemia 1
- Replacement fluid must be sterile 1
Buffer Selection
Choose bicarbonate-buffered solutions over lactate in patients with lactic acidosis, liver failure, or when performing high-volume hemofiltration 1.
The buffer choice follows this algorithm:
- Most CRRT patients: Either lactate or bicarbonate is acceptable (both effectively correct metabolic acidosis) 1
- Lactic acidosis or liver failure: Bicarbonate is preferred because lactate can worsen acidosis in these conditions 1
- High-volume hemofiltration: Bicarbonate is preferred 1
Pre-Dilution vs Post-Dilution Strategy
Post-Dilution (Standard)
Post-dilution provides superior solute clearance because blood is not diluted before entering the filter, maximizing concentration gradients for removal.
Pre-Dilution (Specific Indications)
Pre-dilution enhances achievable ultrafiltration rates and should be used when 1:
- Frequent filter clotting occurs (helps with anticoagulation strategy)
- High-volume CVVH is needed and ultrafiltration rates are limiting
- Blood flow rates are inadequate to achieve desired clearance (can combine pre- and post-dilution)
Critical Caveat
No controlled studies adequately compare these techniques, so the recommendation is based on physiologic principles and expert consensus 1. The choice should be guided by the specific clinical scenario rather than routine practice patterns.
Practical Considerations
Monitoring Requirements
Close monitoring of electrolytes is essential, particularly:
- Potassium and phosphate (risk of hypokalemia and hypophosphatemia with prolonged or high-dose CRRT) 2
- Acid-base status when using citrate anticoagulation 1
Common Pitfall
Avoid using intravenous infusion pumps adapted for CRRT, as these systems have been shown to cause significant fluid balance errors. Use devices specifically designed for CRRT 1.
Temperature Management
While data are insufficient for firm recommendations, body temperature reductions below 35°C should probably be avoided 1.