What is the difference between pre-blood pump rate and replacement rate in Continuous Renal Replacement Therapy (CRRT)?

Difference Between Pre-Blood Pump Rate and Replacement Rate in CRRT

The pre-blood pump rate refers to the rate at which fluid (typically D5W) is administered before the blood enters the filter, while the replacement rate is the rate at which replacement fluid is administered to replace fluid removed during hemofiltration. 1, 2

Pre-Blood Pump Rate

• Pre-blood pump fluid (often D5W) is administered before blood enters the filter to:
  • Enhance the achievable ultrafiltration rate, particularly important in high-volume CVVH 1
  • Help prevent filter clotting in patients with frequent filter clotting issues 1
  • Dilute the blood before filtration, reducing hemoconcentration and improving filter longevity 3
  • Allow for management of specific clinical scenarios such as hyponatremia, where pre-filter D5W can help control the rate of sodium correction 1

Replacement Rate

• Replacement fluid is administered to replace fluid removed during hemofiltration and maintain fluid balance 2
• The recommended replacement fluid rate for CRRT is 20-25 mL/kg/hr according to KDIGO guidelines 2
• For patients with high catabolic states or sepsis, higher rates of 30-35 mL/kg/hr may be considered 3
• Replacement fluid can be administered:
  • Pre-dilution (before the filter): Reduces filter clotting but decreases solute clearance efficiency 3, 2
  • Post-dilution (after the filter): Provides more efficient solute clearance but increases risk of filter clotting 2

Technical Considerations

• The effluent rate in CRRT refers to different parameters depending on the modality:
  • In CVVH: Effluent rate refers to the ultrafiltration rate 2
  • In CVVHD: Effluent rate refers to the dialysate flow rate 2
  • In CVVHDF: Effluent rate is the sum of dialysate and ultrafiltration rates 2

Clinical Impact

• Pre-blood pump dilution reduces the filtration fraction, which helps extend filter life 1, 4
• The filtration fraction should be maintained below 25% to prevent premature filter clotting 4
• Monitoring hematocrit during CRRT can help estimate changes in blood volume and optimize fluid management 5

Common Pitfalls to Avoid

• Failure to account for pre-blood pump dilution when calculating the actual delivered dose of CRRT 2
• Not adjusting the pre-blood pump rate when using it for specific purposes like managing hyponatremia (where the goal is to limit sodium correction to 4-6 mEq/L per 24 hours) 1
• Overlooking the impact of pre-dilution on solute clearance efficiency, which may require adjustment of the overall prescription 2, 4
• Inadequate monitoring of the actual delivered dose, which often falls short of the prescribed dose 2

Practical Application

• For standard CRRT delivery, aim for an effluent volume of 20-25 mL/kg/h as recommended by KDIGO guidelines 2
• Prescribe a higher dose than the target to account for treatment interruptions and ensure adequate delivered dose 2
• Regularly assess the actual delivered dose to ensure therapeutic adequacy 2
• Consider the patient's hemodynamic status when initiating CRRT, as rapid blood flow rate increases may impact stability in some patients 6