Calculating Prismaflex CRRT in Hemodialysis Modality (CVVHD)
For Prismaflex CVVHD (continuous venovenous hemodialysis), the dialysate flow rate directly determines small solute clearance, following the formula: Clearance (K) = Dialysate flow rate (Qd) ÷ 60, where Qd is in mL/hour and clearance is in mL/min. 1
Blood Flow Rate Settings
- Set blood flow (Qb) at 100-200 mL/min for standard CVVHD on the Prismaflex system 2
- Blood flow rates of 120 mL/min are commonly used in clinical practice with the Prismaflex device 3
- For pediatric or low-body-weight patients, blood flow can be reduced to 30-50 mL/min while maintaining efficacy 4
Dialysate Flow Rate Prescription
- Target dialysate flow rate of 1,000-2,000 mL/hour (16.7-33.3 mL/min clearance) for standard CVVHD 2, 1
- The Prismaflex with standard circuits limits dialysate flow to approximately 1,000 mL/hour maximum 4
- To achieve the recommended effluent dose of 20-25 mL/kg/hour, calculate: Qd (mL/hour) = patient weight (kg) × 20-25 5
Calculating Delivered Dose
- For CVVHD, the delivered dose equals the dialysate flow rate since there is minimal ultrafiltration beyond net fluid removal 2
- Example: For a 70 kg patient requiring 25 mL/kg/hour dose:
- Required Qd = 70 kg × 25 mL/kg/hour = 1,750 mL/hour
- This provides clearance of 1,750 ÷ 60 = 29.2 mL/min 1
Small Solute Clearance Prediction
- Clearance of urea nitrogen, creatinine, and uric acid during CVVHD closely approximates the dialysate flow rate divided by 60 1
- The formula Kd = Qd/60 has been validated with very significant correlation (P < 0.001) between calculated and observed clearances 1
- Phosphate clearance may be slightly less predictable than other small solutes during CVVHD 1
Ultrafiltration Rate Calculation
- Net ultrafiltration rate = Patient fluid removal goal (mL/hour) separate from dialysate flow 2
- Set the Prismaflex to achieve desired hourly fluid removal based on patient volume status
- Total effluent = Dialysate flow + Net ultrafiltration rate 2
Key Operational Parameters
- Use PrismOcal or equivalent bicarbonate-based dialysate at the prescribed flow rate 6, 3
- Membrane choice: AN69ST (Prisma Flex ST100) or M150 filters are standard options 3, 7
- Anticoagulation: Regional citrate is preferred if no contraindications exist 6, 5
Critical Pitfall to Avoid
- Do not confuse CVVHD with CVVH (hemofiltration): In CVVH, clearance depends on ultrafiltration rate and blood flow following K(UF) = (Q(UF)/60) × Q(B)/(Q(B) + Q(UF)/60), whereas in CVVHD, clearance is determined solely by dialysate flow rate 1
- CVVHD is more efficient than CVVH for removing small molecular weight solutes but less efficient for larger molecules like beta-2-microglobulin 1