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Dialysate and blood flow dependence of diffusive solute clearance during CVVHD
S Relton1, A Greenberg, P M Palevsky
1Department of Medicine, University of Pittsburgh School of Medicine, Pennsylvania.
Insights
Small molecule clearance in continuous venovenous hemodiafiltration (CVVHD) is limited by dialysate flow. Increasing blood flow above 100 ml/min offers no additional benefit, and polyamide membranes are unsuitable for CVVHD.
Area of Science:
- Nephrology
- Renal Replacement Therapy
- Biomedical Engineering
Background:
- Continuous venovenous hemodiafiltration (CVVHD) is a critical renal replacement therapy.
- Understanding solute clearance (Kd) flow dependence is vital for optimizing CVVHD efficacy.
- Hollow fiber hemofilters (AN69, PAN, PA) are commonly used in CVVHD.
Purpose of the Study:
- To define the flow dependence of diffusive solute clearance (Kd) during CVVHD.
- To evaluate the impact of dialysate flow rate (QDi) and blood flow rate (QB) on Kd.
- To compare the performance of AN69, PAN, and PA hemofilters.
Main Methods:
- Measured urea, creatinine, and phosphate clearance during CVVHD with minimized ultrafiltration.
- Tested AN69, PAN, and PA hollow fiber hemofilters.
- Varied dialysate inflow (QDi) from 8.3 to 33.3 ml/min at constant QB.
- Assessed blood flow dependence by varying QB (50-150 ml/min) at constant QDi (33.3 ml/min).
Main Results:
- Diffusive solute clearance (Kd) increased linearly with dialysate flow for all solutes and membranes.
- For AN69 and PAN membranes, Kd became flow independent at QB > 100 ml/min.
- Polyamide (PA) membranes exhibited significantly lower solute clearance compared to AN69 and PAN (p < 0.01).
- Lactate uptake increased with QDi without causing blood lactate accumulation.
Conclusions:
- Small molecule clearance during CVVHD is primarily dialysate flow-limited.
- Increasing blood flow beyond 100 ml/min does not enhance solute clearance.
- The PA membrane demonstrates poor performance and is unsuitable for CVVHD due to low diffusive solute clearance.
Abstract:
To define the flow dependence of diffusive solute clearance (Kd) during continuous venovenous hemodiafiltration (CVVHD), urea, creatinine, and phosphate clearance were measured during CVVHD, while ultrafiltration was minimized. Studies were performed using AN69, polyacrylonitrile (PAN), and polyamide (PA) hollow fiber hemofilters. To assess the dialysate flow dependence of Kd, clearances were measured at constant blood flow (QB), while dialysate inflow (QDi) was increased incrementally from 8.3 to 33.3 ml/min. Kd increased linearly for all three solutes using all membranes. To assess its blood flow dependence, Kd was measured at blood flows of 50, 75, 100, and 150 ml/min, while QDi was held constant at 33.3 ml/min. The PA membrane showed no flow dependence; for the other membranes, Kd was flow independent for QB > 100 ml/min. At all values of QB and QDi, clearances for the AN69 and PAN membranes were two to three times that of the PA membrane (p < 0.01). Lactate uptake increased with increasing QDi for all three membranes, but did not result in blood lactate accumulation. It is concluded that: 1) small molecule clearance is dialysate flow limited during CVVHD; 2) increasing QB beyond 100 ml/min is unnecessary for maintaining solute clearance; and 3) the PA membrane is unsuitable for CVVHD because of its low diffusive solute clearance.