Dialysate and Blood Flow Dependence of Diffusive Solute Clearance During CVVHD

Abstract

To define the flow dependence of diffusive solute clearance (K_d) 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 K_d, clearances were measured at constant blood flow (Q_B), while dialysate inflow (Q_Di) was increased incrementally from 8.3 to 33.3 ml/ min. K_d increased linearly for all three solutes using all membranes. To assess its blood flow dependence, K_d was measured at blood flows of 50, 75, 100, and 150 ml/min, while Q_Di was held constant at 33.3 ml/min. The PA membrane showed no flow dependence; for the other membranes, K_d was flow independent for Q_B > 100 ml/min. At all values of Q_B and Q_Di, 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 Q_Di 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 Q_B 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.