Dialysate and blood flow dependence of diffusive solute clearance during CVVHD.

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.