Blood-dialysate equilibration during continuous arteriovenous hemodialysis.

In order to determine the degree to which blood and dialysate achieve equilibration of urea concentration during continuous arteriovenous hemodialysis (CAVHD), CAVHD circuits employing the Hospal AN69S dialyzer were perfused with bovine blood (85 to 90 ml/min, hematocrit 28%) containing 50 mg/dl of urea in the presence of countercurrent dialysate flowing at 10, 20, or 30 ml/min. The circuits were constructed and perfusion pressure (66 mmHg) set to yield a near zero net ultrafiltration rate. Urea clearances were measured and found to approximate the dialysate inflow rate, Qd, up to about 15 ml/min. At higher dialysate flow rates, equilibration was incomplete so that urea clearance was 24.7 +/- 0.5 (mean +/- SE) ml/min when Qd was 30 ml/min. A simplified model of urea transport in the dialyzer successfully simulated the data.