Effect of the Direction of Dialysate Flow on the Efficiency of Continuous Arteriovenous Haemodialysis

Abstract

We have investigated the effect of the direction of the dialysate flow during continuous arteriovenous haemodialysis. Under similar conditions countercurrent flow was more efficient than concurrent flow in terms of both urea clearance, (mean ± SEM), 23.5 ± 0.5 compared to 18.4 ± 0.4 ml/min (p < 0.001) and creatinine clearance, 21.1 ± 0.5 compared to 15.6 ± 0.4 ml/min (p < 0.001). There was a greater drop in pressure along the blood compartment of the haemodiafilter during countercurrent flow, 16 ± 0.8 compared to 13 ± 0.3 mm Hg (p < 0.05) during concurrent flow, and this was associated with a greater ultrafiltration rate, 7.2 ± 0.6 compared to 6.0 ± 0.5 ml/min. The differences in diffusion, back diffusion and convection between the two systems resulted in a net gain of lactate/ bicarbonate and a net loss of chloride during countercurrent dialysate flow, and a net loss of lactate/bicarbonate with a gain of chloride during concurrent flow. These losses would have to be corrected in the clinical setting of patients who had been continuously treated by these systems for several days.

How can dialyzer designs improve solute clearances for hemodialysis patients?

“…Diffusional clearance is greatest when blood and dialysate flow in opposite directions, 9 and depends not only upon relative flows, 10 but also the effective concentration gradient, surface area, resistance to solute passage, distance to travel, and temperature. Blood enters and leaves the dialyzer through the dialyzer header.…”

Pre-Dilution or Post-Dilution Fluid Replacement for Continuous Veno-Venous Hemofiltration: That Is the Question

“…Switching the site of replacement fluid from post-filter to pre-filter, so-called predilutional, reduced the risk of filter clotting, but increased costs [2]. Others added complexity to the original simplistic circuit by adding dialysate [3], with greater clearances achieved with countercurrent flow [4]. The development of both hemofiltration machines and membranes designed for CRRT created a very efficient treatment for acute renal failure in the intensive care setting.…”

Choosing a dialyzer: What clinicians need to know

“…In contrast, dialysate enters and leaves via ports in the main body of the dialyzer. Dialyzer designs can have an impact on clearance, in addition to countercurrent blood flow, increase relative flow, concentration gradient resistance to solute passage, hydrophobicity, and electrical charges …”