In vivo measurement of hemodialyzer fiber bundle volume: Theory and validation

Abstract

Fiber bundle volume (FBV), the space within the blood compartment of hollow fiber dialyzers, may decrease during treatment due to clotting. The clots may be flushed out of the dialyzer prior to measurements of FBV by dialyzer reprocessing equipment and a significant drop in FBV during the session may go unrecognized. FBV was measured (1) from the transit time of a saline bolus passing through the dialyzer as recorded by ultrasound dilution sensors placed on the arterial and venous blood lines; (2) from the change in blood concentration induced by a step change in the rate of ultrafiltration as recorded by the venous sensor. In vitro FBV ranged from 47 to 121 ml. Paired absolute differences between the ultrasound and volumetric measurements (flushing saline out of the dialyzer into a graduated cylinder) were 0.16 +/- 4.23% (N = 42) and 2.10 +/- 7.26% (N = 13) for the bolus and ultrafiltration methods, respectively. In vivo reproducibility of the bolus and ultrafiltration methods were 2.65 +/- 2.11% (N = 122) and 3.79 +/- 3.93% (N = 32), respectively. During 31 treatments the FBV by dilution showed an average decrease of 4.17 +/- 8.60%, and in 6 cases FBV fell more than 10%, while measurements of the same FBV by reuse equipment showed an increase of 0.99 +/- 5.82%, P < 0.01. FBV measured by the dilution methods was accurate and reproducible. Preliminary results suggest that in vivo FBV may differ significantly from results reported by reprocessing machines.