Measurement of blood access flow rate during hemodialysis from conductivity dialysance

An on line clearance monitor automated for measurements of conductivity dia lysance (Dcn) was used to measure blood access flow rate (cnQac) during hem odialysis. From mathematical analysis of transport, it was shown that cnQac e = [(Dcn*Decn)/(Dcn - Decn)] [1/bwf], where Dcn is measured with standard cocurrent flow of dialyzer blood (Qb) and Qac; Decn is measured with counte rcurrent Qb/Qac; and bwf is fractional blood water content. An identical eq uation was derived to measure Qac from urea dialysance (uQac, Du, Deu). In vitro studies showed excellent correlation between volumetric measurement o f Qac (vQac) and cnQac, r = 0.98, n = 29, and between uQac and cnQac, r = 0 .97, n = 28. In vivo studies showed comparable agreement between uQac and c nQacc, r = 0.97, n = 14. In two of the patients studied, there was unsuspec ted severe midgraft stenosis (no recirculation in cocurrent flow) with a Qa c of 89 and 202 ml/min disclosed by both cnQac and uQac measurements. Mathe matical analysis also showed that when Qb is greater than Qac and there is recirculation in cocurrent flow, the above equations always return the valu e Qac = Qb. An equation was derived to calculate cnQac without reversal of blood lines in this case, using Dcn calculated from the dialyzer transport coefficient and flow rates.