Variation of intra-access flow early and late into hemodialysis

Intra-access flow (Qac), measured by ultrasound dilution, is a reliable method for screening for access dysfunction. Because of a reduced circulating volume and a relative decrease in blood pressure at the end of hemodialysis (HD), we hypothesized that Qac could be significantly reduced when measured late into HD. Fifty patients were prospectively evaluated for variation in Qac early and late into HD. There were 33 native fistulae and 17 synthetic grafts. Six separate measures of Qac were performed for each patient by ultrasound dilution (Transonic HD01 hemodialysis monitor; Transonic Systems, Inc., Ithaca, NY): three within the first 30 minutes and three within the last 30 minutes of HD. Session time was 3.5 or 4 hours, and mean net ultrafiltration was 3.3 +/- 0.9 L/HD. Early and late into HD, mean arterial pressure (MAP) decreased from 100.0 +/- 14.6 to 92.8 +/- 17.8 mm Hg, heart rate from 73 +/- 11 to 79 +/- 15 bpm, and hematocrit increased from 34 +/- 3 to 38 +/- 4%. For the whole group, mean Qac decreased from 1,101.7 +/- 566.7 to 972.5 +/- 515.6 ml/min (p = NS); when Qac was corrected for a MAP of 100 mm Hg, the reduction remained nonsignificant (from 1,101.7 to 1,048.0 ml/min). When considering native and synthetic fistulae separately, the drop in Qac was still nonsignificant (from 1,098.9 +/- 613.4 to 983.2 +/- 593.2 for native fistulae versus 1,107.2 +/- 480.5 to 999.8 +/- 379.8 ml/min for grafts, p = NS). Overall, the percent reduction in Qac early versus late into HD was 11.7%, whereas it reached only 4.9% when access flows corrected for MAP were considered. We conclude that variation in Qac during HD is relatively small, especially when values are corrected for MAP. Therefore, according to our results, Qac measures by using the ultrasound dilution method made at any time during HD should be reliable for most patients.