Access flow measurement by indicator dilution without indicator injection: effect of switch location

It is well known that the measurement of access flow by one of the various dilution techniques requires the reversal of blood flow drawn from and returned to the peripheral vascular access. But it was only recently recognized that the line switch itself constitutes a dilution experiment for certain blood and dialysate components and properties, so that a subsequent injection of indicator is no longer required. New switches introduced at different locations in the extracorporeal circulation not only simplify manual operation for standard access flow measurement but also provide an essential tool for the new technique, which is based on continuously measuring certain blood and/or dialysate characteristics and their changes caused by switching the bloodlines. In this study, the effects of switching the bloodlines at two different locations were studied when extracorporeal temperatures were used as a marker. The study shows that the temperature changes depend on the location of the switch relative to the extracorporeal temperature sensors, and that different algorithms to calculate access flow have to be used for the two possible switching positions to account for this dependence.