Timing and reproducibility of access flow measurements using extracorporeal temperature gradients

The duration, accuracy, and reproducibility of a new access flow measuring technique was analyzed in a series of in vitro experiments using an extracorporeal line switch that allowed for almost instantaneous reversal of extracorporeal blood flow without disconnecting the blood lines. Access flow was modeled from the magnitude and time course of extracorporeal temperature changes caused by switching the blood lines. Ten tests were done with access flows covering a range from 410 to 1500 ml min. The coefficient of variation of triplicate access flow identifications was 3.8 +/- 1.5%. The mean bias between measured and modeled access flows was 54 +/- 54 ml min and independent of the range of measured access flows. The average time constant for temperatures to stabilize after switching the blood lines was 0.68 +/- 0.11 min. These results show that the instantaneous change in the direction of blood flow in proximal parts of the extracorporeal circulation produces a smooth change in extracorporeal temperatures that can be explained by a mathematical model incorporating access flow and that a reproducible measure for access blood flow can be obtained as one of the model parameters from that fit within a few minutes of switching the blood lines without the injection of indicator.