Continuous measurement of blood volume during hemodialysis by an optical method.

A new method is described to noninvasively and continuously measure changes in blood volume (BV) during hemodialysis by means of an optical reflection method with an optical monitor (950 nm) clipped onto the arterial blood line. The amount of reflected light (L) appeared to be linearly proportional to the erythrocyte concentration (r = 0.91). Changes in L correlated well with changes in erythrocyte concentration during hemodialysis (r = 0.94). A study in 10 patients on regular dialysis was done. The BV decrease after 3 hr of treatment was 17.0 +/- 5.2%, and it correlated with the amount of fluid withdrawn by ultrafiltration (mean, 2,519 +/- 589 ml). Five hypotensive episodes were seen that were characterized by a higher rate of BV fall during the preceding 15 min (9.9 +/- 1.9 versus 3.6 +/- 4.3%/hrp; p < 0.05) and by a lower BV value at that moment (78.2 +/- 3.4 versus 84.5 +/- 4.5%; p < 0.025) than in the other five patients at comparable times. It was concluded that this optical method was a means to detect hypovolemia at an early stage and to prevent ultrafiltration induced hypotension.