Measurement of indocyanine green dye concentration in the extracorporeal circulation.

Indocyanine green (ICG) is a dye used to measure plasma volume and cardiac output and to assess hepatic function by monitoring the ICG plasma disappearance rate. A method for continuous and noninvasive measurement of ICG plasma concentration (c) in the extracorporeal circulation is presented using technology originally developed for on-line measurement of hematocrit. As expected from the ICG absorption maximum at 805 nm, the presence of ICG in blood leads to a pronounced increase in hematocrit (Hct) measured by optical means. This apparent change in hematocrit (Hct(rel) = Hct/ Hct(b) - 1) relative to the baseline hematocrit (Hct(b), where c = 0) showed a strong linear relationship (Hct(rel) = k.c) with respect to ICG concentration. The dependence of k on baseline hematocrit was best described by an exponential relationship (k = ae(bHctb)). The constants a and b were determined in in vitro experiments using bovine blood. With the information on a and b, and when baseline hematocrit is known and can be assumed to remain constant throughout the observation phase, plasma ICG concentration can be noninvasively and continuously measured in the extracorporeal circulation by using available optical techniques without taking blood samples. This approach greatly simplifies the diagnostic use of ICG, which may be of importance in patients treated with extracorporeal blood purification techniques.