[Determination of plasma volume with indocyanine green]

In 31 anesthetized dogs, the distribution volume (Vd) of indocyanine green (ICG, Cardiogreen) was compared to that of Evans blue (EB), which is considered to represent plasma volume. Vd was computed from the injected amount of indicator and the indicator concentration in plasma at time zero, which was obtained by extrapolation of the monoexponential decay of the indicator concentration. The decay was assessed by eight plasma samples from 10-60 min and from 5-12 min after injection of EB and ICG, respectively. EB was measured in a spectrophotometer at 640 and 720 nm; ICG was measured in a spectrophotometer at 805 nm or in a cuvette densitometer with two light sources (LEDs) of 800 and 900 nm. The decay of EB had a half-life of about 330 min, corresponding to a disappearance rate of about 13%/h; the half life of ICG was about 8.0 min, equivalent to a fractional disappearance of about 9%/min. Vd (EB) was 55 +/- 11 (n = 31) ml.kg-1 (mean +/- SD), Vd (ICG) was 55 +/- 12 (n = 20) and 56 +/- 10 (n = 20) ml.kg-1 when ICG was measured in the densitometer or spectrophotometer, respectively. The ICG clearance was about 5 ml.min-1 per kilogram. The accuracy of the determination of plasma volume by ICG was tested in four experiments in two splenectomized dogs. Changes in plasma volume by volume load, enforced diuresis, hemorrhage, and reinfusion were determined by ICG and compared to the theoretical changes calculated from the hematocrit and removed volumes. The mean difference between measured and calculated values was 1.3 +/- 3.4% (n = 14).(ABSTRACT TRUNCATED AT 250 WORDS)