Determination of plasma volume with indocyanine green in man

We investigated the feasibility of using indocyanine green (ICG) for plasma volume (PV) determination in man. Duplicate PV measurements were carried out in 23 healthy subjects to test repeatability. ICG (0.25 mg/kg) was injected intravenously into one arm and venous blood was withdrawn from the opposite arm. Optical density of plasma samples from minute 3 to 9 was measured in a densitometer. ICG concentration at injection time was determined by monoexponential extrapolation. The mean (SD) difference (MD) was -23 ml (183) or -0.6% (5.7%). Linear regression revealed PV2 = 0.92.PV1 + 226 (r = 0.92). The PV values corresponded well with data from other studies. In 26 surgical patients PV was determined using two methods: 1) the same as in healthy subjects and 2) using a modification of this method in whole blood (PVB). For PVB measurement blood was drawn through a cuvette-densitometer from an arterial line. Calculations were the same as in PV determination except for the use of hematocrit to achieve plasma concentrations of ICG from whole blood. In patients MD were -53 ml (144) or -1.3% (4.3) for PV and -19 ml (161) or -0.3% (5.1) for PVB. Comparing PVB and PV revealed MD = -113 ml (149) or -3.3% (4.2). The whole blood method is easier to perform and reduces blood waste to almost zero. In conclusion, ICG is a suitable tracer for PV determination.