Determination of total blood volume by indicator dilution: a comparison of mean transit time and mass conservation principle

Abstract

Objective: Using indocyanine green (ICG), blood volume can be determined within minutes according to the mass conservation principle by back-extrapolation of the concentration/time curve to the time of injection (BV_Tinj) or by the transit time approach (BV_MTT) as the product of cardiac output and mean transit time (MTT) of ICG through the circulation. To see which factor accounts for the difference between the two methods we measured cardiac output and MTT independently and compared the volumes with those obtained by dilution of Evans blue (BV_EB). Design: Prospective animal study. Settings: University department of experimental anaesthesiology. Animals: Six anaesthetised, spontaneously breathing dogs with chronically implanted ultrasound flow probes around the pulmonary artery. Measurements and results: BV_MTT and BV_Tinj agreed closely (48±2 ml·kg^–1 and 49±2 ml·kg^–1), but underestimated blood volume by about 40% compared with BV_EB (75±1 ml·kg^–1). Transit times measured were 33±1 s and should be about 50 s as calculated from the quotient of BV_EB and cardiac output. Conclusions: Both methods underestimate blood volume by about the same extent compared with BV_EB, probably because slowly perfused compartments are not detected during the short measurement period of 4 min. In the case of the transit time approach, rather short transit times result and in the case of the mass conservation principle, back-extrapolation yields rather high plasma concentrations of ICG at the time of injection. Accordingly, the two methods seem to be equivalent for measuring blood volume rapidly, although the absolute volume is underestimated by about 40%.