On Defining the Pulmonary Extravascular Water Volume

To evaluate the effect of diffusion on the pulmonary extravascular water volume determined from indicator dilution studies, simultaneous comparisons of the pulmonary extravascular distribution volumes of tritiated water (THO) and of three lipid soluble substances with higher molecular weights were made. In 21 studies on pentobarbital-anesthetized dogs, the distribution volume of antipyrine (AP) was slightly smaller than that of THO (mean difference=2.86 ml±1.49 SE; 0.1< P <0.05). The results of 21 studies in conscious humans showed a slightly smaller distribution volume for 131 I-iodoantipyrine (IAP) than for THO (mean difference=15.62 ml±3.36 se; P >0.01). Neither the differences between THO and AP in dogs nor those between IAP and THO in humans correlated with either flow (τ=0.25 for dog studies; τ=0.07 for human studies) or the mean transit time of the intravascular indicator (τ=-0.17 for dog studies; τ=0.14 for human studies). No significant difference between the distribution volume of 14 C-ethanol(ETOH) and THO could be shown in either dogs (12 studies) or humans (15 studies). Relative lung-to-blood partitioning of IAP as compared to THO at equilibrium in dogs was 0.92±0.08; the same value for ETOH was 1.18±0.10. The findings strongly suggest that distribution volume for these indicators is independent of their diffusion characteristics and that IAP may be a reasonable substitute for THO in measuring pulmonary extravascular water volume.