THEORETICAL BASIS FOR TIME-0 CORRECTION IN THE REBREATHING ANALYSIS

The uptake of a soluble gas during rebreathing is simulated in a compa rtmental model. The lung is assumed to consist of two parallel units, each one divided into three compartments: a personal dead space, an al veolar space, and a tissue volume. These units are connected to the re breathing bag via a common dead space. Gas exchange is incorporated in to the model for a given cardiac output. Inert and soluble gas concent rations are computed as a function of time in the various compartments by means of differential equations. Using different initial condition s, we simulate time-dependent concentration traces ''at the mouth'' an d estimate the errors made by traditional analysis of the end-tidal ga s concentrations with respect to the ''time O'' correction method as w as first proposed by Sackner et al. (Am. Rev. Respir. Dis. 111: 157-16 5, 1975). We provide a theoretical basis for this correction method an d outline the conditions that need to be fulfilled for its application . We show that the tissue volume and cardiac output estimates are less affected by ventilation and perfusion inhomogeneities when the time O correction method is used. This is particularly relevant for the expe cted increase in tissue volume in microgravity where ventilation and p erfusion inhomogeneities are expected to be attenuated.