Theoretical basis for time 0 correction in the rebreathing analysis

The uptake of a soluble gas during rebreathing is simulated in a compartmental model. The lung is assumed to consist of two parallel units, each one divided into three compartments: a personal dead space, an alveolar space, and a tissue volume. These units are connected to the rebreathing bag via a common dead space. Gas exchange is incorporated into the model for a given cardiac output. Inert and soluble gas concentrations are computed as a function of time in the various compartments by means of differential equations. Using different initial conditions, we simulate time-dependent concentration traces 'at the mouth' and estimate the errors made by traditional analysis of the end-tidal gas concentrations with respect to the 'time 0' correction method as was first proposed by Sackner et al. (Am. Rev. Respir. Dis. 111: 157-165, 1975). We provide a theoretical basis for this correction method and 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 0 correction method is used. This is particularly relevant for the expected increase in tissue volume in microgravity where ventilation and perfusion inhomogeneities are expected to be attenuated.SCOPUS: ar.j