Multiple-breath washout experiments in rat lungs.

Multiple-breath washouts were performed on 30 Wistar rats postmortem in a study in which breaths of 90% O2-5% He-5% SF6 were given. Preliminary comparison of alveolar plateau slopes obtained from anesthetized rats in vivo and postmortem showed that ventilation distribution remains the same within 1 h after the animals were killed. For maneuvers with different preinspiratory lung volumes and end-inspiratory breathholding, we computed the normalized N2 slope (Sn) and Fowler and Bohr dead spaces [VDF(n) and VDB(n), respectively] as a function of breath number (n). For all maneuvers analyzed, Sn of all gases increased in the first two or three breaths and reached a horizontal asymptote thereafter. The value of Sn decreased, both with increasing preinspiratory lung volume and breath hold of 4 s. The fact that the horizontal Sn asymptote is reached after only two or three breaths suggests the absence of convection-dependent inhomogeneities (CDI) in rat lungs. This contrasts with multiple-breath washout experiments in humans, where interregional (gravity-dependent CDI) and intraregional CDI generate a marked increase in Sn throughout the entire washout. Also, in contrast with results in humans, VDF and VDB were independent of n. The present work suggests that rats may be used to study diffusion- and convection-dependent inhomogeneities without the influence of CDI or gas exchange.