Effect of tidal volume on ventilation maldistribution

To examine the effect of changing tidal volume (VT) on ventilation distribution, we studied multiple breath nitrogen washouts in 4 normal subjects breathing with a VT of 0.6, 1.0 or 1.5 L. We used a recently developed technique of analysis (Crawford et al., 1985) that distinguishes inhomogeneity of gas concentrations due to the interaction of convection and diffusion in the lung periphery (DCDI) from ventilation maldistribution among larger units determined at more proximal branchpoints (CDI). The results indicate that increases in VT reduce the inhomogeneity due to DCDI but increase that due to CDI. In view of the VT dependence of physiological dead space, derived from arterial PCO2, we speculate that the increase in CDI has a dominant effect on gas exchange.