Multiple-breath washout and washin experiments in steers

Multiple-breath N-2 washouts (WO) and washins (WI) were performed during regular tidal breathing in 11 unsedated healthy steers approaching pulmonary functional maturity (mean body weight=271 kg). They inspired 20% O-2 in 80% Ar during the WO and air during the WI. For each steer, we computed two indexes of ventilation inhomogeneity from the N-2 WO curves: 1) the curvilinearity of the logarithm of end-tidal N-2 concentrations as a function of cumulative expired volume reflected in the ratio of two slopes fitted between 100 and 50% and between 50 and 10%, respectively, of end-tidal N-2 concentration of the first breath of the WO; and 2) the N-2 phase III slope divided by the mean expired concentration (S-n) of each breath also plotted as a function of cumulative expired volume. Equivalent computation of both parameters was done on WI and WO curves, and similar results were obtained. The mean slope ratio was 0.812+/-0.119 (SD) for all the steers, which is consistent with topographic gravity-dependent specific ventilation distribution inhomogeneity. S-n was independent of the breath number both for WO and WI (mean S-n=0.130+/-0.057 liters(-1)), suggesting that emptying between unequally ventilated units is synchronous. This behavior resembles that observed in rats postmortem (S. Verbanck, E. R. Weibel, and M. Paiva. J. Appl. Physiol. 71: 847-854, 1991) but contrasts with experiments in humans, in whom convection-dependent ventilation inhomogeneities generate a marked increase in S, throughout the entire WO (A. B. H. Crawford, M. Makowska, M. Paiva, and L. A. Engel. J. Appl. Physiol. 59: 838-846, 1985). This is surprising because one would expect gravity-dependent sequential emptying in animals of this size.