Gas mixing in the lungs of dogs and pigs

We performed single breath washouts in 14 dogs and 4 pigs using a gas mixture containing 5% helium (He) and 5% sulphur hexafluoride (SF6) with, and without, an end inspiratory breathhold. In dogs, the slope of the alveolar plateau was steeper for SF6 than for He, the difference decreasing after 20 sec of breathholding. In contrast, in pigs there was no difference between the two slopes although both were steeper than in dogs. Following a 15-sec breathhold, both slopes decreased, that of the He plateau becoming significantly flatter than that for SF6. Furthermore, the expirate became relatively SF6 enriched. The results indicate that in pigs the mechanism responsible for the sloping alveolar plateau is not diffusion dependent, and hence cannot be due to 'stratification'. Neither do the results support diffusive interaction within an asymmetrical acinus as a basis for the slope. Our findings suggest that the latter is due to sequential emptying of relatively large units subtended by a branch point in which gas transport is entirely convective.