This project will study several aspects of low tidal volume (20-100 ml) high frequency (4-40 Hz) ventilation (HFV). Ethane and helium washout from excised dog lungs will be compared to obtain direct evidence supporting or rejecting the augmented diffusion hypothesis as an important transport mechanism during HFV. The regional distribution of tracer washout rates will be compared to the regional distribution of alveolar pressure fluctuations to determine if convective transport processes are significant during HFV. The above studies will be done in normal excised dog lungs, a nitric acid injury model of small airways disease, and a radiation injury model of interstitial fibrosis. The studies in disease models will provide data on the effect of abnormal pulmonary mechanics on gas transport during HFV. The nitric acid injury model will increase the transport resistance in the small airways. Raising the transport resistance of the path where flow is laminar will enhance any density dependent transport due to augmented diffusion that exists. Both disease models will provide a unilateral injury, creating greater inhomogeneity of lung mechanical properties than exists in normal lungs. If pendelluft is an important ventilatory process during HFV, one might see greater ventilation efficiencies in these unilateral injury models than is present in normal lungs. Conversely, convective processes other than pendelluft predict a progressively smaller proportion of total ventilation going to the abnormal lung regions as the regional mechanics become more disparate.