CHANGES IN LUNG-VOLUME, LUNG DENSITY, AND DISTRIBUTION OF VENTILATION DURING HYPOBARIC DECOMPRESSION

The authors studied in effects of decompression on lung volume, the distribution of inspired gas, lung blood volume, and lung mass, as estimated from lung volume and density, in four adult subjects. After control measurements at 90 m, the subjects were decompressed to 4,268 m over 28 min in a hypobaric chamber and the measurements were repeated at intervals during the following 24 hr. Significant increases in total lung capacity (TLC, 21%), 'closing capacity' (CC, 60%), and residual volume (RV, 78%) were found at 5 hr of decompression. The increases in CC and RV were sustained at 20 hr but TLC had returned toward control values and this was associated with a significant (10%, P < 0.05) fall in vital capacity. The slope of the alveolar plateau in the single-breath nitrogen washout test increased progressively (by 36%, P < 0.05 at 20 hr) throughout the period of decompression. Estimated lung mass was a mean 1.3 kg before decompression and 2.0 kg at 10 hr (P < 0.05). Since estimated lung mass increased while lung blood fluid volume remained constant, it is concluded that lung interstitial fluid volume had increased during decompression. These results confirm a previous report indicating increases in lung distensibility during acute hypoxia and strongly support the hypothesis that subclinical pulmonary edema occurs after rapid ascent to high altitude.