Changes in lung volume, lung density, and distribution of ventilation during hypobaric decompression

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Abstract
The effects of decompression on lung volume, the distribution of inspired gas, lung blood volume and lung mass, as estimated from lung volume and density, were studied in 4 adult subjects. After control measurements at 90 m, the subjects were decompressed to 4268 m over 28 min in a hypobaric chamber and the measurements were repeated at intervals during the following 24 h. Significant increases in total lung capacity (TLC, 21%), closing capacity (CC, 60%) and residual volume (RV, 78%) were found at 5 h of decompression. The increases in CC and RV were sustained at 20 h 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 N washout test increased progressively (by 36%, P < 0.05 at 20 h) throughout the period of decompression. Estimated lung mass was a mean 1.3 kg before decompression and 2.0 kg at 10 h (P < 0.05). Since estimated lung mass increased while lung blood volume remained constant, lung interstitial fluid volume had apparently increased during decompression. A previous report indicating increases in lung distensibility during acute hypoxia was confirmed and subclinical pulmonary edema possibly occurs after rapid ascent to high altitude.