Relation between pulmonary gas exchange and closing volume before and after substantial weight loss in obese subjects.

Relations between expiratory reserve volume (ERV), closing volume (CV) as a percentage of vital capacity (CV/VC%), and pulmonary gas exchange have been investigated in the sitting and supine postures in eight severely obese subjects before and after substantial weight loss. CV/VC% exceeded predicted values but did not change with posture or weight loss; the increase could have been owing to the smoking habits of the subjects. Arterial oxygen pressure (PaO(2)) was significantly less in the supine than in the sitting posture and improved after weight loss in six subjects. The reduction in PaO(2) was correlated with the extent to which dependent airways were closed within the range of tidal breathing, shown by increasing negativity of ERV-CV as a percentage of VC. A good correlation was also found between PaO(2) and ERV, an easier measurement for routine use. Improvement of pulmonary gas exchange occurred only in those patients who lost weight to within 30% in excess of ideal.