Effect of posture on lung volume: airway closure and gas exchange in hemidiaphragmatic paralysis.

The effects of posture on lung volume, airway closure, and gas exchange were studied in eight patients with hemidiaphragmatic paralysis. The mean vital capacity in the sitting position was 81% of predicted normal, and in the supine posture fell by a further 19% in right-sided but only 10% in left-sided paralysis. The mean arterial oxygen tension was less than predicted in the sitting posture and fell significantly on lying. Single breath gas transfer factor was normal in all cases whereas the diffusion coefficient was greater than predicted in the sitting posture and rose even further on lying. Closing volume showed no positional change but closing volume as a percentage of vital capacity was higher in the supine position. Regional airways closure was expressed as the relationship expiratory reserve volume minus closing volume. Negative values were found in only two of the subjects in the sitting position but seven had negative values supine, indicating significant airway closure during tidal breathing in this position.