Lung volumes and closing capacity with continuous positive airway pressure.

Total lung capacity, vital capacity, residual volume, and functional residual capacity were determined by body plethysmography and the single-breath oxygen (SBO2) test was performed at 0, 5, and 11 cm H20 continuous positive airway pressure in healthy, awake, seated, spontaneously breathing subjects. Mean values for the absolute lung volume at which phase IV of the SBO2 test begins (closing capacity) did not change significantly with continous positive airway pressure at 5 or 11 cm H2O. Mean total lung capacity, functional residual capacity, and residual volume increased significantly, and the mean closing volume, the lung volume above residual volume at which phase IV begins, decreased significantly with 11 cm H20 continuous positive airway pressure; differences at 5 cm H20 were not significant. The slope of the alveolar nitrogen plateau (phase III) obtained during the SBO-2 test did not change with continuous positive airway pressure.