Influence of expiratory flow on closing capacity at low expiratory flow rates

Single breath oxygen (SBO₂) tests at expiratory flow rates of 0.2, 0.5, and 1.01/s were performed by 10 normal subjects in a body plethysmograph. Closing capacity (CC) the absolute lung volume at which phase IV began increased significantly with increases in flow. Five subjects were restudied with a 200 ml bolus of 100% N₂ inspired from residual volume after N₂ washout by breathing 100% O₂ and similar results were obtained. An additional five subjects performed SBO₂ tests in the standing, supine and prone positions; closing volume (CV), the lung volume above residual volume at which phase IV began, also increased with increases of expiratory flow. The observed increase in CG with increasing flow did not appear to result from dependent lung regions reaching some critical 'closing volume' at a higher overall lung volume. In normal subjects, the phase IV increase in N₂ concentration may be caused by the asynchronous onset of flow limitation occurring initially in dependent regions.