Breathing muscle activity during expiration in patients with chronic airflow obstruction.

During quiet ventilation in 10 patients with severe chronic airflow obstruction (AFO) there were large tidal swings of pleural pressure, 15.8 +/- 5.1 cmH2O, with high negative pressures achieved, 15.2 +/- 5.2 cmH2O. The pattern of pleural pressure mirrored that of tidal expiratory flow with a rapid rise to maximum of pleural pressure and flow at the beginning of expiration, a slow decline of both throughout most of expiration, and just before the onset of inspiration at flow reversal, a sharp fall in pleural pressure and expiratory flow. The shape of expiratory flow and pleural pressure tracings and the pattern of EMG recordings are compatible with loss of post-inspiratory muscle braking of flow. The generally negative pleural pressure and the EMG silence during expiration indicated relaxation of expiratory muscles throughout expiration. Extrapolation of the tidal expiratory flow curve to zero, and calculation of the area under the extrapolated curve showed the volume of dynamic hyperinflation to be a small proportion of the total increase in functional residual capacity above the predicted value in these patients.