Determinants of expiratory airflow in patients with chronic airways obstruction.

This paper examines the relationship between the intrathoracic and bronchial pressures and the flow time course during forceful expiration in patients with severe chronic airways obstruction and tracheobronchial collapse. After an initial peak flow, the flow rate (V) of forceful expirations usually fitted the patient's maximum expiratory flow volume (MEFV) curve. The great reduction in flow after the peak corresponded to a large reduction of the thoracic gas volume (TGV) due to the high intrathoracic pressures compressing the lungs. An occasional slight rise in flow in the middle of the slow phase corresponded to an increase of the TGV due to reduction of the intrathoracic pressure. The early peak flow was found to exceed considerably the ˙V expected from the MEFV curve. Pressure measurements in the right lower lobe bronchus suggest that this could be due to at least two factors: first, a rapid reduction in bronchial volume and, secondly, a short time lag before flow limitation develops, allowing initial rapid transit of air. During a forced expiration, flow limitation was initiated when the equal pressure points were sufficiently upstream to allow compression of small as well as large airways. The evidence suggested that tracheobronchial collapse followed the initiation of flow limitation and was unlikely to have had more than a slight subsidiary effect on the maximum flow. Due to greater compression, the TGV and hence the flow rate were smaller during a forceful expiration than during a less forceful expiration.