Airway compliance and flow limitation during forced expiration in dogs

Abstract

Bronchial pressure measured by means of a Pitot static probe, esophageal pressure and airflow were monitored during forced lung deflations in 6 anesthetized dogs. Dynamic transmural pressure-cross-sectional area curves (Ptm-A curves) were constructed for 3 intrathoracic tracheal positions and 1 right lower lobal bronchial position. From the Ptm-A curves, the maximal possible flow (.ovrhdot.Vmax) through the airways at each of the 4 positions was calculated and compared to the overall maximal flow obtained during the same deflation. The peak of the maximal expiratory flow-volume curve (MEFV curve) equaled the calculated .ovrhdot.Vmax at > 1 position in the trachea, but did not reach the .ovrhdot.Vmax calculated for the more peripheral position. During the transition between the peak and the plateau of the MEFV curve, the Ptm-A curve often changed shape, indicating an abrupt change in the tube law, probably due to changes in axial tension of the airway. During the flow-volume curve plateau, measured flow was near an estimated .ovrhdot.Vmax at a single point in the trachea. At lower lung volumes, where the MEFV curve descends from the plateau, measured flow equaled .ovrhdot.Vmax calculated for the right lower lobe position. This indicates that after an initial period with no localized choking, a choke point develops and eventually moves toward the periphery. Apparently, measurement of dynamic Ptm-A curves allows a precise prediction of maximal expiratory flows from the properties of the airways.