Location of flow limitation in liquid-filled rabbit lungs

The effects of end-inspiratory lung volume (EILV) and expiratory flow rate (Q) on the location of flow limitation in liquid-filled lungs were investigated by measuring pressure along the airways and by radiographic imaging. The lungs of New Zealand white rabbits were filled with perfluorocarbon to the randomly selected EILV of 20, 30, or 40 ml/kg, and the volume was actively drained at one of three Q: 2.5, 5.0, or 7.5 ml/s. The minimum pressures recorded by a movable catheter at locations along the airways show that flow limitation occurred in the main bronchi and trachea, and was independent of EILV and Q. The minimum pressure at the trachea was -80 mm Hg compared with values that were more positive than -10 mm Hg at a location 3 cm distal to the carina for all EILV and Q combinations. This location was confirmed by the lung images. The airway diameters gradually decreased with time, until flow limitation occurred. In airways distal to the collapse, there was not a significant decrease in diameter. Based on these data, we conclude that flow limitation in liquid-filled lungs occurs in the trachea and main bronchi and its location is independent of EILV or Q.