The influence of bronchial smooth muscle tone on critical narrowing of dependent airways.

Critical narrowing of the dependent airways was examined in anaesthetized dogs with closed chests. Two techniques were used, (a) "closing" volume (CV), measured from the expired nitrogen plateau and (b) tantalum bronchography, to measure the calibre of airways of 3-8 mm diameter in the upper (UZ), middle (MZ) and lower (LZ) zones. The point of airway "closure" at the junction of phases III and IV of the alveolar plateau coincided with an inflection in the diameter-lung volume curve of the airways in the most basal zone of the lung. This was accompanied by a sudden large increase in the calculated resistance of the airways in LZ compared with UZ. Following stimulation of the vagi CV increased and there was increased narrowing of airways, particularly in LZ. The addition of 0.5% halothane to the anaesthetic abolished the effect of vagal stimulation on CV and on airway resistance.