Isovolume bronchoconstriction by vagal stimulation in dogs: effects of lung inflation pressure

The changes elicited in lung mechanics by a given stimulus to the airway smooth muscle depend significantly on end-expiratory lung volume. However, the precise quantitative relationship between volume (or inflation pressure) and airway responsiveness remains to be elucidated. We measured the changes in lung elastic recoil pressure and impedance at 1 and 8 Hz, produced in anesthetized, paralyzed, open-chest dogs over periods of 32 sec, when the vagus nerves were continuously electrically stimulated at constant lung volume. The increases in lung elastic recoil pressure increased with PEEP, which we interpret as being due to parenchymal distortions produced by the contracting airways acting against parenchymal attachment forces that increase with lung volume. In contrast, the increases in lung resistance at 1 and 8 Hz and elastance at 1 Hz all decreased by several-fold as PEEP was increased, which we interpret as reflecting the decreased airway smooth muscle shortening that was achieved with increasing parenchymal load.