Effect of vagal tone on airway diameters and on lung volume in anesthetized dogs.

In 18 open-chest dogs we obtained pressure-diameter (P-D) curves from tantalum bronchograms and pressure-volume (P-V) curves by plethysmography. After vagotomy most of the decrease in diameter with decreasing transpulmonary pressure (Ptp) occured below 10 cmH2O and there was no P-D hysteresis. Smaller airways narrowed more with decreasing Ptp than larger ones. Bronchodilators did not increase diameters after vagotomy (P less than 0.2). With vagi intact, diameters were smaller at all Ptp (P less than 0.01) and exhibited hysteresis, but the lung P-V curve was unchanged. Vagal stimulation narrowed airways further at all Ptp and hysteresis was marked. Smaller airways narrowed more with vagal stimulation than larger ones. Vagal stimulation did not change the deflation limb of the P-V curve but decreased inflation volumes slightly at all Ptp (P less than 0.01). We conclude that in vivo tone is vagal and that it affects the physical properties of airways, but not of lungs, making the airways remarkably independent from lung parenchyma.