Response to external inspiratory resistive loading and bronchospasm in anesthetized dogs.

Mouth occlusion pressure (P0.1) and breathing-pattern responses to external inspiratory resistive loading and methacholine chloride-induced bronchospasm were assessed in six dogs under pentobarbital sodium anesthesia. There was no change in P0.1 with external loading, but, in response to bronchospasm, we observed a P0.1 increase proportional to the change in lung resistance. These results indicate that, unlike external loading, the ventilatory-drive adaptation to bronchospasm does not require consciousness of the animal. The breathing-pattern response to bronchospasm consisted of tachypnea associated with decreased tidal volume (VT), decreased inspiratory duration (TI), and unchanged mean inspiratory flow (VT/TI). In response to resistive loading there was no tachypnea, VT decreased, TI was unchanged, and VT/TI decreased. We suggest that in response to resistive loading there was no modification of vagal activity, whereas in bronchospasm there was an increase of vagal activity, which was responsible for the changes in breathing pattern and, at least in part, for the changes in P0.1.