The roll of the vagus nerves in the respiratory response to CO 2 under hyperoxic conditions.

Abstract

In anesthetized rabbits rebreathing oxygen the respiratory and circulatory responses to increased $$P_{A_{{\text{CO}}_{\text{2}} } } $$ were determined repeatedly in the same animal before and after inactivation of the vagus nerves by cold blocking and finally sectioning. In some animals responses were also obtained by blocking the vagi during respiration of constant mixtures of CO2 and O2. With intact vagi the $$\dot V/P_{A_{{\text{CO}}_{\text{2}} } } $$ response curve was, on the average, practically linear up to an increase of the $$P_{A_{{\text{CO}}_{\text{2}} } } $$ by 27 Torr. Inactivation of the vagi caused a small increase of the resting ventilatory minute volume during the initial breathing of pure oxygen, and a corresponding decrease of the resting $$P_{A_{{\text{CO}}_{\text{2}} } } $$ . In the range of small increases of $$P_{A_{{\text{CO}}_{\text{2}} } } $$ the $$\dot V/P_{A_{{\text{CO}}_{\text{2}} } } $$ response curve was significantly steepened and shifted to higher ventilatory values by the inactivation. With greater increases, the slope of the response with inactivated vagi was flatter and shifted to lower ventilatory values as compared with intact vagi. These effects were the result of corresponding changes of the response curves for tidal volume and respiratory rate: Inactivation considerably steepened the $$\dot V_T /P_{A_{{\text{CO}}_{\text{2}} } } $$ response at low values of $$\Delta P_{A_{{\text{CO}}_{\text{2}} } } $$ while flattening the response at high values, whereas the $$f/P_{A_{{\text{CO}}_{\text{2}} } } $$ curve was only depressed at all stages of hypercapnia. The results were principally the same whether the carotid sinus nerves were intact or sectioned. Vagal inactivation had only little influence on the reactions of systemic blood pressure and heart rate to the increase of $$P_{A_{{\text{CO}}_{\text{2}} } } $$ . It is concluded that vagal reflexes, possibly from stretch receptors of the lungs, modify the ventilatory efficacy of the central chemosensitive drive of respiration in the rabbit.