Brain hypoxia and control of breathing: Role of the vagi

Vagally mediated reflexes play an important role in the generation of respiratory responses to various stimuli. This study examined the role of vagally mediated mechanisms in the generation of the respiratory responses to progressive brain hypoxia secondary to carboxyhemoglobinemia (HbCO 0-55%) in six unanesthetized goats. Ventilation, respiratory cycle timing, and the lung inflation reflex were measured before and during CO inhalation in intact and bilaterally vagotomized animals. Our results indicate that vagal reflexes contribute a small magnitude of the hyperpnea caused by carboxyhemoglobinemia. Furthermore, in contrast to that reported for CO₂ inhalation, the tachypneic nature of the ventilatory response to CO is not a vagally mediated phenomenon. CO inhalation had a biphasic influence on the strength of the lung inflation reflex measured as the ratio of inspiratory time during occlusion (TI(occl)) to inspiratory time of the preceding spontaneous breath (TI(spont)). At HbCO levels of 35%, TI(occl)/TI(spont) was enhanced, whereas at HbCO levels of 55% the ratio fell to unity, indicating abolition of the reflex. After vagotomy, this ratio was unity at all levels of carboxyhemoglobinemia.