Influence of hypoxia on ventilatory responses to intralaryngeal CO 2 in cats

Abstract

In decerebrate, vagotomized cats, induction of CO 2 into the isolated laryngeal airway while systemic P CO2 is held constant evokes dose-related reflex changes in ventilatory activity. Because systemic hypoxia is known to exaggerate ventilatory responses to other types of laryngeal chemo- stimulation in neonates, we have compared the responses of phrenic and hypoglossal nerve activities to ventilation of the larynx with 10% CO 2 during systemic hyperoxia (Fi o 2 = 1.00) to those during hypoxia (Fi o 2 = 0.12). Compared with hyperoxic baseline condition, hypoxia stimulated phrenic activity but attenuated the reduction in phrenic activity evoked by intralaryngeal CO 2. Hypoglossal activity was increased by intralaryngeal CO 2 and this response appeared to be reduced by hypoxia, but neither of these findings was statistically significant. The response of phrenic activity to intralaryngeal CO 2 during systemic hypercapnia was similar to that during hypoxia. The increase of phrenic activity in response to hypoxia was prevented by carotid body resection. Similarly, the hypoxic attenuation of the phrenic response to intralaryngeal CO 2 appeared to be absent after carotid body resection, although this finding was not established statistically. These results differ from previous reports of exaggerated laryngeal chemoreflex responses during hypoxia. The difference may reflect differences in the receptors and synaptic mechanisms of the reflexes, the severity and time course of hypoxia or the presence or depth of general anesthesia or sleep.