INFLUENCE OF HYPOXIA ON VENTILATORY RESPONSES TO INTRALARYNGEAL CO2 IN CATS

In decerebrate, vagotomized cats, introduction of CO2 into the isolate d laryngeal airway while systemic P(CO2) is held constant evokes dose- related reflex changes in ventilatory activity. Because systemic hypox ia is known to exaggerate ventilatory responses to other types of lary ngeal chemostimulation in neonates, we have compared the responses of phrenic and hypoglossal nerve activities to ventilation of the larynx with 10% CO2 during systemic hyperoxia (FI(O2) = 1.00) to those during hypoxia (Fi(O2) = 0.12). Compared with the hyperoxic baseline conditi on, hypoxia stimulated phrenic activity but attenuated the reduction i n phrenic activity evoked by intralaryngeal CO2. Hypoglossal activity was increased by intralaryngeal CO2 and this response appeared to be r educed by hypoxia, but neither of these findings was statistically sig nificant. The response of phrenic activity to intralaryngeal CO2 durin g systemic hypercapnia was similar to that during hypoxia. The increas e of phrenic activity in response to hypoxia was prevented by carotid body resection. Similarly, the hypoxic attenuation of the phrenic resp onse to intralaryngeal CO2 appeared to be absent after carotid body re section, although this finding was not established statistically, Thes e results differ from previous reports of exaggerated laryngeal chemor eflex responses during hypoxia. The difference may reflect differences in the receptors and synaptic mechanisms of the reflexes, the severit y and time course of hypoxia or the presence or depth of general anest hesia or sleep.