Differences in the effects of hypercapnia and hypoxia on the swallowing reflex in cats

The effects of changes in PaCO₂ and PaO₂ on the swallowing reflex were studied in anaesthetized, vagotomized, paralysed and artificially ventilated cats. The swallowing reflex was induced by electrical stimulation of the superior laryngeal nerve (SLN). This initially suppressed activity in the phrenic nerve (PN). The swallowing reflex was then identified by a characteristic brief burst of PN activity and a large amplitude burst of hypoglossal nerve (HN) activity. Steady-state responses to constant SLN stimulation for 60 s were measured at four carbon dioxide tensions (PaCO₂ 3.9, 5.1, 6.3 and 7.8 kPa) with hyperoxia (PaCO₂ > 51 kPa) and at four values of Pa_O2 (PaO₂ 56, 11.3, 6.9 and 4.8 kPa) at a fixed PaCO₂ (PaCO₂ 4.2 kPa). Although both hypercapnia and hypoxia increased the spontaneous respiratory activity in PN and HN, the number of swallows elicited during SLN stimulation was not influenced by PaCO_2′, whereas a progressive decrease in the number of swallows with decreasing PaO₂ was observed consistently. These results indicate that the swallowing reflex is independent of the background respiratory activity and that hypoxia depresses the swallowing reflex, whereas hypercapnia has no effect.