Chemical respiratory control in chronically hyperoxic cats

Chemical control of respiration in cats after chronic normobaric hyperoxia (NH; inhalation of 100% O2 for 60-67 h) was compared with that of control rats, anesthetized with pentobarbital. After chronic hyperoxia, induction of moderate hypoxia (PaO2 = 50-60 Torr) increased inspiratory time (TI) often without increasing tidal volume (VT). More intense hypoxia (PaO2 = 40-50 Torr) depressed tidal volume and further increased TI, diminishing the respiratory drive (VT/TI). Hypercapnia, on the other hand, increased tidal volume and shortened respiratory cycle time; but these responses were subnormal. The normal stimulatory effects of intravenous nicotine and inhibitory effect of dopamine on carotid chemo-receptor activity and ventilation were preserved in the NH cats. Cyanide, however, did not stimulate carotid chemoreceptor activity and ventilation. Thus, the changes in the carotid and aortic chemosensory activities elicited appropriate reflex ventilation responses, indicating that the central component of the chemoreflex was not impaired. The ventilatory depression during hypoxia despite an active chemosensory input is consistent with the lack of carotid chemosensory response to and a central depressant effect of hypoxia in the NH cats, and was presumably associated in part with an increased responsiveness of airway reflexes. We conclude that chronic hyperoxia selectively attenuated carotid chemosensory and chemoreflex responses to hypoxia.