Hyperoxic ventilatory responses of high altitude acclimatized cats

We have examined the effect of steady-state hyperoxia on the ventilation of sea level (SL) cats and cats acclimatized to simulated high altitude (HA) at 5500 m for three weeks. Three groups of cats were studied. In group I, the ventilatory responses to 10%, 21% and 100% O2 were studied at SL, and after acclimatization to HA, the ventilatory responses to 10% and 100% O2 were measured. In group II the ventilatory responses and femoral artery and superior sagittal sinus blood gases were measured in two sets of cats, one at SL and one at HA, during exposure to the gases outlined in group I. In group III, we examined the effect of chronic vagotomy on the ventilatory responses to the gas mixtures outlined in group I. Breathing 100% O2 at SL had no significant effect on ventilation, tidal volume, respiratory frequency, or cerebral blood flow (inferred from the cerebral veno-arterial CO2 difference). Ventilation was constant in the HA acclimatized cats while breathing 10% and 100% O2, but the ventilatory pattern changed dramatically during hyperoxia: respiratory frequency increased and tidal volume fell. Breathing 100% O2 was associated with changes in CBF, and venous PCO2 that might be expected to stimulate ventilation, but the change in ventilatory pattern suggests to us that hyperoxic disinhibition of central respiratory processes (which were modified by HA acclimatization) is the mechanism whereby ventilation is sustained during hyperoxia at HA. After vagotomy at HA, ventilation remained constant while breathing 100% O2, but the changes in respiratory pattern were no longer apparent. Therefore, vagal afferents seems to have a role in determining the pattern, but not necessarily the absolute level, of ventilation during hyperoxia. Cats vagotomized at SL prior to HA exposure did not show any evidence of HA ventilatory acclimatization; thus, the vagi may also play a heretofore unrecognized role in the process of acclimatization.