Hypoxic ventilatory response of cats at high altitude: an interpretation of 'blunting'

Cats acclimatized to a simulated altitude of 5500 m developed attenuated ventilatory response in the hypoxic test range of PAO2 = 60-45 torr, but their CO2 response remained normal, although the curve was shifted to a lower PACO2 range. The acclimatized cats a high respiratory frequency and maintained hyperventilation under normoxia. Cats from 3100 m altitude had hypoxic reponses which were, on the average, slightly below sea level standards, but the difference was not statistically significant. Two cats raised at 4640 m had a normal hypoxic ventilatory response even though the frequency response was 'blunted'. These data suggest the possibility of hypoxic 'threshold' near 5500 m to produce an attenuation effect. Another series of cats acclimatized to 5500 m were tested with more severe hypoxia, and they exhibited brisk ventilatory response in range PAO2, 40-25 torr, although they showed typical 'blunting' in the range PAO2, 60-45 torr. These results suggested that the phenomenon of attenuated hypoxic response at high altitude was a reflection of shift of hypoxic set point to a lower PAO2. Finally, it was shown that the hypoxic responses of 'blunted' animals were restored to normal after mid-collicular decerebration; and that decortication resulted in a typical hyperexcitability of the hypoxic response. These results are discussed in terms of hypothesized suprapontine modulating influences on the control of breathing, and possiblities for a contribution of these mechanisms in pathogenesis of hypoxic 'blunting' are raised.