Measuring ventilatory acclimatization to hypoxia, comparative aspects

Acclimatization to hypoxia increases the hypoxic ventilatory response (HVR) in mammals. The literature on humans shows that several protocols can quantify this increase in HVR if isocapnia is maintained, regardless of the exact level of PaCO2. In rats, the isocapnic HVR also increases with chronic hypoxia and this cannot be explained by a non-specific effect of increased ventilatory drive on the HVR. Changes in arterial pH are predicted to increase the HVR during chronic hypoxia in rats but this has not been quantified. Limitations in determining mechanisms of change in the HVR from reflex experiments are discussed. Chronic hypoxia changes some, but not all, indices of ventilatory motor output that are useful for normalization between experiments on anesthetized rats. Finally, ducks also show rime-dependent increases in ventilation during chronic hypoxia and birds provide a good experimental model to study reflex interactions. However, reflexes from intrapulmonary CO2 chemoreceptors can complicate the measurement of changes in the isocapnic HVR during chronic hypoxia in birds. (C) 2000 Elsevier Science B.V. All rights reserved.