VENTILATORY RESPONSE TO SUSTAINED EUCAPNIC HYPOXIA IN THE ADULT CONSCIOUS DOG

The ventilatory response to 20 min sustained isocapnic hypoxia (Sa(O2), 80 +/- 2%) was examined in 5 trained unanesthetized adult dogs breathing through an endotracheal tube. End tidal P(CO2) was maintained at the resting levels. The dogs' conscious status was monitored by recording EEG and EOG on a chart recorder. The room temperature was kept between 19 and 21-degrees-C. All tests were repeated in each dog on 2 occasions: (1) unloaded tracheal breathing or (2) resistive loaded breathing. During unloaded tracheal breathing, the average ventilation in response to sustained hypoxia rose from a control of 5.1 +/- 0.3 L/min (mean +/- within-dog SE) to 19.2 +/- 1.1 L/min at the initial stage of hypoxia. Ventilation remained at 20.7 +/- 1.3 L/min at 10 min, and then 19.7 +/- 1.4 L/min at the completion of the 20 min hypoxic exposure. There was no ventilatory adaptation observed (P > 0.05). After release from hypoxia, the ventilation fell abruptly to 7.6 +/- 0.8 L/min, which was higher than the resting baseline level (P < 0.05), and then gradually returned to the resting baseline within 10 min. Experiments exposing the dogs to 40 min sustained hypoxia also failed to elicit significant adaptation. During resistive loading, the pattern of average ventilation in response to sustained hypoxia was similar to that observed in unloaded breathing tests. But the ventilatory recovery was longer than unloaded breathing, returning to the resting baseline within 20 min. Again, there was no ventilatory adaptation observed. These findings indicate that ventilatory adaptation in the awake dog does not occur during exposure to a sustained hypoxia (80 +/- 2% saturation) over 40 min.