Pattern of breathing and mouth occlusion pressure during acclimatization to high altitude.

Pattern of breathing and mouth occlusion pressure have been investigated during a 6-day sojourn at an altitude of 3457 m in six resting subjects breathing ambient air or an hyperoxic mixture. It has been shown that ventilation increases and alveolar PCO2 decreases, both variables reaching a steady-state after about 4 days at altitude. Changes in ventilation are caused by modifications, at first, in the timing component of the respiratory cycle and later on, in the mean inspiratory airflow. This suggests that the output of the respiratory center is qualitatively modified during the acclimatization period. Mouth occlusion pressure, an index of neuromuscular inspiratory drive, increased on the first day at altitude, remaining constant thereafter. The mean inspiratory flow did not change on the first day, but progressively increased in the subsequent 3 days. The observation that flow increased in the face of a constant neuromuscular inspiratory drive suggests that during acclimatization to altitudes, the progressive increase in ventilation is due to changes in mechanics of the ventilatory pump rather than changes in central neural drive.