Breathing pattern in hypoxic exposures of varying duration.

The relative contributions of breathing frequency and tidal volume to the increase in ventilation during acute or prolonged exposure to hypoxia is uncertain. We examined the changes in breathing pattern during hypoxic exposures lasting minutes, hours, and days using data from previous studies. Increased tidal volume accounted for the increased ventilation during 7-10 and 30 min of isocapnic and poikilocapnic (no CO2 added) hypoxic exposures as well as during 7 h of poikilocapnic hypobaric hypoxia (4,800 m). Tidal volume was also a greater overall contributor than frequency to increased ventilation in sea-level residents during 3 days of isocapnic hypobaric hypoxia (4,100-4,600 m) and in Denver (1,600 m) residents during 5 days on Pikes Peak (4,300 m). In sea-level residents during 3 days of poikilocapnic hypobaric hypoxia (3,600-4,300 m) and during 7-8 days on Pikes Peak, increased frequency accounted for the rise in ventilation. Tidal volume thus contributed more than frequency to increasing ventilation during brief hypoxia, whereas the contribution of frequency was increased in prolonged hypoxia involving a 4,300-m altitude ascent and hypocapnia.