Objective.-The present study assessed the effects of acute hyperoxia on resting-minute ventilation (VE) during altitude acclimatization to 4300 m.

Methods.-Resting-minute ventilation, end-tidal partial pressure carbon dioxide (PETCO2) and oxygen (PETO2), and arterial oxygen saturation (SpO(2)) were measured during chronic poikilocapnic hypobaric hypoxia, supplemental oxygen breathing, and the subsequent return to hypobaric poikilocapnic hypoxia at altitude. Fifteen adult male lowlanders were studied at sea level and on the 3rd and 12th days at 4300 m. At sea level, subjects first breathed room air that was followed by 25-minute steady-state poikilocapnic hypoxia (FIO2 = 0.125). Ventilatory responses to acute poikilocapnic hypoxia (APH) were collected over the first 1-10 minutes, and responses to chronic poikilocapnic hypoxia (CPH) were collected over the last 3 minutes of the hypoxia exposure. At altitude, CPH was provided by ambient-air breathing (PIO2 = 86 mm Hg) that was interrupted by 10 minutes of oxygen breathing (FIO2 = 1.0, PIO2 = 460 mm Hg) and then a subsequent return to ambient air to measure APH ventilatory responses.

Results.-Between day 1 and day 12, during CPH, VE and SpO(2) increased (P < .05) by 46% and 6%, respectively, whereas PETCO2 decreased. On day 3 and day 12, breathing oxygen did not lower VE compared with CPH. However, the VE during APH immediately after oxygen breathing at high altitude was always greater (P < .05) than during CPH and did not change with duration of residence at altitude.

Conclusions.-These results show that short-duration oxygen breathing increases the subsequent ventilatory response to poikilocapnic hypoxia in altitude-acclimatized lowlanders, resulting in a transient elevation of SpO(2).

Muza, S.R., Young, A.J., Sawka, M.A., Forte, V.A., Rock, P.B., Fulco, C.S., Cymerman, A. (2004).Wilderness and Environmental Medicine. 15, 1, 18-24.

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