Cardiopulmonary responses of resting man during early exposure to high altitude

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Abstract
Sixteen young male subjects were exposed to simulated altitudes of 2,000, 11,000, and 15,000 ft. in a hypobaric chamber and studied at sitting rest after 10, 20, 30, and 40 hr. of exposure. Of those measurements made, only PaO2 [arterial O2 tension] changed significantly with time of exposure. Cardiac output rose from 71 at 2,000 ft. to 84 and 123 ml/min. per kg at 11,000 and 15,000 ft., respectively. This was due to significant Increases In heart rate at both higher elevations with no alteration in stroke volume. Mean arterial blood pressure was unaltered and, therefore, peripheral resistance fell concomitant with the rise in cardiac output. Mean PaO2 for the 3 altitudes was 94, 63, and 44 mm Hg; and percent SaO2 99. 92d 78- Largest changes in PaCO2 [arterial CO2 tension] and pH were observed between 2,000 and 11,000 ft. and much less between 11,000 and 15,000 ft. Changes in cardiac output appeared best related to alterations in arterial O2 content or saturation rather than tension, suggesting a peripheral rather than a central origin for the stimulus for the cardiovascular response to high altitude. The cardiac output response appears to follow an exponential relationship with altitude.

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