Abstract
In order to assess the effects of hypobaric hypoxia on early cardiac responses, cardiac output during rest and work during acute and graded exposure to simulated altitudes of 3, 000, 4, 500, and 6, 000 m, was estimated in 18 healthy male volunteers by impedance cardiography. The subjects were divided into two groups; 6 who had worked mainly at a desk without any accompanying sports activities (Group A) and 12 who had trained daily for their overseas high-altitude expeditions (Group B). The cardiac output (CO) at rest while sitting, at 3, 000 m, showed a tendency toward a decrease for Group A, but not for Group B. At 4, 500 m, the CO change was not significant in Group A, but showed an increase in Group B. At 6, 000 m, the CO increase was about 30 % in both groups. The consistent increase of heart rate (HR) with altitude was accompanied by a reduction of the stroke volume (SV), the remaining CO change being relatively small. The above discrepancy in the CO change at lower altitudes may represent a difference in sensitivity to hypoxia between the groups and may partly clarify the previous disagreement concerning the resting cardiac function at altitude.
A moderate workload at 0 m was still moderate at 3, 000 m, where the moderate HR acceleration was well compensated for a reduction in SV leaving CO unchanged, but the exercise was almost at its maximum at 4, 500 m, since HR approached a ceiling level (150-160 beats/min) which was attained at 6, 000 m and CO increased remarkably.
Of 2 male volunteers (Group C) who slept overnight at 4, 500 m, one showed a depression pattern in CO similar to 0-m sleep, while the other did not. In this CO reduction, the SV change was the determining factor as indicated previously for a 3, 500-m sleep.
