Reduced autonomic activity during stepwise exposure to high altitude

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Abstract
Several studies have shown increased sympathetic activity during acute exposure to hypobaric hypoxia. In a recent field study we found reduced plasma catecholamines during the first days after a stepwise ascent to high altitude. In the present study 14 subjects were exposed to a simulated ascent in a hypobaric chamber to test the hypothesis of a temporary reduction in autonomic activity. The altitude was increased stepwise to 4500 m over 3 days. Heart rate variability (HRV) was assessed continuously in seven subjects. Baroreceptor reflex sensitivity (BRS) was determined in eight subjects with the ‘Transfer Function’ method at baseline, at 4500 m and after returning to baseline. Resting plasma catecholamines and cardiovascular- and plasma catecholamine- responses to cold pressor- (CPT) and mental stress-test (MST) were assessed daily in all and 12 subjects, respectively. Data are mean ± SEM. Compared with baseline at 4500 m there were lower total power (TP) (35 457 ± 26 302 vs. 15 001 ± 11 176 ms2), low frequency (LF) power (3112 ± 809 vs. 1741 ± 604 ms2), high frequency (HF) power (1466 ± 520 vs. 459 ± 189 ms2) and HF normalized units (46 ± 0.007 vs. 44 ± 0.006%), P ≤ 0.001. Baroreceptor reflex sensitivity decreased (15.6 ± 2.1 vs. 9.5 ± 2.6 ms mmHg–1, P=0.015). Resting noradrenaline (NA) decreased (522 ± 98 vs. 357 ± 60 pmol L–1, P=0.027). The increase in systolic blood pressure (SBP) and NA during mental stress was less pronounced (21 ± 4 vs. 10 ± 2% and 25 ± 9 vs. –2 ± 8%, respectively, P < 0.05). The increase in SBP during cold pressor test decreased (16 ± 3 vs. 1 ± 6%, P=0.03). Diastolic blood pressure, HR and adrenaline displayed similar tendencies. We conclude that a transient reduction in parasympathetic and sympathetic activity was demonstrated during stepwise exposure to high altitude.