Reduced autonomic activity during stepwise exposure to high altitude

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