Reduced autonomic activity during stepwise exposure to high altitude

Area of Science:

• Physiology
• Altitude Medicine
• Autonomic Nervous System Research

Background:

• Previous research indicates increased sympathetic activity during acute hypobaric hypoxia.
• A recent field study observed reduced plasma catecholamines during early high-altitude ascent.
• This study investigates the hypothesis of a temporary reduction in autonomic activity during simulated high-altitude exposure.

Purpose of the Study:

• To test the hypothesis of a temporary reduction in autonomic activity during simulated high-altitude ascent.
• To assess changes in heart rate variability (HRV) and baroreceptor reflex sensitivity (BRS).
• To evaluate cardiovascular and plasma catecholamine responses to stress tests.

Main Methods:

• 14 subjects were exposed to a stepwise simulated ascent to 4500 m over 3 days in a hypobaric chamber.
• Heart rate variability (HRV) was continuously monitored.
• Baroreceptor reflex sensitivity (BRS) was measured using the 'Transfer Function' method.
• Resting plasma catecholamines and responses to cold pressor test (CPT) and mental stress test (MST) were assessed daily.

Main Results:

• At 4500 m, significant reductions in total power, low frequency, and high frequency of HRV were observed compared to baseline.
• Baroreceptor reflex sensitivity (BRS) decreased significantly.
• Resting noradrenaline levels were reduced, and cardiovascular and catecholamine responses to CPT and MST were less pronounced.

Conclusions:

• Stepwise exposure to high altitude induces a transient reduction in both parasympathetic and sympathetic autonomic activity.
• Findings contrast with previous studies suggesting increased sympathetic activity under hypobaric hypoxia.
• The results highlight a complex autonomic response to high-altitude acclimatization.