Relation of sympathetic activation to ventilation in man at 4300 m altitude

The sympathetic nervous activity increases at high altitude but is not maximal initially when hypoxemia is most severe.

The sympathetic activation would correlate better to the ventilatory response to chronic hypoxia than to the severity of hypoxia per se.

Eleven healthy male volunteers (27 +/- 1 yr) had measurements from the abdominal aorta of pressure, catecholamines, and blood gases at sea level, on arrival at 4300 m, and after 21 d of residence. Additionally, we measured 24-h urinary catecholamine excretion at sea level and each day at altitude, and made serial measurements of resting ventilatory parameters.

Arterial norepinephrine (NE) concentrations on arrival at 4300 m were little changed from sea level, but were increased following acclimatization at 21 d. Arterial oxygenation was decreased on arrival, but improved with acclimatization. Arterial epinephrine (E) concentrations were increased on arrival, and returned to an intermediate level by 21 d. The urinary NE excretion was increased along with the increase in VE (p < 0.01) and the fall in end-tidal PCO2 (p < 0.001), but not with the decrease in end-tidal PO2 during the sojourn at 4300 m. Excretion of E did not relate to any ventilatory parameters. Propranolol (240 mg.d-1), which was given to 6 of 11 subjects, did not affect any relationships.

The sympathetic activation was related to the ventilatory response but not to measures of hypoxemia at 4300 m. We conclude that factors related to ventilatory acclimatization, possibly increased chemoreceptor activity, contribute to the development of sympathetic activation at high-altitude.