RELATION OF SYMPATHETIC ACTIVATION TO VENTILATION IN MAN AT 4300 M ALTITUDE

Background. The sympathetic nervous activity increases at high altitud e but is not maximal initially when hypoxemia is most severe. Hypothes is: The sympathetic activation would correlate better to the ventilato ry response to chronic hypoxia than to the severity of hypoxia per se. Methods: Eleven healthy male volunteers (27 +/- 1 yr) had measurement s from the abdominal aorta of pressure, catecholamines, and blood gase s at sea level, on arrival at 4300 m, and after 21 d of residence. Add itionally, we measured 24-h urinary catecholamine excretion at sea lev el and each day at altitude, and made serial measurements of resting v entilatory parameters. Results: Arterial norepinephrine (NE) concentra tions on arrival at 4300 m were little changed from sea level, but wer e increased following acclimatization at 21 d. Arterial oxygenation wa s decreased on arrival, but improved with acclimatization. Arterial ep inephrine (E) concentrations were increased on arrival, and returned t o an intermediate level by 21 d. The urinary NE excretion was increase d along with the increase in VE (p < 0.01) and the fall in end-tidal P CO2 (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 p arameters. Propranolol (240 mg . d(-1)), which was given to 6 of 11 su bjects, did not affect any relationships. Conclusion: The sympathetic activation was related to the ventilatory response but not to measures of hypoxemia at 4300 m. We conclude that factors related to ventilato ry acclimatization, possibly increased chemoreceptor activity, contrib ute to the development of sympathetic activation at high-altitude.