Cardiovascular responses to upright tilt at a simulated altitude of 3,700 m in men.

To examine the effects of high altitude on cardiovascular responses to orthostasis, 11 healthy males were tested at a 10-min passive 70 degrees head-up tilt at sea level and at a simulated high altitude of 3,700 m. During the control period in the supine position, heart rate and forearm blood flow were higher at high altitude (p < 0.05). Mean arterial pressure remained unchanged during head-up tilt at sea level, but it reduced from 82 mm Hg to 72 mm Hg (p < 0.05) during head-up tilt at high altitude. There were no altitude-related changes in the magnitude of the increase in forearm vascular resistance and the reduction in cardiac output and laser-Doppler skin blood flow in response to head-up tilt. The total peripheral resistance increased from 14.4 to 20.5 mm Hg.L-1 x min-1 (p < 0.05) during head-up tilt at sea level, but the change was not significant at high altitude. The lack of altitude-related changes in forearm vascular resistance and laser-Doppler skin blood flow during head-up tilt in the presence of attenuated total peripheral resistance response at high altitude may suggest that the orthostatic hypotension at high altitude is associated with a lower magnitude of vasoconstrictor response in the regions other than the limbs and the skin.