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Objective: Hemodynamic changes in response to the hypoxic environment of high altitude are vascular bed-specific. The aim of the present study was to investigate diameter and blood flow changes in conduit vessels in response to hypobaric hypoxia. Methods: Eleven healthy subjects ascending Mount Everest to base camp participated in this study. Vessel diameter and blood velocity for brachial, carotid, common femoral, superficial femoral, and deep femoral arteries were measured by portable Doppler ultrasound. Blood flow was calculated from these values. Measurements were taken at sea level, at increasing altitudes on ascent to base camp (1310 m, 3470 m, 5330 m), and repeated on descent to lower altitude (1310 m). Results: For all vessels except carotids, both vessel diameter and blood flow decreased between sea level and initial ascent to altitude, with subsequent persistence of these decreased values; there was no further significant change with continued ascent to higher altitude. Blood flow for all arteries (except carotids) increased significantly on descent to lower altitude, with an associated nonsignificant increase in velocity and decrease in diameter. Conclusions: This study showed that there is vasoconstriction of limb conduit vessels at altitude, which persists upon descent to lower altitude. Blood flow in these vessels also decreases with initial exposure to high altitude, yet increases when returning to lower altitude, reflecting variations in blood velocity. Carotid arteries responded differently to the stimulus of hypobaria than limb conduit vessels; there was no change in diameter seen on ascent or descent, but there was a progressive decrease in blood flow on ascent, with no change on subsequent descent. [Copyright &y& Elsevier]