During hypoxic exercise some vasoconstriction is needed to match O2delivery with O2demand at the microcirculatory level

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Abstract
To test the hypothesis that the increased sympathetic tonus elicited by chronic hypoxia is needed to match O2delivery with O2demand at the microvascular level eight male subjects were investigated at 4559 m altitude during maximal exercise with and without infusion of ATP (80 μg (kg body mass)−1min−1) into the right femoral artery. Compared to sea level peak leg vascular conductance was reduced by 39% at altitude. However, the infusion of ATP at altitude did not alter femoral vein blood flow (7.6 ± 1.0versus7.9 ± 1.0 l min−1) and femoral arterial oxygen delivery (1.2 ± 0.2versus1.3 ± 0.2 l min−1; control and ATP, respectively). Despite the fact that with ATP mean arterial blood pressure decreased (106.9 ± 14.2versus83.3 ± 16.0 mmHg,P< 0.05), peak cardiac output remained unchanged. Arterial oxygen extraction fraction was reduced from 85.9 ± 5.3 to 72.0 ± 10.2% (P< 0.05), and the corresponding venous O2content was increased from 25.5 ± 10.0 to 46.3 ± 18.5 ml l−1(control and ATP, respectively,P< 0.05). With ATP, leg arterial–venous O2difference was decreased (P< 0.05) from 139.3 ± 9.0 to 116.9 ± 8.4−1and legwas 20% lower compared to the control trial (1.1 ± 0.2versus0.9 ± 0.1 l min−1) (P= 0.069). In summary, at altitude, some degree of vasoconstriction is needed to match O2delivery with O2demand. Peak cardiac output at altitude is not limited by excessive mean arterial pressure. Exercising legis not limited by restricted vasodilatation in the altitude‐acclimatized human.