Hyperbaric hyperoxia reduces exercising forearm blood flow in humans

Hypoxia during exercise augments blood flow in active muscles to maintain the delivery of O2 at normoxic levels. However, the impact of hyperoxia on skeletal muscle blood flow during exercise is not completely understood. Therefore, we tested the hypothesis that the hyperemic response to forearm exercise during hyperbaric hyperoxia would be blunted compared with exercise during normoxia. Seven subjects (6 men/1 woman; 25 ± 1 yr) performed forearm exercise (20% of maximum) under normoxic and hyperoxic conditions. Forearm blood flow (FBF; in ml/min) was measured using Doppler ultrasound. Forearm vascular conductance (FVC; in ml·min^-1·100 mmHg^-1) was calculated from FBF and blood pressure (in mmHg; brachial arterial catheter). Studies were performed in a hyperbaric chamber with the subjects supine at 1 atmospheres absolute (ATA) (sea level) while breathing normoxic gas [21% O2, 1 ATA; inspired PO₂ (PI_O2) ≈ 150 mmHg] and at 2.82 ATA while breathing hyperbaric normoxic (7.4% O₂, 2.82 ATA, PI_O2 ≈ 150 mmHg) and hyperoxic (100% O₂, 2.82 ATA, PI_O2 ≈ 2,100 mmHg) gas. Resting FBF and FVC were less during hyperbaric hyperoxia compared with hyperbaric normoxia (P < 0.05). The change in FBF and FVC (Δ from rest) during exercise under normoxia (204 ± 29 ml/min and 229 ± 37 ml·min^-1·100 mmHg^-1, respectively) and hyperbaric normoxia (203 ± 28 ml/min and 217 ± 35 ml·min^-1·100 mmHg^-1, respectively) did not differ (P = 0.66- 0.99). However, the ΔFBF (166 ± 21 ml/min) and ΔFVC (163 ± 23 ml·min^-1·100 mmHg^-1) during hyperbaric hyperoxia were substantially attenuated compared with other conditions (P < 0.01). Our data suggest that exercise hyperemia in skeletal muscle is highly dependent on oxygen availability during hyperoxia.