Phenylephrine increases near-infrared spectroscopy determined muscle oxygenation during head-up tilt in men

Phenylephrine is an α-adrenergic agent and yet seems to increase near-infrared spectroscopy determined muscle oxygenation (S_mO₂) that reflects the ratio between oxygenated (O₂Hb) and deoxygenated (Hb) haemoglobin/myoglobin. We examined whether the increase in S_mO₂ by phenylephrine reflects veno-constriction and to secure filling of the veins, subjects were exposed to head-up tilt (HUT). Phenylephrine (0.2 mg) was administered to 10 healthy males (24 years (22-27; median with interquartile range)) during 40° HUT with S_mO₂, O₂Hb and Hb determined for the biceps brachii and vastus lateralis muscles. Changes in red cell volume within the thorax and thigh were evaluated by electrical admittance and brachial vein diameter determined by ultrasound. HUT accumulated blood in arms and legs as indicated by reduced thoracic and conversely enhanced thigh electrical admittance. Both over the arm and leg, HUT reduced S_mO₂ as a consequence of reduced O₂Hb and increased Hb (p < .05). After 5 min HUT, phenylephrine increased MAP and total peripheral resistance, and both arm and thigh S_mO₂ increased due to a decrease in Hb and an increase in O₂Hb. The results confirm that S_mO₂ decreases during HUT and demonstrate venous filling in the limbs. Furthermore, vasoconstriction during HUT is indicated by a decrease in O₂Hb. Conversely, phenylephrine increased S_mO₂ likely illustrating increased muscle blood flow and venoconstriction as O₂Hb increased while Hb was reduced.