Phenylephrine increases near-infrared spectroscopy determined muscle oxygenation in men.

Phenylephrine increases mean arterial pressure (MAP) by enhanced total peripheral resistance (TPR) but near-infrared spectroscopy (NIRS) determined muscle oxygenation (Sm O2 ) increases. We addressed that apparent paradox during supine rest and head-up tilt (HUT). Variables were determined ± phenylephrine in males during supine rest (n = 17) and 40° HUT (n = 7). MAP, stroke volume (SV), heart rate (HR), and TPR were derived by Modelflow® and NIRS determined biceps Sm O2 and (tibial) bone oxygenation (Stibial O2 ). For ten subjects, cardiac filling and the diameter of the inferior caval vein (ICV collapsibility index: ((ICVexpiration  - ICVinspiration )/ICVexpiration ) × 100) were assessed by ultrasound. Pancreatic polypeptide (PP) and atrial natriuretic peptide (proANP) in plasma were determined by immunoassay. Brachial artery blood flow was assessed by ultrasound and skin oxygenation (Sskin O2 ) monitored by white light spectroscopy. Phenylephrine increased MAP by 34% and TPR (62%; P < 0.001) during supine rest. The ICV collapsibility index decreased (24%; P < 0.001) indicating augmented cardiac preload although volume of the left atrium and ventricle did not change. SV increased (18%; P < 0.001) as HR decreased (24%; P < 0.001). ProANP increased by 9% (P = 0.002) with unaffected PP. Brachial artery blood flow tended to decrease while Sskin O2 together with Stibial O2 decreased by 11% (P = 0.026) and 20% (P < 0.001), respectively. Conversely, phenylephrine increased Sm O2 (9%) and restored the HUT elicited decrease in Sm O2 (by 19%) along with SV (P = 0.02). Phenylephrine reduces skin and bone oxygenation and tends to reduce arm blood flow, suggesting that the increase in Sm O2 reflects veno-constriction with consequent centralization of the blood volume.