Phenylephrine decreases frontal lobe oxygenation at rest but not during moderately intense exercise

Whether sympathetic activity influences cerebral blood flow (CBF) and oxygenation remains controversial. The influence of sympathetic activity on CBF and oxygenation was evaluated by the effect of phenylephrine on middle cerebral artery (MCA) mean flow velocity (Vmean) and the near-infrared spectroscopy-derived frontal lobe oxygenation (ScO2) at rest and during exercise. At rest, nine healthy male subjects received bolus injections of phenylephrine (0.1, 0.25, and 0.4 mg), and changes in mean arterial pressure (MAP), MCA Vmean, internal jugular venous O2 saturation (SjvO2), ScO2), and arterial PCO2 (PaCO2) were measured and the cerebral metabolic rate for O2 (CMRO2) was calculated. In randomized order, a bolus of saline or 0.3 mg of phenylephrine was then injected during semisupine cycling, eliciting a low (approximately 110 beats/min) or a high (approximately 150 beats/min) heart rate. At rest, MAP and MCA Vmean increased approximately 20% (P