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

Brassard P, Seifert T, Wissenberg M, Jensen PM, Hansen CK, Secher NH. Phenylephrine decreases frontal lobe oxygenation at rest but not during moderately intense exercise. J Appl Physiol 108: 1472-1478, 2010. First published March 11, 2010; doi:10.1152/japplphysiol.01206.2009.-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 (V-mean) and the near-infrared spectroscopy-derived frontal lobe oxygenation (Sc-O2) 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 O-2 saturation (Sjv(O2)), Sc-O2, and arterial PCO2 (Pa-CO2) were measured and the cerebral metabolic rate for O-2 (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 (similar to 110 beats/min) or a high (similar to 150 beats/min) heart rate. At rest, MAP and MCA Vmean increased similar to 20% (P < 0.001) and similar to 10% (P < 0.001 for 0.25 mg of phenylephrine and P < 0.05 for 0.4 mg of phenylephrine), respectively. Sc-O2 then decreased similar to 7% (P < 0.001). Phenylephrine had no effect on Sjv(O2), PaCO2, or CMRO2. MAP increased after the administration of phenylephrine during low-intensity exercise (similar to 15%), but this was attenuated (similar to 10%) during high-intensity exercise (P < 0.001). The reduction in Sc-O2 after administration of phenylephrine was attenuated during low-intensity exercise (-5%, P < 0.001) and abolished during high- intensity exercise (-3%, P = not significant), where PaCO2 decreased similar to 7% (P < 0.05) and CMRO2 increased 17% (P < 0.05). These results suggest that the administration of phenylephrine reduced Sc-O2 but that the increased cerebral metabolism needed for moderately intense exercise eliminated that effect.