The effect of phenylephrine on arterial and venous cerebral blood flow in healthy subjects

Aim: Sympathetic regulation of the cerebral circulation remains controversial. Although intravenous phenylephrine (PE) infusion reduces the near‐infrared spectroscopy (NIRS)‐determined measure of frontal lobe oxygenation (S_cO₂) and increases middle cerebral artery mean blood velocity (MCA V_mean), suggesting α‐adrenergic‐mediated cerebral vasoconstriction, this remains unconfirmed by evaluation of arterial and venous cerebral blood flow. Methods: We determined S_cO₂, MCA V_mean, and right internal carotid artery (ICA) and internal jugular venous (IJV) blood flow (duplex ultrasound) during infusion of PE in eight supine young healthy men [26 (3) years, 177 (7) cm and 74 (8) kg; mean (SD)]. Results: Compared with saline, during infusion of PE, mean arterial pressure increased 26 ± 3% (mean ± SE) and MCA V_mean by 4·8 ± 1·9% (P<0·05), while S_cO₂ decreased by 13·7 ± 3·7% (P<0·05) with no significant changes in the arterial oxygen or carbon dioxide tensions. ICA blood flow did not change significantly in response to PE administration (351 ± 12 versus 373 ± 21 ml min^?1; P = 0·236), while IJV blood flow increased (443 ± 57 versus 507 ± 58 ml min^?1; P = 0·023). Conclusions: These findings confirm that PE induces a reduction in S_cO₂ measured by NIRS and causes an increase in MCA V_mean indicative of cerebral arterial vasoconstriction, although ICA was preserved and IJV increased. These results suggest that a decrease in S_cO₂ during infusion of PE reflects an altered cerebral contribution of arterial versus venous blood to the NIRS signal, although we cannot rule out that an effect of PE on skin blood flow is important.