The impact of systemic vasoconstrictors on the cerebral circulation of anesthetized patients

BACKGROUND. The effect of vasoconstrictors on intracerebral hemodynamics in anesthetized patients is controversial. The influence of phenylephrine and norepinephrine on the cerebral circulation was investigated in isoflurane- or propofol-anesthetized patients using transcranial Doppler ultrasonography.

Forty patients were randomly assigned to have vasoconstrictor tests with norepinephrine or phenylephrine during either isoflurane or propofol anesthesia. Blood flow velocities were simultaneously measured in the middle cerebral artery and ipsilateral extracranial internal carotid artery. Baseline recordings were done during stable anesthesia in a supine position (test 0). A second series of measurements were performed after norepinephrine or phenylephrine had increased mean arterial blood pressure by about 20% (test 1). With maintained norepinephrine or phenylephrine infusion, a final series of results were obtained after the increased mean arterial blood pressure was counteracted by a slightly head-up patient position (test 2).

Both vasoconstrictors significantly increased mean flow velocities in the middle cerebral artery (norepinephrine: 43 +/- 11 cm/s to 49 +/- 11 cm/s; phenylephrine: 43 +/- 8 cm/s to 48 +/- 9 cm/s; +/- SD) and internal carotid artery (norepinephrine: 27 +/- 7 cm/s to 31 +/- 8 cm/s; phenylephrine: 27 +/- 9 cm/s to 31 +/- 10 cm/s) in the isoflurane-but not in the propofol-anesthetized patients. In the head-up position, only small and insignificant flow velocity changes were observed in both cerebral arteries independent of the vasoconstrictor or background anesthetic.

The results of the present study indicate that norepinephrine and phenylephrine do not directly affect intracranial hemodynamics in anesthetized patients, but rather that hemodynamic changes observed with vasoconstrictors reflect the effect of the background anesthetic agents on cerebral pressure autoregulation.