Concurrent increases in brain electrical activity and intracranial blood flow velocity during low-dose ketamine anaesthesia

The purpose of the present study was to assess the effects of low-dose ketamine on spontaneous brain electrical activity (EEG) and intracranial blood flow velocity. Twenty healthy volunteers were divided into two groups: Group I (n=10) received 0.25 mg·kg^-1 ketamine iv; Group II (n=10) received 0.5 mg·kg^-1 ketamine iv. Mean arterial blood pressure (MAP), heart rate (HR), end-tidal PCO₂ (P etCO₂), and arterial oxygen saturation (SaO₂) were measured. The EEG was recorded from temporo-occipital recording sites over both hemispheres. Blood flow velocity in the middle cerebral artery was measured using a transcranial Doppler ultrasound system. All variables were evaluated at baseline and for 60 min following ketamine. Administration of ketamine resulted in increases of MAP and HR in both groups to a similar degree. The PetCO₂ and SaO₂ did not change in either group over time. Ketamine caused a dose-dependent, transient shift in the EEG to synchronous high-voltage slow waves with an increase in total power (Group 1: 301±38%; Group II: 104±28%). These changes were associated with dose-dependent increases in mean blood flow velocity (Group I: 35±7%; Group II: 68±10%). Our data suggest that increases in intracranial blood flow velocity are closely correlated to increases in neuronal activity and are not secondary to changes in systemic haemodynamic variables.