Cerebral blood flow velocity in relation to cerebral blood flow, cerebral metabolic rate for oxygen, and electroencephalogram analysis during isoflurane anesthesia in dogs

The purpose of this study was to correlate changes in cerebral blood flow velocity (V(mean)) with cerebral blood flow (CBF) during isoflurane anesthesia in dogs. The relation between cerebral oxygen consumption (CMRO₂) and electroencephalogram (EEG) analysis also was investigated. Blood flow velocity was measured in the middle cerebral artery using a pulsed transcranial Doppler (TCD). CBF was measured with radioactive microspheres. EEG was measured over both hemispheres and median EEG frequency (median frequency) was calculated after fast Fourier transformation. Baseline anesthesia was maintained with 50% nitrous oxide in oxygen and 50 μg · kg^{- 1} · h^-1 fentanyl. Animals of Group 1 (control, n = 6) were not given isoflurane. Data were recorded at baseline, and at 30, 60, and 90 min. There was no significant change in any variable over time. In Group II (n = 7), data were recorded at baseline and at 1%, 2%, and 3% end-tidal isoflurane. Mean arterial pressure was maintained at baseline levels by phenylephrine infusion. CBF increased from 70.8 ± 10.6 mL · 100 g^-1 · min^-1 at baseline to 146.1 ± 36.9 mL · 100 g^-1 · min^-1 with 3% isoflurane (P < 0.01). V(mean) increased from 38.3 ± 6.7 cm/s to 65.6 ± 9.7 cm/s (P < 0.01). The correlation between relative changes in CBF and V(mean) was r = 0.94 (P < 0.01). With 1% isoflurane the EEG shifted to slow-wave, high- voltage activity, and median frequency decreased from 5.9 ± 0.7 Hz to 1.4 ± 0.4 Hz (P < 0.05). Median frequency was not decreased further during 2% and 3% isoflurane anesthesia. In contrast, CMRO₂ did not change with 1% isoflurane, but decreased 50% with 3% isoflurane (P < 0.05). The correlation between median frequency and CMRO₂ was not significant (r = 0.19, P > 0.05). A negative correlation was found between periods of electrical silence and CMRO₂ (r = -0.73, P < 0.05). We conclude that relative increases in CBF can be assessed by V(mean). In contrast, even though EEG patterns indicate changes in anesthetic depth, depression of cerebral metabolism is not closely related to changes in median frequency.