Cerebral Responses in Horses to Halothane and Isoflurane Anaesthesia: Eeg Power Spectrum Analysis and Differences in Arteriovenous Oxygen Content

In a prospective study we compared the EEG variables total amplitude (TA), 80% spectral edge frequency (SEF-80), the ratio of fractional amplitudes distributed into the BETA and DELTA frequency band (BETA/DELTA-ratio), and differences in arteriovenous oxygen content (AVD02), obtained from horses anaesthetized with either halothane (H; n=4) or isoflurane (I; n=4) in oxygen. All horses underwent orthopaedic procedures. After premedication with xylazine (0.88 mg/kg IV), anaesthesia was induced with diazepam (0.033 mg/kg IV) and ketamine (2.2 mg/kg IV). During anaesthesia horses were ventilated using IPPV. EEG variables and AVD02 were recorded at equal levels of surgical anaesthesia (stage III/1–2), as determined by clinical signs and a dominant delta activity in the EEG power spectrum. PaC02was kept between 35 mmHg and 45 mmHg, PaO2above 100 mmHg, and mean arterial blood pressure (MAP) was adjusted to at least 80 mmHg. The average body temperature was 35.4 ± 1.1°C (H) and 35.6 ± 0.7°C (I), respectively. In horses anaesthetized with I, TA was significantly higher (P<01) (I: 3533 ± 70 γV; H: 235.9 ± 63.4 γV), whereas SEF-80 (I: 10.7 ± 0.7 Hz; H: 12.4 ± 0.7 Hz) and BETA/DELTA-ratio (I: 035 ± 0.06; H: 0.53 ± 0.12) were significantly lower (P<01) compared with H. We also observed significantly lower (P<05) AVD02 values with I (1.5 ± 0.5 Vol%) than with H (2.0 ± 1.2 Vol%). Since a depression in cerebral activity during anaesthesia is characterized by a decrease in EEG frequency content and a concomitant increase in EEG amplitude, the authors conclude that at equal levels of surgical anaesthesia, isoflurane exerts a more pronounced depression in cerebral electrical and metabolic activity in horses.