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Introduction: Quantitative measures derived from the electroencephalogram (qEEG) have been used extensively in human medicine to assess the function of the brain under clinical manipulation (such as anesthesia and sedation), as well as in disease states (such as head trauma, coma and stroke). The goal of this study was to determine if qEEG measures could be used to predict the response of cats under isoflurane concentration to noxious stimuli.

Methods: Twelve cats aged 1–2 years were included. Neurological and physical examination, blood chemistries and CBCs were normal in all cats. Anesthesia was induced with isoflurane (ISO) in O2, the cat was intubated and the femoral artery catheterized. The following physiologic data were collected continuously throughout the course of the experiment: temperature, blood pressure, respiratory rate, heart rate, and end-tidal CO2 and ISO concentrations. An Aspect Medical Systems A-1050 monitor was used to continuously record the raw electroencephalogram (EEG) and several qEEG measures including bispectral index (BIS), spectral edge frequency (SEF), burst suppression ratio (BSR), and total EEG power (POW). ISO concentration was maintained constant for 15 minutes, after which a tail clamp was applied for 1 minute. If the cat responded, the concentration was increased 10% and the procedure repeated. If the cat did not respond, the ISO concentration was decreased 10% and the procedure repeated. The minimum alveolar concentration (MAC) was the midpoint between a positive and negative response and, for each cat, this was determined three times.

Results: BIS was significantly lower in cats at the lowest concentration at which there was no response (MAC−, 21.3±28.3) than at the highest concentration at which there was a response (MAC+, 63.4±22.1; p<.01, Wilcoxon signed rank). However, in 3/12 cats (25%), BIS was higher at MAC− than at MAC+. Of the other qEEG measures, only BSR was significantly different between MAC− and MAC+ (50.4±34.4 vs. 6.1±19.4, p<.01). Of the physiologic data collected, only respiratory rate was different between MAC− and MAC+ (29.4±10.0 vs. 45.3±17.9, p<.05).

Conclusions: These results suggest that qEEG measures may be useful for anesthesia monitoring in cats, but measures that are less species-specific than the BIS may hold more promise. In addition, this work suggests that further studies utilizing qEEG measures for evaluation of sedation during ventilation as well as outcome prediction in animals with head trauma and critical illnesses with CNS complications are indicated.