Abstract

Bispectral index (BIS) is a numerical value derived from the EEG that is used clinically by anesthesiologists to assess CNS depression in human beings. In a previous study, canine BIS values were inversely related to end-tidal sevoflurane (SEVO) concentration: BIS = 102.7 − (14.2 × [SEVO]); R2 = 0.754. The purpose of this study was to determine the relationship between BIS and end-tidal isoflurane (ISO) in the same dogs used in the SEVO study. ISO minimum alveolar concentration (MAC) was determined using the tail-clamp technique for six English pointer dogs (three males and three females) with a mean ± SD age and weight of 3.7 ± 2.0 years and 20.2 ± 4.8 kg, respectively. One week after MAC determination, BIS was measured at 0.8, 1.0, 1.5, and 2.0 MAC multiples of ISO. Ventilation was controlled and atracurium (0.2 mg kg−1 followed by 6 µg kg−1 minute−1 IV) was administered to eliminate EMG artefact from the EEG. The BIS was determined using an A-2000 monitor (Aspect Medical Systems Inc.). After a 15-minute equilibration period, BIS data were collected for 5 minutes and median BIS values were calculated. Heart rate, direct mean arterial blood pressure (MAP), esophageal temperature, and arterial blood gases were measured at each BIS collection period. End-tidal CO2 and percentage [ISO] were monitored using a calibrated infra-red gas analyzer. Data were analyzed using RM anova (p < 0.05). The mean ISOMAC value was 1.3 ± 0.2%. The BIS values were inversely related to [ISO]: BIS = 93.9 − (25.2 × [ISO]); R2 = 0.614. Mean ± SD BIS values at 0.8, 1.0, 1.5, and 2.0 MAC were 65 ± 8, 60 ± 7, 52 ± 3, and 15 ± 27, respectively. At 2 MAC, three dogs had burst suppression that prevented reliable BIS determination. The BIS values were significantly higher during SEVO at 0.8 and 1.0 MAC compared to ISO. Minor, yet statistically significant, differences between and within anesthetic groups were observed for cardiovascular and arterial blood gas data. We conclude that a predictive relationship exists between BIS and end-tidal [ISO]. However, BIS correlated with end-tidal [SEVO] better than with [ISO]. The BIS appears to be somewhat agent dependent in this group of six dogs. Clinical use of BIS monitoring in dogs may require evaluation of specific anesthetic drug combinations.

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