Comparison of surface and subdermal electrodes for monitoring bispectral index in anesthetized dogs

Bispectral index (BIS) is a value derived from the EEG used to assess central nervous system depression. In a previous report, canine BIS values using adhesive surface electrodes were inversely related to sevoflurane (SEVO) minimum alveolar concentration (MAC)-multiples. However, use of subdermal needle electrodes would allow more practical measurement of BIS in animals. The purpose of this study was to compare canine BIS values obtained using both surface and subdermal electrodes. SEVO MAC was determined in eight English pointer dogs (four males, four females; mean ± SD age and weight of 3.9 ± 2.2 years and 20.7 ± 4.1 kg, respectively). One week later, BIS was determined at 0.8, 1.0, 1.5, and 2.0 MAC multiples using both electrode sets applied in randomized order. Ventilation was controlled and atracurium (0.2 mg kg−1 followed by 6 µg kg−1 minute−1 IV) was administered to eliminate EMG artifact from the EEG. BIS was determined using an A-2000 monitor connected to a computer for data collection at 5-second intervals. After a 15-minute equilibration period at each MAC multiple, BIS data were collected for 5 minutes and median BIS values 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 [SEVO] were continuously monitored using an infrared gas analyzer. Median BIS values were analyzed by simple linear regression and other variables were analyzed using one-way analysis of variance (p < 0.05). Mean ± SD SEVO MAC was 2.1 ± 0.3%. Mean ± SD BIS values at 0.8, 1.0, 1.5 and 2.0 X MAC were 77 ± 3, 73 ± 5, 57 ± 7 and 53 ± 7, respectively, for surface electrodes and 80 ± 6, 72 ± 7, 56 ± 4 and 50 ± 5, respectively, for subdermal electrodes. At 2X MAC, BIS could not be determined in six dogs due to burst suppression. The regression equation comparing electrodes was: BIS (subdermal) = − 5.5 + [1.1 X BIS (surface)]; R2 = 0.846. Of the other measured variables, only mean MAP was significantly different among MAC-multiples. We conclude that use of subdermal needle electrodes for measurement of BIS in anesthetized dogs is a reliable and practical alternative to use of adhesive surface electrodes.