Relationship of Canine Bispectral Index to Multiples of Sevoflurane Minimal Alveolar Concentration, Using Patch or Subdermal Electrodes

Purpose: The purpose of the study reported here was to determine the relationship of bispectral index (BIS) to multiples of sevoflurane minimal alveolar concentration (MAC), using proprietary patch or subdermal needle electrodes in dogs.

Methods: Eight English Pointers (4 males, 4 females; mean ± SD age and body weight of 3.9 ± 2.2 years and 20.7 ± 4.1 kg, respectively) were studied. Sevoflurane MAC was determined in each dog, using the standard tail clamp technique. One week later, BIS was determined in each dog at 0.8, 1.0, 1.5, and 2.0 MAC multiples of sevoflurane, using proprietary patch electrodes and 29-gauge platinum needle electrodes applied in randomized order. Ventilation was controlled, and atracurium (0.2 mg/kg of body weight, followed by 6 g/kg/min, i.v.) was administered to eliminate the electromyographic artifact from the electroencephalogram. The BIS was determined, using an A-2000 BIS monitor connected to a computer for data logging at five-second intervals. After a 15-min equilibration period at each sevoflurane MAC-multiple, BIS data were collected for five minutes, and median BIS values were calculated. Heart rate, direct mean arterial blood pressure, esophageal temperature, and arterial pH and blood gas tensions were measured immediately after each BIS collection period. End-tidal CO₂ and sevoflurane concentrations were continuously monitored, using an infrared gas analyzer. Data were analyzed, using one-way repeated measures analysis of variance (P < 0.05). Agreement of BIS values from each electrode type was determined.

Results: Mean ± SD sevoflurane MAC was 2.1 ± 0.3%. Mean ± SD BIS values at 0.8, 1.0, 1.5, and 2.0 MAC were 77 ± 3, 73 ± 5, 57 ± 7, and 53 ± 7, respectively, for patch electrodes and 80 ± 6, 72 ± 7, 56 ± 4, and 50 ± 5, respectively, for subdermal needle electrodes. At 2 MAC, BIS could not be determined in six dogs due to presence of burst suppression in the EEG. The regression equation comparing electrodes was: BIS (subdermal) = −5.5 + (1.1 × BIS [patch]); R² = 0.846; bias = −0.192, with a 95% confidence interval of −9.96 to 9.56.

Of the other measured variables, none were significantly different between electrode types. Within each group of electrode type, MAP was significantly different among MAC multiples. Within the patch electrode group, PaO₂, bicarbonate concentration, and base excess were significantly different among MAC multiples.

Conclusions: Bispectral index significantly decreased with increasing sevoflurane MAC multiples over the range of 0.8 to 2.0 MAC, using patch or subdermal electrodes in dogs. Use of subdermal needle electrodes is a reliable and practical alternative to use of patch electrodes for measurement of BIS in dogs.