Evaluation of the bispectral index as an indicator of degree of central nervous system depression in isoflurane-anesthetized horses

Objective-To determine whether the bispectral index (BIS) can be used, as an indicator of degree of CNS depression in isoflurane-anesthetized horses. Animals-10 Standardbred and 6 Norwegian cold-blooded trotter stallions admitted for routine castration. Procedure-A 2-channel referential electrode configuration was used to record EEG for calculation of BIS by the EEG monitor. The BIS was calculated before (awake) and after (sedated) administration of detomidine (0.01 mg/kg of body weight, IV) and butorphanol (0.01 mg/kg, IV). Anesthesia was induced with ketamine hydrochloride (2,5 mg/kg, IV) and diazepam (0.04 mg/kg, IV) and maintained with isoflurane delivered in oxygen. The BIS was calculated after 30 minutes of equilibration, at an end-tidal isoflurane concentration of 1.4% (n = 8) or 1.9% (8) and recorded continuously during surgery. Results-Bispectral index was significantly less in sedated and anesthetized horses, compared with awake horses. However, BIS was, not significantly different between sedated and anesthetized horses. Mean BIS in horses anesthetized at 1.9% isoflurane was significantly greater, compared with horses anesthetized at an endtidal concentration of 1.4%.. Four horses in the 1.4% group moved during surgery and BIS increased immediately prior to movement in 2 of these horses. Conclusions and Clinical Relevance-BIS is not a precise indicator of degree of CNS depression in isoflurane-anesthetized horses. Thus, determination of BIS may not be a useful technique for monitoring anesthetic depth in isoflurane-anesthetized horses.