The bispectral index during recovery from halothane and sevoflurane anaesthesia in horses

To record the bispectral index (BIS) when horses moved during either halothane or sevoflurane anaesthesia and when they made volitional movements during recovery from these anaesthetics.

Randomized prospective clinical study.

Twenty-five client-owned horses undergoing surgery aged 8.8 (+/- 5.3; 1-19) years (mean +/- SD; range).

Baseline BIS values were recorded before pre-anaesthetic medication (BIS(B)) and during anaesthesia (BIS(A)) maintained with halothane (group H; n = 12) or sevoflurane (group S; n =13) at approximately 0.8-0.9 x minimum alveolar concentrations (MAC). Bispectral indices were recorded during the surgery when unexpected movement occurred (BIS(MA)), during recovery when the first movement convincingly associated with consciousness was observed (BIS(M1)) and once sternal recumbency was achieved (BIS(ST)).

No significant difference in BIS(M1) was found between halothane- (85 +/- 7; 75-93) and sevoflurane- (87 +/- 10; 70-98) anaesthetized horses although BIS(A) was significantly (p = 0.0002) lower in group S (62 +/- 7; 53-72) than group H (74 +/- 7; 60-84). Differences between BIS(M1) and BIS(A) were significant in sevoflurane (p = 0.00001) and halothane recipients (p = 0.002) but were greater in group S (25 +/- 9; 4-38) compared with group H (12 +/- 10; -9-25). In six of eight horses, BIS(MA) values ranged between those recorded during anaesthesia and at first movement.

Bispectral indices appear to approximate levels of unconsciousness, suggesting that monitoring the BIS may assist equine anaesthesia. However, it does not predict intra-operative movement.