Bispectral index and the clinically evaluated anaesthetic depth in dogs

Objective This study investigated whether the bispectral index (BIS monitor) corresponded with the clinical assessment of anaesthetic depth in dogs. Study design Prospective clinical study. Animals Sixty-five dogs undergoing anaesthesia for surgery. Methods Dogs were assigned to one of three different anaesthetic techniques. A three point scale was devised to determine the clinical depth of anaesthesia (CDA); CDA 1 represented light, CDA 2 surgical and CDA 3 excessive depth of anaesthesia. BIS values were recorded and CDA assessed at specific times and points throughout surgery. Data were statistically analysed using mixed model regression. Results Clinical depth of anaesthesia was assessed as CDA 1 on 68, 2 on 748 and 3 on four occasions. The BIS recorded for CDA 1 differed significantly from that for CDA 2 (p < 0.001). However, individual BIS values measured at light and surgical levels of anaesthesia overlapped considerably. The sensitivities and specificities calculated for BIS to diagnose CDA 1 compared to CDA 2 in the three anaesthetic protocols were 28-86% and 55-85%. The accompanying positive predictive value was 0.08-0.29 and the negative predictive value was 0.95-0.97. End-tidal isoflurane concentrations (anaesthetic techniques 1 and 3) and propofol infusion (technique 2) at CDA 1 was significantly lower than those at CDA 2 (p = 0.001). Conclusions Although BIS values overall distinguished between CDA scores, the calculated specificities, sensitivities and predictive values were low, and there were anomalous results in individual cases. Clinical relevance The use of the BIS as the sole method to determine anaesthetic depth in dogs is imprudent.