Spectral entropy and bispectral index (BIS) can differentiate between conscious and unconscious levels in horses. This study aimed to determine whether spectral entropy and BIS parameters can distinguish between different end-tidal isoflurane concentrations (EtIso, %), and whether these parameters change when a nociceptive stimulus is applied. Ten clinically healthy horses were anaesthetized using xylazine, diazepam, ketamine, and isoflurane. The following parameters were monitored: EtIso, response entropy (RE), state entropy (SE), RE-SE difference, BIS, and electromyography (EMG). Three EtIso concentrations were randomly assigned: 0.9% (EtIso0.9), 1.2% (EtIso1.2), and 1.5% (EtIso1.5). Ten minutes after reaching the first EtIso, the parameters were recorded at baseline (Baseline) and for 5 minutes following the application of an electrically nociceptive stimulus (first two minutes, EarlyP
the last three minutes, LateP). This procedure was repeated three times for each EtIso. A two-way repeated measures ANOVA was performed. The RE, SE, BIS, and EMG parameters exhibited a significant increase upon the application of a painful stimulus. Notably, only the EMG parameter demonstrated significant differences between EtIso0.9 and EtIs1.5 during both EarlyP and LateP phases (EarlyP: EtIso0.9, 72±23 vs EtIso1.5, 48 ± 23, (P=0.011)
LateP: EtIso0.9, 65 ± 27 vs EtIso1.5, 43 ± 23, (P=0.022)). Currently, these monitors do not appear to be useful for assessing anaesthetic depth in horses under the studied conditions, although they do seem to be useful for detecting induced nociceptive stimuli.