Electroencephalographic Narcotrend index, spectral edge frequency and median power frequency as guide to anaesthetic depth for cardiac surgery in laboratory sheep

In order to provide objective measures of anaesthetic depth for periods without clinical signs (e.g., cardiopulmonary bypass [CPB]), the correlation between the electroencephalographic Narcotrend index (NI), 95% spectral edge frequency (SEF95), median power frequency (MPF) and clinical stages of anaesthesia was investigated in 16 juvenile sheep. Data were recorded during recovery from anaesthesia for pulmonary or aortic valve replacement.A significant (P < 0.05) negative correlation was found between clinical stages of anaesthesia and NI (rs = −0.534) and SEF95 (rs = −0.543). No significant correlation existed between anaesthetic stages and MPF (rs = −0.292, P > 0.05). The sensitivity of NI, SEF95 and MPF to assure an adequate level of anaesthesia was 71.43%, 66.67% and 44.44%, respectively, while the specificity of the descriptors ranged between 97.44 and 92.31%. No significant age-related effect on EEG data and stages of anaesthesia was detected when data from sheep between 4 and 6 months of age were compared with data obtained from 8- to 12-month old sheep. In conclusion, NI seems to be the most appropriate EEG descriptor to assure adequate depth of anaesthesia in juvenile, isoflurane-anaesthetized sheep.