Comparison of bispectral index and spectral entropy during isoflurane and medetomidine general anaesthesia in horses.

Abstract

The bispectral index (BIS) has been evaluated as an indicator of central nervous system (CNS) depression in horses during general anaesthesia. The spectral entropy is another electroencephalographic device and it has not been evaluated yet in horses. To determine if spectral entropy can assess anaesthetic depth during the different phases of anaesthesia, define the value of state and response entropy during surgical plane of anaesthesia and compare them with BIS. Clinical, prospective, non-blinded observational study. Thirty-five horses ASA I or II undergoing scheduled surgical procedure were used. BIS and electromyography (EMG) with a BIS monitor and state and response entropy with a spectral entropy monitor were recorded at baseline after receiving 5µg/kg bwt i.v. of medetomidine (sedation period), during the anaesthetic maintenance with isoflurane and medetomidine (intraoperative period) and once the trachea was extubated (recovery period). A general linear model for repeated measurements was employed. Correlation and agreement between methods were also assessed. Data are presented as mean±SD. State entropy, response entropy and EMG showed significant differences according to the anaesthetic period (P<.001). There was no significant difference in BIS between baseline and sedation period, but there were differences between the remainder of the periods (P<.001). BIS (53.4±11.2) was significantly higher (P<.001) than response entropy (35.1±7.1) and state entropy (27.4±4.8) during surgical plane of anaesthesia. The ICC between BIS and response entropy was 0.56 and between BIS and state entropy was 0.43, without agreement between them. The need to shave the skin in contact with the sensors and the difficulty in taking measurements during recovery period. Spectral entropy can be used to detect the different periods of an anaesthetic protocol, with the lowest values during the intraoperative period. A low correlation and no concordance were observed between both methods.