Prediction of motor responses to surgical stimuli during bilateral orchiectomy of pigs using nociceptive flexion reflexes and the bispectral index derived from the electroencephalogram.

Movement responses to noxious stimuli during anaesthesia indicate nociception. Under experimental conditions consistent prediction of such movement responses has been achieved by the use of nociceptive flexion reflexes (NFRs). The aim of this study was to investigate the reliability of NFRs and the electroencephalogram bispectral index (BIS) in predicting motor responses to real surgical stimuli in pigs. The study was undertaken in 30 pigs undergoing bilateral orchiectomy under ketamine/azaperone anaesthesia. During the operation, movement responses to seven distinct surgical steps that provided noxious stimuli of different intensity were evaluated. Any movement response of limbs or the head was considered as a positive response. For each surgical step the values of NFRs and the BIS obtained just prior to the step were tested as predictors of movement responses. The prediction probability for movement responses was 0.58±0.04 for the BIS and 0.76±0.03 for the NFRs. It was concluded that NFRs but not the BIS can predict the effectiveness of ketamine anaesthesia in terms of the suppression of movement responses to surgical stimuli.