Comparison of the nociceptive flexion reflex threshold and the bispectral index as monitors of movement responses to noxious stimuli under propofol mono-anaesthesia

Prediction of movement responses to noxious stimuli during anaesthesia is of clinical importance. Susceptibility of a parameter of immobility to both hypnotic and analgesic influences could pose an advantage. Here, nociceptive reflexes might be useful, but data regarding the suppression by hypnotic substances are scarce. Therefore, we compared the prediction of movement responses by the RIII reflex threshold and the bispectral index (BIS) during propofol mono-anaesthesia. Fifteen male volunteers were included. Propofol effect compartment concentration was increased every 15 min in steps of 1 μg ml−1 (max 7 μg ml−1). Every 5 min, the reactions to trapezius squeezes and 30 s tetanic stimulations (80 mA) of the right ulnar nerve were tested. The RIII reflex threshold was estimated continuously using an automated threshold tracking system that analyses the nociceptive RIII response at the left biceps femoris muscle to stimulation of the left sural nerve. Twelve subjects completed the study. RIII threshold values were normalized by subtraction of the first threshold that was estimated after the subject's loss of consciousness. The population prediction probability PK amounted to 0.84 for the RIII threshold and to 0.86 for the BIS (difference not significant). Movement responses to noxious stimuli under propofol can be predicted by the RIII threshold with a comparable accuracy as the BIS. Therefore, the RIII threshold seems to be influenced by hypnotic effects. Since susceptibility of the RIII threshold to analgesic influences is well established, an advantage for the RIII threshold in the prediction of motor responses could be expected when analgesic substances are used in addition to propofol.