Neuromuscular block and the electroencephalogram during sevoflurane anaesthesia

The effect of neuromuscular block on the anaesthetic depth of hypnosis is an elusive question. We simultaneously investigated the influence of neuromuscular block on the bispectral index, a measure of hypnosis during general anaesthesia, and on the electroencephalogram. Patients were anaesthetized with sevoflurane. Noxious tetanic electrical stimulation was applied on two occasions: before and after profound neuromuscular block achieved with rocuronium. Neuromuscular block significantly attenuated the effect from noxious stimulation on electroencephalogram power and synchrony in the gamma band (P < 0.05), and the corresponding effect on bispectral index (P < 0.02). These findings are probably due to the reduced arousing afferent input from paralysed muscles, and not to changes in the frontal electromyogram.