Intraoperative neurophysiological monitoring of the afferent pain pathway in cardiac surgery patients.

Objective: No specific and sensitive method is available in routine clinical practice to detect pain in anaesthetised patients during surgery. The main aim of the present study was to investigate whether intraoperative somatosensory evoked potentials, the bispectral index, electroencephalogram, blood pressure and heart rate change during painful stimulation in cardiac surgery patients. Methods: After induction of anaesthesia, 37 cardiac surgery patients were subjected to increasing electrical stimulation of both the median nerves and subsequent intravenous infusion of remifentanil to suppress this painful stimulation. Results: The higher intensities of electrical stimulation significantly modified the cortical evoked potentials, the electroencephalogram spectral edge frequency and blood pressure. We also observed the appearance of a middle-latency component in the somatosensory evoked cortical potentials between 60 and 70 ms. These neurophysiological and clinical responses were significantly reduced by remifentanil administration. Conclusions: The data suggest that somatosensory evoked potentials might be used to detect and monitor painful stimulation during surgery, unlike the bispectral index, which does not seem to be highly sensitive to intraoperative pain. Significance: Measurement of intraoperative somatosensory evoked potentials provides a specific and sensitive method to monitor the afferent pain pathway in anaesthetised patients.