Influence of different remifentanil concentrations on the performance of the surgical stress index to detect a standardized painful stimulus during sevoflurane anaesthesia.

BACKGROUND Although measurement of cerebral hypnotic drug effect and muscle relaxation is common clinical routine during anaesthesia, a reliable measurement of the neurophysiological effects evoked by a painful stimulus is still missing. Recently, the surgical stress index (SSI) has been introduced as a surrogate measure of 'nociception'. The present study aimed to examine the influence of increasing remifentanil concentrations on the ability of SSI to detect a standardized painful stimulus during sevoflurane anaesthesia. METHODS Twenty-four patients received incremental or decremental doses of 0, 2, and 4 ng ml(-1) remifentanil effect-site concentration (Ce(remi)) during 0.7 MAC sevoflurane. Painful tetanic stimulation was applied at least 5 min after changing Ce(remi). SSI, heart rate (HR), response entropy (RE), state entropy (SE), RE-SE difference, and bispectral index (BIS) were obtained in each patient before and after stimulation. Further prediction of an author-defined response to painful stimulus was analysed. RESULTS SSI and BIS, but not HR, SE, RE, or RE-SE difference were significantly altered after stimulation. Change in SSI (Delta SSI) was significantly dependent on Ce(remi), as Delta SSI was [median (inter-quartile range)] 20 (15-31), 10 (1-19), and 3 (1-10) at 0, 2, and 4 ng ml(-1) Ce(remi). In 10 out of 63 cases, SSI detected response to stimulation, not detected by another variable. SSI was unable to predict movement after stimulation as P(K) value is 0.59 (0.09). CONCLUSIONS The SSI response to tetanic stimulation was dependent on the remifentanil concentration.