State index of nociception correlates both with remifentanil concentration and level of painful stimulation during surgery

Background and Goal of Study. Today no numeric measures of nociception during general anaesthesia are available. In search of such readily useable measure both predictive characteristics of the drug effect and responses to surgical stimulation are needed. We analyzed the alterations of photoplethysmographic waveform amplitude (PPGA) and heart rate (HR) during changing surgical stimuli and different target site levels of remifentanil. Our purpose was to develop a State Index of Nociception (SN) based on PPGA and HR derived information, which could measure the adequacy of analgesia during general anaesthesia and surgery. Materials and Methods. Physiological parameters were measured from 53 female patients anaesthetized with propofol and remifentanil, and paralysed with rocuronium. Remifentanil target site concentration was adjusted to 1, 3, or 5 ng/ml during surgery. SN was used to explain the combination of stimulation and remifentanil effect site concentration. SN was correlated with stimulation and remifentanil for each patient and the significance was calculated using Wilcoxon sign rank test over the subjects . Results and Discussions. SN correlated positively with stimulation (median r=0.42, P<0.0001) and negatively with remifentanil level (median r=-0.33, P<0.0001). Median correlation with their difference was 0.50 (P<0.0001). High SN values indicated presence of noxious stimuli and low remifentanil levels. Conclusions. SN is sensitive to low analgesia levels during noxious stimulation, and provides information of the adequacy of anesthesia.