Intraoperative Nociception Monitoring Using the NoL Index: Phase-Specific Assessment of Nociceptive Responses During Spinal Surgery

<b>Background</b>: Quantifying intraoperative nociceptive responses under general anesthesia remains challenging, particularly during complex procedures such as spinal surgery. The Nociception Level (NoL) index is a multiparametric tool designed to reflect the dynamic balance between nociception and analgesia in anesthetized patients. This study aimed to evaluate NoL fluctuations during predefined phases of spinal surgery and assess their relationship to anesthetic administration. <b>Methods</b>: This prospective observational study enrolled 44 adult patients undergoing lumbar discectomy, laminectomy, or spinal fusion under remifentanil-propofol anesthesia. Continuous NoL monitoring was performed using the PMD100™ system. Sixteen anatomically and procedurally defined surgical phases were analyzed. The primary outcome was the mean NoL value in each phase. The secondary outcome was the association between NoL values and intraoperative infusion rates of remifentanil and propofol. Repeated-measures ANOVA with Bonferroni correction was used for phase comparisons. <b>Results</b>: Mean NoL values remained within the target range (10-25) in most phases. However, significant elevations were observed during pedicle screw insertion (mean 27.9, SD ± 17.7), cage insertion (27.6, SD ± 10.5), and flavectomy (28.0, SD ± 27.0), indicating increased nociceptive burden. The lowest NoL values occurred during skin closure (16.6, SD ± 11.2) and discectomy (18.0, SD ± 2.8). Propofol and remifentanil infusion rates remained within standard clinical ranges but were slightly elevated during high-NoL phases. <b>Conclusions</b>: Despite standardized anesthesia, distinct nociceptive peaks were observed during specific stages of spinal surgery. These findings suggest that NoL monitoring may help identify high-nociception phases and guide tailored analgesic strategies. Future randomized trials are warranted to assess whether protocolized NoL-guided anesthesia improves intraoperative management and postoperative outcomes.