Utility of the nociception level index to monitor intraoperative nociception and predict acute postoperative pain in thoracic surgery.

Purpose: Postoperative pain remains a significant adverse event after thoracic surgery, to which intraoperative nociception contributes. To measure intraoperative nociception, the Nociception Level (NOL) monitor is one option. This study aims to assess the NOL's utility for measuring intraoperative nociception and predicting acute postoperative pain and opioid consumption.

Methods: This observational study included 114 thoracic surgery patients (37 thoracotomy, 77 minimally invasive surgery) utilizing two analgesic approaches (40 peridural anaesthesia, 74 intercostal nerve block). NOL's utility to monitor responses to incision and nerve block was assessed at population (Wilcoxon-signed-rank tests) and individual level (ROC-analyses). NOL's predictive utility for postoperative pain and opioid consumption was analysed at population (Mann-Whitney-U tests) and individual level (multivariable linear regression).

Results: Population NOL significantly increased after incision and decreased after nerve block (p < 0.01/ p < 0.01) and individual detection of nociception was significantly better than chance (AUCs: 0.68 [95%CI 0.61-0.75] / 0.62 [95%CI 0.53-0.72]). However, NOL did not differ significantly between thoracotomy and minimally invasive surgery (p = 0.12) or peridural anaesthesia and nerve block (p = 0.16), despite significantly different postoperative pain and opioid consumption (p < 0.01). Multivariable analyses showed no significant effect of NOL on postoperative pain or opioid consumption.

Conclusion: NOL captures intraoperative stress with an accuracy allowing to differentiate large nociception changes at both the population and individual level after skin incision and nerve block. However, NOL was unable to differentiate between patients with plausibly different nociception levels, like patients undergoing different surgical or analgesic techniques. Consequently, while NOL can detect large nociceptive changes, its current accuracy may be insufficient to reliably guide individual analgesia in clinical practice.