Identification of the intraoperative antinociceptive effect of intravenous fentanyl using the Nociception Level (NOL) index versus clinical parameters in patients undergoing gynecological laparoscopic surgery: A secondary analysis of the NOLGYN study

Background: While we typically assess nociception balance during general anesthesia through clinical parameters such as heart rate (HR) and mean arterial pressure (MAP) variation, these parameters are not specific to nociception.

Objective: We hypothesized that using the Nociception Level (NOL) index to assess the analgesic effect of a fentanyl bolus would be superior to standard clinical parameters.

Design: Ancillary study of the NOLGYN study, a randomized controlled trial comparing intraoperative NOL-guided administration of fentanyl (NOL group) versus standardized care (SC group).

Setting: University hospital in Montréal, Canada between November 2018, and December 2019.

Patients: Women undergoing gynecological laparoscopic surgery.

Intervention: In our evaluation of intraoperative nociception, we analyzed the analgesic effect of fentanyl using three parameters: MAP, HR, and the Nociception Level (NOL) index. All fentanyl injection events were extracted from the database.

Main outcome measure: The primary endpoint was the difference between values before and after each injection.

Results: The median of the NOL index before fentanyl injection was 30.5 (IQR 19.4 to 40.7) versus 18.9 (IQR 11.5 to 27.4) after (P < 0.001). The median of MAP was 106.4 mmHg (IQR 99.9 to 113.4) before injection versus 103.2 mmHg (IQR 97.5-110.7) after (P < 0.001). The median of HR before injection was 74.2 (IQR 64.2-83.8) versus 72.4 (IQR 63.4-81.3) after (P < 0.001).

Conclusions: The NOL index, HR, and MAP all statistically discriminated the analgesic effect of fentanyl but only the NOL index proved clinically relevant to identify the analgesic effect of one fentanyl injection.

Trial registration: www.

Clinicaltrials: gov (NCT03776838) registered in October 2018.