A Comparison of the Analgesia Nociception Index and Nociception Level Index Under General Anesthesia With Propofol and Remifentanil

BACKGROUND: Adequate analgesia during general anesthesia is fundamental to minimize stress responses and optimize perioperative outcomes. Traditional monitoring based on nonspecific autonomic responses may be confounded by nonnociceptive factors. Newer nociception monitors, including the Analgesia Nociception Index (ANI) and the Nociception Level Index (NOL), offer alternative approaches, yet direct comparisons in the same patient population remain limited. METHODS: In this prospective study, 30 adult patients undergoing general anesthesia with propofol and remifentanil target-controlled infusion were monitored concurrently with ANI and NOL. Recorded values were categorized into insufficient, adequate, or excessive analgesia according to manufacturer thresholds. Agreement between monitors was evaluated. A 1-category disagreement was where monitors disagreed by one level (eg, insufficient vs adequate or adequate versus excessive). A 2-category disagreement was where they disagreed by 2 levels (eg, insufficient versus excessive). RESULTS: Across more than 38 hours of time-matched recordings, the monitors agreed in 43.7% of the monitoring time. One-category and 2-category disagreements were observed in 37.1% and 8.7% of recordings, respectively. Excessive analgesia was noted in 37.6% (ANI) and 38.1% (NOL) of surgical time. Each monitor reported "optimal analgesia" on around 31% of the time and insufficient analgesia 25% of the time. CONCLUSIONS: The modest agreement between ANI and NOL underscores discrepancies likely stemming from their different methodological approaches. These findings suggest that a multimodal strategy integrating various monitoring modalities may enhance intraoperative analgesic management.