Background: The NOL index is based on multiparametric analysis of heart rate (HR), skin conductance, wave plethysmography, and their time derivative. The aim of this study was to evaluate the NOL to detect standardized nociceptive stimuli with various remifentanil dosages under general anesthesia.
Methods: A prospective, observational study at a single center (NCT02602379) included 40 ASA I to III patients undergoing laparotomy under remifentanil-desflurane anesthesia with epidural analgesia. A tetanic stimulation was applied (forearm) at remifentanil intravenous (IV) infusion of 0.005, 0.05, 0.1, and 0.15 µg/kg/min. NOL and its variations were compared with other parameters namely heart rate, mean arterial pressure, Bispectral Index, and Analgesia Nociception Index (ANI). Receiver operating characteristic (ROC) curves were plotted to assess the response to both intubation and standardized stimulus under remifentanil infusion of 0.005 µg/kg/min.
Results: The post-stimulation NOL values at remifentanil doses of 0.005, 0.05, 0.1 and 0.15 µg/kg/min (39 [23-55], 15 [7-30], 8 [4-14] and 8.5 [4-15]) were significantly higher than pre-stimulation counterparts (P<0.0001). For all other parameters, there was also significant difference between pre- and post-stimulation values at all remifentanil dosages (P<0.0001). Area under the ROC curve (AUC) for the NOL during standardized stimulation was larger than for all other parameters at the exception of ANI (P=0.94). The AUC of NOL for nociception during tracheal intubation was greater (0.93 vs. 0.84 and 0.64 for ANI and HR, respectively).
Conclusions: NOL monitoring is a promising index to assess the level of nociception in patients under general anesthesia.