Prospective observational study of the non-invasive assessment of immediate postoperative pain using the analgesia/nociception index (ANI).

BACKGROUND The analgesia/nociception index (ANI), a 0-100 non-invasive index calculated from heart rate variability, reflects the analgesia/nociception balance during general anaesthesia. The aim of this study was to evaluate the ANI in the assessment of immediate postoperative pain in adult patients undergoing general anaesthesia. METHODS Two-hundred patients undergoing scheduled surgery or endoscopy with general anaesthesia were included in this prospective observational study. Pain intensity was assessed using a 0-10 numerical rating scale (NRS) after arousal from general anaesthesia. Receiver-operating characteristic (ROC) curves were built to assess the performance of ANI to detect patients with NRS>3 and NRS ≥ 7 on arrival in the postoperative care unit. RESULTS A negative linear relationship was observed between ANI and NRS (ANI=-5.2 × NRS+77.9, r(2)=0.41, P<0.05). At the threshold of 57, the sensitivity and specificity of ANI to detect patients with NRS>3 were 78 and 80%, respectively, with a negative predictive value of 88%, corresponding to an area under the ROC curve (AUC) of 0.86. At the threshold of 48, the sensitivity and specificity of ANI to detect NRS ≥ 7 were 92 and 82%, respectively, with a negative predictive value of 99%, corresponding to a ROC curve AUC of 0.91. CONCLUSIONS A measurement of ANI during the immediate postoperative period is significantly correlated with pain intensity. The measurement of ANI appears to be a simple and non-invasive method to assess immediate postoperative analgesia.