Is acute postoperative pain reflected by a change in parasympathetic cardiac tone? Evaluation of the Analgesia Nociception Index

Background: The Analgesia Nociception Index (ANI; Metrodoloris, France) is a 0-100 score which is calculated via the heart rate variability-based assessment of cardiac parasympathetic tone. As the latter is reduced with pain/nociception, ANI could potentially be a tool for monitoring acute postoperative pain. However, the score has not yet been validated for this purpose. Thus the aim of our trial was to investigate the relationship between ANI and acute postoperative pain. Methods: After institutional approval, acute pain was repeatedly assessed in 50 recovery room patients via a 0-10 point numeric rating scale (NRS), while ANI was simultaneously measured using a stand-alone Metrodoloris ANI monitor which derived patient electrocardiogram data via two single use ANI-electrocardiogram electrodes applied in V1 and V5 position. Results: A total of 443 NRS-ANI were obtained from 46 patients (31+14 years). ANI and NRS showed a small, but significant, negative correlation (Spearman's rho: -0.13; P=0.017). A trend towards lower ANI readings in higher levels of pain was observed (mean [standard error of the mean]; NRS 0: ANI 60.2 [2.2] vs NRS 6-10: ANI 56.8 [2.1]; not significant). Receiver operating curve analysis for ANI as a detector for severe pain (NRS >5) did not show a significant predictive value (sensitivity/specificity). Conclusion: Despite a significant correlation, ANI differences between states of pain were small and unlikely of clinical value. As confounders such as anxiety or agitation cannot be excluded in recovery room patients, any monitor based on sympatho-vagal balance may be more valuable in anaesthetised subjects.