Autonomic Parameters Correlated to Acute Postoperative Pain in the Postanesthesia Care Unit: A Systematic Review

Current pain scoring systems are being replaced by sensor-derived physiological parameters responding to autonomic changes (i.e., autonomic parameters such as heart rate variability and pupillary diameter) that quantify pain intensity at a specific moment. However, evidence supporting their use in postoperative patients is scarce. This review aims to analyze the correlation between autonomic parameters and self-reported pain intensity.

PubMed/Medline, Embase, Scopus, and Google Scholar were inquired for studies evaluating autonomic parameters in the assessment of acute postoperative pain in the postanesthesia care unit and measuring the correlation between these and pain intensity. The keywords used were related to heart rate variability, photoplethysmography (PPG), electrodermal activity, pupillary reflexes, postoperative pain, and pain measurement.

Out of 3,204 studies, 23 fulfilled the eligibility criteria. PPG features and indices had the strongest correlations with pain scoring systems, with correlation coefficients of -0.738 and 0.778 for the ratio of alternating current and direct current and perfusion index in obese patients, respectively. In addition, the variant coefficient of pupillary diameter and the analgesia nociception index had the highest sensitivity, while the pupillary light reflex amplitude had the highest specificity.

PPG and pupillary features are promising candidates for the objective assessment of postoperative pain. Studying these variables and the addition of machine learning algorithms for feature extraction will provide supporting data for their translation into the clinical setting.