The Analgesia Nociception Index: a pilot study to evaluation of a new pain parameter during labor

BackgroundObjective pain assessment that is not subject to influences from either cultural or comprehension issues is desirable. Analysis of heart rate variability has been proposed as a potential method. This pilot study aimed to assess the performance of the PhysioDoloris™ analgesia monitor which calculates an Analgesia Nociception Index derived from heart rate variability. It was compared with visual analogical pain scores.MethodsForty-five parturients who requested epidural analgesia were recruited. Simultaneous couplets of pain scores and Analgesia Nociception Index values were recorded every 5 min regardless of the presence or absence of uterine contractions. The relationship between indices was characterized, and a cut-off value of Analgesia Nociception Index corresponding to a visual analogical score >30 (range 0–100) was used to determine the positive and negative predictive value of the Analgesia Nociception Index.ResultsThere was a negative linear relationship between visual analogical pain scores and Analgesia Nociception Index values regardless of the presence of uterine contractions (regression coefficient ± SEM = −0.18 ± 0.032 for entire dataset). Uterine contraction significantly reduced the Analgesia Nociception Index (P < 0.0001). Using a visual analogical pain score >30 to define a painful sensation, the lower 95% confidence limit for the Analgesia Nociception Index score was 49.ConclusionThe Analgesia Nociception Index has an inverse linear relationship with visual analogical pain scores. Further studies are necessary to confirm the results of this pilot study and to look at the influence of epidural analgesia on the Analgesia Nociception Index.