Objective assessment of the immediate postoperative analgesia using pupillary reflex measurement: a prospective and observational study.

BACKGROUND The evaluation of pain intensity during the immediate postoperative period is a key factor for pain management. However, this evaluation may be difficult in some circumstances. The pupillary dilatation reflex (PDR) has been successfully used to assess the analgesic component of a balanced anesthetic regimen. We hypothesized that PDR could be a reliable index of pain intensity and could guide morphine administration in the immediate postoperative period. METHODS One hundred patients scheduled to undergo general surgery were included in this prospective observational study. Pain intensity was assessed by using a simple five-item verbal rating scale (VRS). After patients awoke from general anesthesia, those experiencing mild or more severe pain (VRS more than 1) received intravenous morphine titration. Before and after intravenous morphine titration, the PDR induced by a standardized noxious stimulus was measured with a portable pupillometer. A receiver-operating curve was built to estimate the accuracy of PDR in objectively detecting patients requiring morphine titration. Results are given as median (95% CI). RESULTS On the initial evaluation, a correlation was found between VRS and PDR (ρ = 0.88 [0.83-0.92], P < 0.0001). In the 39 patients that had a VRS more than 1, PDR before and after morphine titration was respectively 35% (31-43) versus 12% (10-14); P < 0.0001. The PDR threshold value corresponding to the highest accuracy to have VRS more than 1 was 23%, with 91% and 94% sensitivity and specificity, respectively. CONCLUSION In the immediate postoperative period, the PDR is significantly correlated with the VRS. The pupillometer could be a valuable tool to guide morphine administration in the immediate postoperative period.