The value of pupillary diameter in evaluating pain perception after awakening in patients undergoing general anesthesia during orthopedic surgery

Abstract Background The pupillary response to tetanic electrical stimulation has been used to reflect the balance between nociceptive stimuli and analgesia. Although pupillary pain index (PPI) was utilized to predict postoperative pain, it depends on tetanic stimulation and is complex. Our aim was to describe the potential relationship between PD in the presence of surgical stimulation and pain levels after awakening. Methods According to the Verbal Rating Scale (VRS) score after extubation, the patients were divided into painless group (VRS = 0) and pain group (VRS ≥ 1). Pupillary diameter (PD) and pupillary light reflex velocity (PLRV) were compared between two groups when patients entered the operating room (T1), before incision (T2), 10 s after incision (T3), 30 s after incision (T4), 1 h after incision (T5), at the end of surgery (T6), shortly after extubation (T7), and when patients expressed pain clearly (T8). The magnitude of PD change (ΔPD) compared to the baseline value after anesthesia induction (T2) is calculated. The correlations between pupillary parameters and pain after awakening was calculated Results Patients with VRS ≥ 1 had greater PD than painless patients at T3-7 (P = 0.04, 0.04, 0.003, <0.001, <0.001), and it was positive correlated with VRS score after awakening at T4-7 (r = 0.188, 0.217, 0.684, 0.721). The ability of T6ΔPD to predict VRS ≥ 1 was strong [threshold: 20.53%, area under the curve (AUC): 0.93, 95% confidence interval (CI): 0.89–0.97 ]. Conclusion Our study suggested that PD could be a useful index to direct the individualized analgesics used during operation, in order to better avoid the occurrence of pain during the postoperative emergence period. Trial registration: This study was registered with the Chinese Clinical Trial Registry (registration number: ChiCTR2000040908, registration date: 15/12/2020).