Relationship Between Local Anesthetic Infiltration Pain Before Spinal Anesthesia and the Analgesia Nociception Index for Women Undergoing Elective Cesarean Delivery: A Prospective Observational Study

Background Heart rate variability-derived indices, such as the high-frequency vitality index (HFVI)/analgesia nociception index (ANI), have been proposed as objective pain assessment tools. However, their ability to predict pain perception during local anesthetic injection (LAI) remains unclear. Methods This single-center observational study included 44 pregnant women scheduled for cesarean delivery under spinal anesthesia. The HFVI/ANI was measured for five minutes before LAI using a Root® monitor (Masimo Corp., Irvine, CA). The primary outcome measure was the mean HFVI/ANI before LAI during spinal anesthesia. The secondary outcomes included the area under the receiver operating characteristic curve to determine the cut-off and predictive accuracy. Results There was no significant difference in the pre-LAI HFVI/ANI between the patients with visual numeric rating scale (VNRS) of ≤3 and those with VNRS of >3 (HFVI: 64.25 ± 11.8 vs. 62.64 ± 12.3, P = 0.66). ROC analysis revealed poor predictive ability (AUC = 0.54, 95% CI: 0.36-0.72). No significant correlation was found between HFVI/ANI and VNRS. Conclusion Pre-LAI HFVI/ANI values were not associated with pain perception during LAI during cesarean delivery. Further studies are needed to explore optimal indicators for HFVI/ANI-based pain assessment.