Effect of postoperative peripheral nerve blocks on the analgesia nociception index under propofol anesthesia: an observational study

The analgesia nociception index (ANI), also referred to as the high frequency variability index (HFVI), is reported to be an objective measure of nociception. This study investigated changes in ANI after peripheral nerve blocks (PNB) under general anesthesia. Understanding these changes could enhance assessment of PNB efficacy before emergence from general anesthesia. This study enrolled 30 patients undergoing elective upper limb surgery. After surgery, median and maximum ANI values were recorded during two periods: a 5-minute period before PNB and a 20-minute period after PNB. The numeric rating scale (NRS) for pain was assessed twice: immediately after emergence from general anesthesia (N1) and the maximum pain experienced by the following morning after PNB effects subsided (N2). The difference in ANI before and after PNB was tested using the Wilcoxon signed-rank test. Statistical significance was set at P < 0.05. The ANI significantly increased after PNB in both the median (pre vs. post PNB value: 53.5 [44.0–68.0] vs. 59.0 [47.0-78.3], median [interquartile range]; P < 0.05) and maximum values (64.0 [56.3–79.5] vs. 74.5 [61.5–85.3]; P < 0.01). Secondary analysis revealed that significant ANI increases in both median (48.0 [42.3–66.5] vs. 61.0 [50.0-76.5]; P < 0.01) and maximum values (58.5 [50.3–75.3] vs. 76.0 [71.8–83.5]; P < 0.01) in the 18 cases with N2 ≥ 4 whereas no statistical differences were observed in the 12 cases with N2 < 4. The increased ANI value after PNB under propofol anesthesia may be a valuable indicator for assessing PNB efficacy. UMIN000050334. February 28, 2023.