Analgesia-nociception index accurately predicts inadequate pectoralis muscle fascia block (PECS) in patients undergoing breast surgery: A prospective observational study

Background Postoperative opioid administration has been largely replaced by regional anesthesia techniques. We aimed to determine whether intraoperative Analgesia-Nociception Index (ANI) can aid in early evaluation of the effectiveness of regional blocks such as the pectoralis muscle fascia block (PECS, pectoserratus and interpectoral plane blocks) and predicting the need for analgesics postoperatively. Methods This prospective observational study enrolled 30 women (age: 20–80 years) undergoing unilateral, non-intubated, breast tumor excision alone or in conjunction with sentinel lymph node biopsy. PECS block was performed following sedation. ANI readings were obtained at 1-min intervals, and polar coordinates were assigned to the distance from the nipple (0.5-cm intervals) and o'clock position (15-min intervals) for each reading. Pain scores were assessed using a numeric rating scale from 0 to 10, and analgesics were administered depending on pain score post-operatively. Results 8 (27%), 19 (63%), and 3 (10%) patients received morphine, tramadol, and no analgesics, respectively. In total, 954 ANI measurements were obtained. At the proposed cut-off of 50, the sensitivity and specificity of the ANI nadir for need of post-operative opioids were 0.875 and 0.932, respectively. Block effectiveness was most satisfactory in the upper lateral quadrant of the breast with nipple–areolar complex (NAC) sparing effect. Most average ANI measurements for the NAC were <50. No patient experienced postoperative nausea/vomiting, although one reported dizziness. Conclusions The intraoperative ANI nadir <50 was strongly correlated with need for postoperative opioids. The ANI may aid in objectively evaluating the effectiveness of pectoralis muscle fascial blocks and predicting postoperative need for analgesics.