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Table 1 Study protocol

From: Differences in heart rate variability may be related to the appearance of postoperative pain in patients undergoing breast cancer surgery

1. The study sample comprised 82 patients who underwent breast cancer surgery between 2015 and 2016 in our institution.
2. Patients were excluded if they met our exclusion criteria.
3. We then selected 20 patients who had no pain after surgery immediately upon admission to the post-anesthesia care unit (PACU) (with “no pain” defined as a Numerical Rating Scale (NRS) value between 0 and 2).
4. These 20 patients were divided into two groups: Group A (n = 16) had no pain upon admission to PACU and remained pain free upon discharge from PACU (12 h after surgery); group B (n = 4) had no pain immediately upon admission to PACU and experienced increasing pain 1 h after surgery while in PACU, requiring analgesic therapy.
5. NRS was measured immediately upon arrival in PACU, then 1 and 2 h after surgery.
6. Heart rate variability (HRV) was measured upon arrival in PACU and then 2 h after surgery.
7. These 20 patients (both group A (n = 16) and group B (n = 4) remained in PACU for 12 h after surgery.