This study aims to investigate whether heart rate variability (HRV), a marker of autonomic nervous system activity, can predict the occurrence of rebound pain after total knee arthroplasty (TKA). Rebound pain is defined as a sudden and intense pain episode (NRS ≥7) that typically arises after the resolution of peripheral nerve blocks used for postoperative analgesia.

Patients undergoing elective unilateral TKA under spinal anesthesia with peripheral nerve blocks will be included. HRV will be measured both before and after surgery using a chest-worn heart rate monitor. Pain levels, analgesic consumption, sleep quality, and patient satisfaction will also be recorded. The primary goal is to determine whether perioperative HRV values can serve as a predictive biomarker for rebound pain.

Secondary outcomes include the relationship between HRV and pain intensity, opioid use, sleep quality, and length of hospital stay. The findings may contribute to developing individualized pain management strategies for TKA patients.

Inclusion Criteria:

• Age between 18 and 70 years
• American Society of Anesthesiologists (ASA) physical status I-III
• Scheduled for elective, unilateral total knee arthroplasty under spinal anesthesia
• Planned use of multimodal analgesia including peripheral nerve blocks
• Ability to provide written informed consent
• Ability to understand and complete pain diaries and questionnaires

Exclusion Criteria:

• Known allergy to local anesthetics
• Contraindications to regional anesthesia
• Contraindications to spinal anesthesia
• ASA physical status IV or higher
• Pregnancy or breastfeeding
• Neurological or psychiatric disorders impairing ability to participate or report outcomes
• Use of anticoagulant therapy or presence of coagulation disorders
• Infection at the proposed injection site
• Emergency (non-elective) surgical procedures

Adult patients aged 18 to 70 years scheduled for elective unilateral total knee arthroplasty under spinal anesthesia at a tertiary care hospital. All participants will receive multimodal analgesia including peripheral nerve blocks as part of standard care.

The individual participant data will not be shared due to institutional policy and patient confidentiality considerations.