Thoracotomy is known as one of the most painful surgical procedures, with up to 65% of patients developing chronic post-thoracotomy pain syndrome (PTPS), and approximately 10% experiencing pain that significantly impacts quality of life. Video-assisted thoracoscopic surgery (VATS) has become increasingly common over the past decade and offers reduced postoperative pain, morbidity, and length of hospital stay compared to open thoracotomy. However, VATS can still cause moderate to severe postoperative pain and a high risk of chronic pain. Optimizing analgesia after VATS remains critical. With advances in ultrasound technology, several regional anesthesia techniques such as serratus anterior plane block (SAPB), erector spinae plane block (ESPB), and thoracic epidural analgesia (TEA) have shown comparable efficacy. In 2023, Tulgar et al. described the serratus posterior superior intercostal plane block (SPSIPB), which demonstrated dermatomal coverage from C3 to T10. This randomized controlled trial aims to compare the analgesic efficacy of SPSIPB and SAPB in patients undergoing VATS procedures.

The patients will be divided into two groups and all will receive general anesthesia. Before extubation, one group will receive a Serratus Anterior Plane Block, and the other group will receive a Serratus Posterior Superior Intercostal Plane Block. The randomization of the study will be performed by a physician who will not be involved in the patient follow-up, using a computer-generated randomization code (generated by the computer). The interfascial plane block (Serratus Anterior Plane Block or Serratus Posterior Superior Intercostal Plane Block) will be given to the anesthesiologist in a sealed envelope by an independent assistant personnel outside the study. The patient will not be aware of which block is being applied. The anesthesiologist performing the block will not participate in the patients' pain monitoring. Postoperative pain assessment and data collection will be performed by another anesthesiologist who is unaware of the study.

For standardization purposes, the block procedure will be performed by an experienced anesthesiologist who has performed at least 20 successful and uncomplicated procedures before.

Inclusion Criteria:

• BMI < 35 kg/m² Patients with ASA scores I, II, III

Exclusion Criteria:

• Patients who do not want to be included in the study Psychiatric and neurological disease with blurred consciousness Patients with ASA > 3 BMI > 35 kg/m² Abnormality in coagulation parameters History of allergy to local anesthetic drugs Infection at the injection site Patients whose surgical procedure time exceeds 150 minutes

The anesthetist who will perform the blocks will not participate in the pain follow-up of the patients. Postoperative pain assessment and data collection will be performed by another anesthetist blinded to the study.