This prospective, single-center observational study compares the effects of general anesthesia and spinal anesthesia on tissue perfusion in patients undergoing lower extremity surgery with tourniquet use. Tissue oxygenation in the limb distal to the tourniquet is monitored noninvasively using near-infrared spectroscopy (NIRS), and perfusion loss is quantified using an area-under-the-curve (AUC) approach. The primary objective is to evaluate whether spinal anesthesia better preserves distal tissue oxygenation during tourniquet inflation compared with general anesthesia. Secondary objectives are to assess reperfusion response after tourniquet release using changes in NIRS values at 20 minutes relative to baseline, the presence of early hyperemia (rSO₂ overshoot), and the association between tourniquet duration and perfusion loss. Additional exploratory analyses evaluate selected metabolic and inflammatory markers, including pH, lactate, and potassium.

Inclusion Criteria:

• Age 18 to 65 years
• Scheduled for elective lower extremity orthopedic surgery
• ASA physical status I or II
• Expected operation duration 30 to 120 minutes
• Expected tourniquet duration 30 to 90 minutes
• Informed about the anesthesia method and agreed to participate voluntarily
• No contraindication to neuraxial block

Exclusion Criteria:

• Refusal to participate
• History of peripheral arterial disease, diabetic neuropathy, or advanced vascular insufficiency
• Severe or uncontrolled systemic disease, including ASA III or higher
• Coagulopathy, local infection, or contraindication to spinal anesthesia
• Body mass index greater than 35 kg/m²
• Pregnancy or impaired consciousness
• Intraoperative hemodynamic instability or anticipated instability
• Need for additional intraoperative sedative or analgesic medication
• Inability to obtain adequate and reliable NIRS signal for technical reasons

Adults aged 18 to 65 years with ASA physical status I-II who were scheduled for elective lower extremity orthopedic surgery with tourniquet use at Prof. Dr. Cemil Taşcıoğlu City Hospital between 01 March 2025 and 30 September 2025. Eligible patients were assessed consecutively and enrolled after providing informed consent. Patients were analyzed in 2 observational cohorts according to anesthesia method used in routine clinical practice: general anesthesia and spinal anesthesia.

Individual participant data will not be shared because no formal data-sharing plan was established for this thesis-based observational study, and sharing is limited by institutional and ethics requirements.