Participants for the study will be selected based on a thorough preoperative assessment and will be scheduled for lower abdominal or lower limb surgeries. After obtaining informed consent, each participant will undergo a sensory evaluation to establish baseline sensory function. In the operating room, an 18G IV catheter will be inserted, and standard monitoring, including non-invasive blood pressure, ECG, and SpO2, will be implemented. A bolus of 0.9% normal saline will be administered to stabilize hemodynamics before the subarachnoid block. Monitoring will include a pulse oximetry probe on the non-operating limb and skin temperature measurements, with the operating room maintained at 22-24°C. Participants will receive 0.02 mg/kg IV midazolam for sedation. During the subarachnoid block procedure, participants will be positioned sitting, and a subarachnoid puncture will be performed at the L3-L4 or L4-L5 intervertebral spaces with 12.5 to 15 mg of bupivacaine administered. Perfusion Index (PI) will be recorded immediately after the needle is removed. Post-administration, PI, Mean Arterial Pressure (MAP), sensory response to a pin prick test, and motor blockade will be monitored at intervals of 2, 4, 6, 8, 10, 12, 16, and 20 minutes. Surgery will proceed only if a sensory block at the T10 level is confirmed; otherwise, general anesthesia will be used.