The goal of this observational study is to clarify the relationship between upper and lower limb blood pressure in thyroid surgery patients under general anesthesia. The hemodynamic state during anesthesia is different from that during the awake state. Therefore, to assess whether the difference in blood pressure between the upper and lower limbs observed in the awake state is consistent with that observed under general anesthesia, the investigators will compare the inter-limb blood pressure differences before anesthesia induction and during general anesthesia.

Non-invasive blood pressure measurement during surgery is typically conducted on the upper arm, and previous studies about the relationship between intraoperative blood pressure fluctuation and postoperative mortality and morbility have also based on the upper arm measurements. However, in clinical practice, if monitoring blood pressure in the upper arm is not feasible during surgery, the cuff usually be placed on the ankle.

According to a previous study, the systolic blood pressure at the ankle is approximately 17.0 mmHg higher than the systolic blood pressure at the arm in the supine position (95% CI 15.4-21.3 mmHg), while the diastolic blood pressure did not show significant differences in awake state. Since the hemodynamic state of patients under anesthesia differs from that of awake patients, there is insufficient evidence to directly apply the known differences between upper and lower limb blood pressures in awake patients to set target blood pressure ranges during anesthesia. As a result, anesthesiologists often rely on clinical judgment in these situations.

Therefore, this study sets the null hypothesis that 'the differences in blood pressure between the upper and lower limbs before and after anesthesia are the same.' The goal is to determine whether the previously known relationship between upper and lower limb blood pressures in awake patients can be applied to patients under general anesthesia. Additionally, this study aims to investigate the relationship between upper and lower limb blood pressures when the lower limb blood pressure is outside the normal range during general anesthesia.

Inclusion Criteria:

• Patients having elective thyroid surgery under general anesthesia
• Aged between 20 to 80 years old
• American Society of Anesthesiologist(ASA) Physical Status Class I or II
• Individuals who have voluntarily given written consent to participate in this clinical study

Exclusion Criteria:

• Patients with Diabetes Mellitus or Peripheral vascular disease
• Patients with a history of Heart disease; Myocardial infarction, Heart failure, etc.
• Patients with Arrhythmia
• Patients with musculoskeletal abnormalities
• Patients for whom the placement of a non-invasive blood pressure cuff is contraindicated: inflammatory diseases or wounds causing pain at the cuff site, severe edema, thrombophlebitis, etc.
• Other cases deemed unsuitable for the study by the researcher: absence of appropriately sized blood pressure cuffs, placement of a pressurized blood pressure cuff expected to affect the induction and maintenance of general anesthesia, history of mental illness, etc.

Patients receiving elective thyroid surgery under general anesthesia