The beach chair position for shoulder operation induces hypotension during general anesthesia. Hypotension is induced by decreased preload and contractility due to anesthesia. Masimo is a pulse oximetry which shows pleth variability index noninvasively. The pleth variability may predict the hypotension during beach chair position.

The beach chair position for shoulder operation induces hypotension during general anesthesia. Hypotension is induced by decreased preload and contractility due to anesthesia. Masimo is a pulse oximetry which shows pleth variability index noninvasively. The investigators put the masimo probe before general anesthesia and recorded the values before and after beach chair position, The values include arterial pressure, heart rate, blood pressure, perfusion index, pleth variability index.

The primary end point was the predictive value and threshold or pleth variability index for hypotension induced by the beach chair position. The occurrence of hypotension was counted by using vasopressor. The use of vasopressor was performed when mean blood pressure less than 65 mmHg or 20% of baseline.

The investigators planned to recruit 57 patients.

Inclusion Criteria:

• elective shoulder surgery under general anesthesia

Exclusion Criteria:

• right heart failure
• morbid obesity (BMI > 30)
• arrhythmia on preoperative electrocardiography
• pulmonary hypertension
• peripheral vascular disease or pulmonary disease
• pregnant

age between 20 and 70 years who is planning to undergoing elective shoulder surgery under general anesthesia