Appropriate indices to guide fluid therapy are essential for effective hemodynamic management during the perioperative period. Values derived from respiratory changes in arterial pressure waveform or plethysmographic waveform such as stroke volume variation and pleth variability index have been demonstrated to be able to predict fluid responsiveness in surgical or critically ill patients. Their ability to predict fluid responsiveness can be affected by factors that influence the arterial tone or the compliance of the respiratory system by position change. The aim of this prospective trial is to investigate the ability of stroke volume variation and pleth variability index to predict fluid responsiveness in the reverse trendelenberg position.

Inclusion Criteria:

1. Age ≥ 20
2. elective robot assisted thyroidectomy

Exclusion Criteria:

1. arrhythmia
2. BMI>30
3. valvular heart disease
4. pul hypertension
5. peripheral artery occlusive disease
6. lung disease
7. right heart failure