Perioperative fluid management is crucial for patients' outcome. Muller et al developed a "Mini-fluid challenge method " to predict fluid responsiveness and the efficacy. The investigators design the study to investigate the effectiveness of mini-fluid challenge test in video assisted thoracic surgery.

Perioperative fluid management is crucial for patients' outcome. Series of studies have indicated that adequate fluid management optimizes the cardiac out put, improves tissue perfusion, thus decrease the risk of postoperative morbidity. Muller et al developed a "Mini-fluid challenge method " to predict fluid responsiveness and the efficacy. Fluid status and proper perfusion condition in patients undergo thoracic surgery are especially crucial for the vulnerability of lung toward fluid overload. To the best of our knowledge, the efficacy of the test was not discussed in the perioperative care in video assisted thoracic surgery. The investigators design the study to investigate the effectiveness of mini-fluid challenge test in video assisted thoracic surgery.

Inclusion Criteria:

• Patients receiving scheduled video assisted thoracic surgery
• BMI 18.5~30 kg.m-2

Exclusion Criteria:

• age younger then 20 yrs or elder than 80 yrs
• pregnant women
• patients in intensive care units
• patients with the underlying disease including respiratory failure(FEV1/FVC < 70 % and FEV1 < 50%), heart failure(NYHA score =III、IV), kidney failure(eGFR< 60 ml.min-1.1.73m-2), liver failure
• patients with ongoing infection
• patient allergic to voluven