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 prone position 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. To the best of our knowledge, the efficacy of the test was not discussed in the perioperative care in prone position surgery. The investigators design the study to investigate the effectiveness of mini-fluid challenge test in prone position surgery.

Inclusion Criteria: meet both of

1. Patients receiving scheduled prone position spine surgery
2. BMI 18.5~30 kg.m-2

Exclusion Criteria:

1. age younger then 20 yrs or elder than 80 yrs
2. pregnant women
3. patients in intensive care units
4. 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
5. patients with ongoing infection
6. patients allergic to voluven