As in previous studies,14-15 the sample size is determined by considering that an area under the ROC curve ≥ 0.8 is clinically reliable to predict fluid responsiveness. To detect a 0.3 difference from the null hypothesis of 0.5, 28 patients will be required in each group with a type-I error of 0.05 and a power of 80% under the ROC curve, assuming equal number of responders and non-responders. To compensate for a dropout rate of 10%, 31 patients will be included in each group.