Fluid responsiveness (FR)refers to the ability of heart to increase its stroke volume in response to volume load.Low tidal volume and high PEEP exerts contrast effect on the prediction of fluid responsiveness, the aim of this study is to compare the relative predicting power of the dynamic preload indicator (PPV, SVV), passive leg raising test, and pleth variability index (PVI) on the fluid responsiveness of acute respiratory distress syndrome ventilated with various PEEP levels or various tidal volumes.