 Pulse pressure variation PPV is a well-known and widely used dynamic preload indicator based on heart-lung interaction to predict fluid responsiveness. Generally, patients are considered to be fluid-responsive when the PPV value larger than 11-13. However, several previous researches demonstrated that there is a zone of uncertainty grey zone in PPV. To predict fluid-responsiveness accurately in the patients with PPV within grey zone 9-13, the investigators would evaluate the augmented PPV using augmented ventilation. 