This is an observational study and as a diagnostic intervention, subjects in the study would receive mechanical ventilation with augmented tidal volume of 12ml/kg for 2min. Augmented ventilation is performed when the patient's PPV is within grey zone (9-13%). The investigators perform this procedure to every patients and do not assigh this intervention to the subjects of the study. Then, 6ml/kg of ballanced crystalloid loading will be infused to every patient.

When the patient's PPV is within grey zone, patient's tidal volume is maintained with augmented tidal volume of 12 ml/kg (from normal ventilation of 8ml/kg) for 2min duration.

Augmented ventilation (12ml/kg) will be performed when the participant's PPV is within grey zone (9-13%). PPV value will be collected automatically by Intelivue philips patient monitor. Percentage changes in stroke volume index by EV1000 according to fluid loading were used as principal indicators of fluid responsiveness. Patients were classified as responders or non-responders when increases in SVI were ≥ 10% or <10% after volume loading (crystalloid iv 6ml/kg). To test the abilities of augmented PPV to predict fluid responsiveness, areas under the receiver operating characteristics (ROC) curves of the responders [area under the curve (AUC) = 0.5: no better than chance, no prediction possible; AUC = 1.0: best possible prediction] will be calculated.

PPV value will be collected automatically by Intelivue philips patient monitor.