This study evaluates predictability of fluid responsiveness of pressure-based dynamic variables such as pulse pressure variation and systolic pressure variation, according to tidal volume change in patients undergoing cardiac surgery.

Pressure-based dynamic variables such as pulse pressure variation(PPV) and systolic pressure variation(SPV) are known to be unreliable for prediction of fluid responsiveness in children.

The hypothesis is that tidal volume change in mechanically ventilated children undergoing anesthesia would affect reliability of aforementioned dynamic variables in prediction of fluid responsiveness, especially in the way that reliability increases for high tidal volume.

In children undergoing cardiac surgery, tidal volume is changed to 6mL/kg, 10mL/kg and 14mL/kg after closure of sternum, followed by measurement of PPV, SPV. We also measure the respiratory variation of aortic blood peak velocity(△Vpeak) via transesophageal echocardiography, which is known to best predict fluid responsiveness.

Afterward, 10mL/kg of crystalloid solution is administered for fluid loading. 'Fluid responder' is defined as subjects with increase of stroke volume index more than 15% after fluid loading of 10mL/kg.

With these data, whether the predictability of fluid responsiveness of PPV and SPV changes according to change in tidal volume is evaluated by comparing the area under the curve of the receiver-operating characteristics curve between themselves and △Vpeak.

Inclusion Criteria:

• Children younger than 6 years old planned to undergo ventricular septal defect closure or atrial septal defect closure under general anesthesia

Exclusion Criteria:

• Children with other complex cardiac defects
• Children with arrhythmia
• Children with preoperatively measured ejection fraction of less than 30%
• Children with underlying pulmonary disease