The purpose of the study is to verify the efficacy of using end-expiratory and end-inspiratory occlusion tests as an index of fluid responsiveness in mechanically ventilated patients with cardiac surgery.

Patients with hypotension after anesthetic induction for cardiac surgery, who requires fluid resuscitation based on clinical judgement by the anesthesiologists are enrolled in this study. Patients are monitored by transesophageal echocardiography and FloTrac/Vigileo. Hemodynamic variables (heart rate, systolic blood pressure, diastolic blood pressure, mean artery pressure, central venous pressure, stroke volume variation, cardiac index, velocity time integral of the aorta etc) are measured at baseline, after end-expiratory occlusion (EEO) test, after end-inspiratory occlusion (EIO) test, after passive legs raising test, and after fluid challenge respectively. Responders are defined by an increase in velocity time integral over 15% after infusion of 5ml/kg of crystalloid solution.

Inclusion Criteria:

• cardiac surgical patients
• hypotension after induction of anesthesia
• required volume expansion by clinical judgement of the anesthesiologist

Exclusion Criteria:

• younger than 18 years
• severe valve regurgitation or systolic dysfunction of the right ventricle
• contraindication of the transesophageal echocardiography examination
• cardiac arrhythmia
• left ventricular ejection fraction less than 30% before surgery