The purpose of this study is to determine the difference in the impact of moderate positive end-expiratory pressure (PEEP) on hepatic venous flow Doppler in patients undergoing cardiac surgery: conventional versus protective ventilation strategy The possible changes in forward and backward flows (Doppler profiles) of hepatic venous flow at different degrees of PEEP in conventional and protective ventilation strategies are analyzed by using intraoperative transesophageal echocardiography (TEE) in patients undergoing cardiac surgery .

Following data would be determined after 5 min-exposure volume controlled ventilation employing tidal volume of 10 ml/kg and 6 ml/kg with low PEEP (with 2 mmHg) and moderate PEEP (7 mmHg) during remifentanil-based anesthesia (1.0 mcg/kg/min) for cardiac surgery (n=12)

1. sum of forward hepatic venous flow
2. sum of backward hepatic venous flow
3. peak velocity of diastolic hepatic venous flow
4. lung compliance
5. peak airway pressure
6. mean airway pressure
7. other pressure derived hemodynamic parameters:heart rate; systolic, diastolic, and mean blood pressures; systolic, diastolic, and mean pulmonary artery pressures; central venous pressure (CVP), pulmonary capillary

Inclusion Criteria:

• Patients undergoing cardiac surgery
• Patients provide written informed consents

Exclusion Criteria:

• Re-do or tri-do surgery
• Patients with active infection
• Patients with endocrine disease
• Patients ischemic heart disease
• patients with tricuspid valve regurgitation greater than mild degree