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 .