Fluid Responsiveness in Post-cardiac Surgery

Inclusion Criteria: * Patients on mechanical ventilation * Patients ≥18 years of age * patients undergoing cardiac surgery (coronary artery bypass graft, valve repair/ replacement, combined cardiac surgery) * patients with normal systolic function of the left and right ventricle * patient is mechanically ventilated with a protective lung strategy Exclusion Criteria: * Patients with Spontaneously breathing activity * Patients undergoing emergent cardiac surgery. * Patient with severe peripheral arterial occlusive disease * Pregnant women * Contraindication of passive leg raising test * unstable post-operative course * Post-operative complications such as uncontrolled bleeding, severe neurologic impairment, or accidental mechanical complications; * presence of residual severe tricuspid or any valvular regurgitations * low cardiac output, low ejection fractions (EF ≤45%) * open chest,Pao2/Fio2 ≤ 200