the study aims to examine the validity of combined end-expiratory and end-inspiratory occlusion test and tidal volume challenge test for prediction of fluid responsiveness in immediate post-cardiac surgery patients

multiple studies prove that dynamic variables based on heart-lung interactions accurately can predict fluid responsiveness in mechanically ventilated patients, including pulse pressure variation (PPV) and stroke volume (SV) variation.

Furthermore, passive leg raising (PLR), end-expiratory occlusion test, and tidal volume challenge test, "Mini"-fluid challenge are currently available to assess fluid responsiveness.

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