1. To evaluate the consistency of cardiac output measured by pulmonary artery catheter and LiDCO in cardiac surgical patients
2. To evaluate whether different hemodynamic monitoring methods can accurately detect the trend of cardiac output changes

Close monitoring of cardiac output (CO), especially in patients before and after intervention(such as volume expansion, diuresis, vasoconstriction, vasodilation therapy), could help to adjust the treatment strategy in cardiac surgical patients. Pulmonary artery catherization (PAC) has been used for hemodynamic monitoring for more than four decades. In spite of its invasiveness, it remains the clinical reference method for the assessment of CO at the bedside. Nowadays, many less invasive alternatives, such as LiDCO Rapid Pulse Contour Analysis System (LiDCO Ltd., Cambridge, United Kingdom), are already available on market. However, consistency between different hemodynamic monitoring results still raise concern. And whether hemodynamic monitoring could accurately detect the changes of these parameters before and after intervention is an important issue in clinical practice. In this study, the investigators are going to collect CO and changes of CO from PAC and LiDCO before and after intervention (passive leg raising and dobutamine stress test) in cardiac surgical patients. Our resulst could provide important reference for cardiac surgical patients how to choose appropriate hemodynamic monitoring tools and how to interpret the results of hemodynamic monitoring.

Inclusion Criteria:

1. cardiac surgery
2. hemodynamic monitoring (PAC, LiDCO)
3. mechanical ventilation

Exclusion Criteria:

1. cardiac arrhythmia
2. moderate to severe aortic, mitral and tricuspid regurgitation
3. IABP
4. ECMO