As considering the close relationship between systolic dysfunction and diastolic dysfunction, the EAS index e'/(a' x s') including systolic and diastolic function could be expected to predict hemodynamic changes during off-pump coronary artery bypass surgery(OPCAB). Exposure of the grafting site during OPCAB needs displacement of the heart, resulting in hemodynamic instability.

Therefore the investigators are going to evaluate EAS index with preoperative Transthoracic echocardiography(TTE) and want to know the relationship between preoperative EAS index and intraoperative hemodynamic changes in patients undergoing OPCAB, in a prospective observational study.

Inclusion Criteria:

1. Age ≥ 20
2. Patients undergoing multivessel off-pump coronary artery bypass surgery

Exclusion Criteria:

1. Age < 20
2. Patients with valvular heart disease
3. Congestive heart failure, NYHA class ≥III
4. Emergency surgery
5. Patients with arrhythmia
6. ST segment elevation myocardial infarction

patients with coronary artery obstructive disease planned to have multivessel OPCAB