Background: The oxygen saturation of the central vein (ScVO2) has been regarded as a surrogate of tissue perfusion in patients of severe sepsis and major surgery. However, ScVO2 in acute coronary syndrome has not been addressed. We tried to delineate the trend of ScVO2 in patients of acute coronary syndrome. Methods and Patients: This was a prospective observational study in the coronary care unit of a medical center. Patients of acute coronary syndrome with acute lung edema or cardiogenic shock were enrolled. Blood samples from central vein (via 3-lumen catheter in superior vena cava) and peripheral artery immediately after admission, 24 and 48 hours later were analyzed by co-oxymetry method. The primary endpoint was