Low cardiac syndrome( LOS) is the imbalance of the oxygen supply and demand at the tissue due to reduced oxygen supply by decreased cardiac function. The causes of the cardiac dysfunction are insufficient cardioplegia, elongation of aortic clamp or cardiopulmonary bypass (CPB) duration, incomplete revascularization, and so on. The preventions of LOS are myocardial protections intraoperatively. The most important myocardial protection is to accomplish more quickly, accurately and safety to operation. And today, cardioplegia is essential for surgery requiring CPB. The integrated myocardia protection is likely to be effective to the patients who suffer low cardiac function. LOS is commonly defined the decreased cardiac index (CI)[<2.0~2.2 l/min/m2], systolic blood pressure (<80~90 mmHg) and the signs of tissue malperfusion, in the absence of hypovolemia. The treatment of LOS is, at first, the correction of hypovolemia, then rhythm control, adjustment of preload and afterload. When it does not recover from LOS despite adequate treatment, the mechanical circulatory assist devices should be introduced without hesitation.