Based on the aim of blood transfusion that maintain the balance of oxygen supply and oxygen consumption, the investigators hypothesize that index of transfusion trigger for patients with hemoglobin level between 6g/dL and 10g/dL could be calculated by parameters including infusion rate of inotropic drugs for maintaining normal cardiac output, fraction of inspired oxygen for maintaining saturation of oxygen≧95%, core temperature, and torlerance to anemia.