Tolerance to anemia in healthy subjects and in the critically ill patient Tolerance to anemia is highly dependent on the volume status of the patient, physiological reserve, and the dynamics of the anemia (for example, chronic, such as the anemia of sepsis, versus acute, such as hemorrhagic conditions). Normovolemic anemia is better tolerated than anemia in hypovolemic states (e.g., acute bleeding in trauma patients or surgery) in which cardiac output acutely decreases. In healthy subjects submitted to normovolemic hemodilution, cardiac output increases because of decreased blood viscosity (especially relevant in severe anemia) and increased adrenergic response, allowing tachycardia and increased myocardial contractility. Other phenomena include blood flow redistribution (to heart and brain) and an increased oxygen extraction ratio (reflected by a decrease in mixed venous saturation [SvO 2 ]). These mechanisms allow healthy humans to tolerate severe degrees of normovolemic anemia [14, 15] , although side effects, such as arrhythmias or ST changes, can be observed in extreme cases [16, 17] . The myocardium is the organ at risk in cases of acute anemia in which both tachycardia and increased ventricle contractility may increase myocardial oxygen demand. Because myocardial oxygen extraction is already almost maximal at rest, every increase in myocardial oxygen demand must be accompanied by increased coronary blood flow [18] . This can become problematic in patients with stenotic coronary arteries especially when tachycardia is present, which can decrease diastoledependent left ventricle perfusion. Therefore, in critically ill patients, especially those with heart failure or coronary artery disease (CAD), the myocardium may not tolerate such low hemoglobin levels [19] . In acute myocardial infarction, anemia may worsen myocardial ischemia, generate arrhythmias, and potentially increase infarct size [20] . In patients with acute coronary syndrome or heart failure, anemia increases morbidity and mortality [21, 22] . For these reasons, patients with cardiac problems should be managed with a more liberal approach to transfusion than other patients [23, 24] .