Hemodilution and anemia in patients with cardiac disease: what is the safe limit?

Anemia is associated with increased perioperative morbidity and mortality, especially in patients with cardiovascular disease. Although blood transfusions are commonly used in these patients, there is little evidence that such an approach improved patients' clinical outcome.

From an experimental point of view, tolerance to acute anemia is significantly reduced in animals with externally applied coronary artery stenosis. In rats subjected to transient coronary occlusion, however, moderate isovolemic hemodilution attenuated myocardial damage and improved survival. Tolerance to severe anemia could also be improved by hyperoxic ventilation and maintenance of adequate coronary perfusion pressure. From the clinical perspective, tolerance to anemia in patients with cardiovascular disease closely depends on myocardial oxygen demand, in particular on the level of heart rate. A reduction in heart rate as observed in anesthetized patients undergoing moderate hemodilution may confer some cardioprotection.

The absolute lowest threshold for anemia in patients with cardiac disease cannot be established. Further studies are needed to develop evidence-based transfusion guidelines for these patients. Rather than primarily focusing on transfusion, physicians should first administer therapies that have been shown to improve outcome. In the setting of coronary artery disease, control of heart rate appears of paramount importance.