The red blood cell transfusion trigger. Physiology and clinical studies.

The need for red blood cell transfusion is determined by the etiology, chronicity, and severity of anemia, the patient's ability to compensate for decreased oxygen carrying capacity, and tissue oxygen requirements. The compensatory mechanisms invoked by acute blood loss differ from the physiologic adaptations to chronic anemia. Factors such as myocardial depression and inadequate ventilation can also decrease oxygen delivery, but their occurrence cannot be reliably predicted, particularly in surgical patients. A margin of safety is therefore necessary to prevent inadequate oxygen delivery and potential morbidity and mortality in anemic patients. Appropriate use of pharmacologic agents can obviate the need for red blood cell administration in some chronically anemic patients. There is no single red blood cell transfusion trigger. The entire clinical picture not just the hemoglobin, must be considered in each patient.