19 Chronic anemia is better tolerated than acute anemia. Patients with cardiac disease or atherosclerotic flow-restricting lesions have limited adaptability to anemia. Indeed, many factors should be considered in the transfusion decision, including the patient's current intravascular volume, the duration and extent of anemia, and the presence of pathology that affects cardiopulmonary function and circulation. Suggested guidelines for transfusion must balance the risk of transfusion, including possible transmission of hepatitis and HIV, versus the need for oxygen-carrying capacity in recovery from trauma, surgery or illness. While waiting for crossmatched blood to be available, emergency transfusion needs can be met in various ways, depending on the clinical status of the patient and the equipment available.
Most patients can tolerate an acute decrease in hemoglobin and therefore oxygen-carrying capacity.
Type O Rh-positive Red Blood Cells for males or postmenopausal females can be transfused in this setting as well. The patient can be switched back to his or her inherent type-specific blood after subsequent testing by the blood bank indicates it is safe to do so. Calcium-containing solutions must not be added to blood, particularly at slow infusion rates, because small clots may form due to the presence of calcium in excess of the chelating ability of the citrate anticoagulant. Although an anticoagulant is added to blood when it is collected, small clots are occasionally present in donor units, requiring filtration at the time of infusion. Standard blood administration sets usually have a filter screen with a pore size of 170 mm.