How risky is an aortic valve replacement without a blood transfusion?

Depends: Many patients can have aortic surgery without anemia or need for transfusion. Large, young, good hemoglobin and careful primary surgery without anemia will do well. If small, old, anemic, frail one can die without blood availability and use. If one has substantial intraoperative, or postoperative bleeding transfusion and blood or products are necessary to save the patient.