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Background: The impact of anemia on free tissue transfer remains controversial, as surgeons weigh the benefit of decreased blood viscosity in an anemic, hemodiluted state against the risks of diminished oxygen carrying capacity. Common surgical practice is to transfuse at hemoglobin (Hb) < 10 or hematocrit (Hct) < 30.  There are no large-scale studies evaluating the impact of pre-operative anemia in patients receiving a free flap. It is essential to optimize transfusion and resuscitation practice since anemia and transfusion may impact both flap survival and patient morbidity. The objective of this study is to determine whether pre-operative anemia increases the risk of flap morbidity.
Methods: A retrospective review included all free flaps performed by the plastic surgery service at a major, academic medical center from December 2005 to December 2009.  Pre-operative hemoglobin (Hb) and hematocrit (Hct) were recorded, along with demographics, recipient site and indication, donor tissue transferred, comorbid diseases, American Society of Anesthesiology (ASA) status and perioperative information including transfusion, estimated blood loss, operative length and perioperative complications.  Primary outcomes were defined as flap failure and need for re-operation at the recipient site.  Secondary outcomes included complications typically associated with patient and flap morbidity, classified respectively into medical and surgical complications. Medical complication included death, myocardial infarction, pulmonary complication (pneumonia, atelectasis, pulmonary edema), stroke and dysrhythmia. Surgical complication included necrosis, infection, thrombosis, bleeding and hematoma. Statistical significance was determined by Fisher’s exact test.
Results: 123 free flaps were performed among 116 patients. 105 cases met the inclusion criteria.  Hb < 10 was present in 29[28%] cases. 20 of these patients had Hb < 9, defined as profoundly anemic.   Overall, there was an 11% (12/105) flap failure rate. The most common cause of failure was vascular thrombosis. Patients with profound anemia, defined as Hb < 9, experienced a 3-fold increased risk of flap failure (RR = 3.04, p < 0.05). Patients just below the transfusion threshold (9 < Hb < 10) did not experience a statistically significant increased risk of failure. 24[23%] patients underwent re-operation at the recipient site. The most common indication was subtotal flap necrosis debridement. ASA class IV patients were 3 times more likely to undergo re-operation (RR = 3.3, p < 0.05) but not at increased risk of flap failure. Anemia with Hb < 10 was not associated with medical complication. However, patients ASA Class III and above experienced more post-operative medical complications (RR = 4.4, p < 0.05) but no statistically significant increase in surgical complication rates.
Conclusions: Profound preoperative anemia with Hb < 9 is a predictor of free flap failure. No significant risk is observed at Hb between 9 and 10, therefore the common transfusion threshold (Hb < 10) is not the tipping point for compromised flap viability. Patients undergoing free flap tolerate pre-operative anemia with regard to post-operative medical morbidity. Thus, pre-operative anemia is well tolerated in free flaps with no increased risk of flap or medical morbidity until patients are well beneath the typically held transfusion threshold.