Optimal Hemoglobin Concentration in Patients with Subarachnoid Hemorrhage, Acute Ischemic Stroke and Traumatic Brain Injury

Although patients with acute ischemic stroke (AIS), subarachnoid hemorrhage, (SAH), and traumatic brain injury (TBI) have high red blood cell transfusion (RBCT) needs, some evidence indicates that RBCTs in these patients can lead to poor clinical outcomes. In this review, Leal-Noval and colleagues discuss the effects of RBCT on cerebral function and clinical outcomes in patients with AIS, SAH, and TBI. The brain can compensate to maintain cerebral blood flow (CBF) and oxygen delivery during anemia and in normal individuals, these mechanisms are disrupted only at very low hemoglobin levels. Studies on the effects of transfusion on cerebral function indicate that baseline cerebral oxygenation is important for RBCT success. However, RBCT may exert its influence by mechanisms different from those used in the oxygen-carrying capacity of hemoglobin because it increases blood viscosity, which may, therefore, actually be the trigger for transfusion. RBCT is associated with a poor clinical outcome in patients who have suffered an SAH. Several studies have corroborated findings that RBCT can lead to a 3-fold increase in the risk of intracerebral hemorrhage. Two studies found that a hemoglobin level >11 g/dL seemed to protect the patient against cerebral ischemia, stroke, and cerebral infarction; conversely, low a hemoglobin level was associated with an increased risk of death and poor functional outcome. To date, RBCT has not been considered as part of the protocol for treatment of patients with AIS. However, the hematocrit should be assessed in these patients because, as a measure of blood viscosity, it has an inverse relationship with CBF. Precise control of the hematocrit is necessary; low levels lead to decreased oxygen transport and high levels to increased blood viscosity and impaired microcirculation. These effects are exacerbated by the use of RBCT, especially stored RBCs. Mid-range hematocrit levels are associated with good outcomes. To avoid hypoxia n patients with TBI, oxygen delivery must be maximized by increasing CBF and oxygen transport. Anemia and RBCT must be avoided because of their adverse effect on clinical outcomes. RBCT based on hemoglobin levels could result in excessive or inadequate transfusions in patients with AIS, TBI, or SAH. More physiologic transfusion triggers using direct signals coming from the brain may, in the future, replace arbitrary hemoglobin-based triggers for RBCT.

Curr Opin Crit Care. 2008 Apr;14(2):156-162