Red cell exchange transfusions lower cerebral blood flow and oxygen extraction fraction in pediatric sickle cell anemia

Blood transfusions are the mainstay of stroke prevention in pediatric sickle cell anemia (SCA), but the physiology conferring this benefit is unclear. Cerebral blood flow (CBF) and oxygen extraction fraction (OEF) are elevated in SCA, likely compensating for reduced arterial oxygen content (CaO 2 ). We hypothesized that exchange transfusions decrease CBF and OEF by increasing CaO 2 , thereby relieving cerebral oxygen metabolic stress. Twenty-one children with SCA on chronic transfusion therapy (CTT) had MRIs before and after exchange transfusion. Arterial spin labeling and asymmetric spin echo sequences measured CBF and OEF, respectively, which were compared pre- and post-transfusion. Volumes of tissue with OEF above successive thresholds (36, 38, 40%), as a metric of regional metabolic stress, were compared pre- and post-transfusion. Transfusion increased Hb (9.1 to 10.3 g/dL, p