Low‐shear red blood cell oxygen transport effectiveness is adversely affected by transfusion and further worsened by deoxygenation in sickle cell disease patients on chronic transfusion therapy

BACKGROUND: Simple chronic transfusion therapy (CTT) is a mainstay for stroke prophylaxis in sickle cell anemia, but its effects on hemodynamics are poorly characterized. Transfusion improves oxygen‐carrying capacity, reducing demands for high cardiac output. While transfusion decreases factors associated with vasoocclusion, including percent hemoglobin (Hb)S, reticulocyte count, and circulating cell‐free Hb, it increases blood viscosity, which reduces microvascular flow. The hematocrit‐to‐viscosity ratio (HVR) is an index of red blood cell oxygen transport effectiveness that varies with shear stress and balances the benefits of improved oxygen capacity to viscosity‐mediated impairment of microvascular flow. We hypothesized that transfusion would improve HVR at high shear despite increased blood viscosity, but would decrease HVR at low shear. STUDY DESIGN AND METHODS: To test this hypothesis, we examined oxygenated and deoxygenated blood samples from 15 sickle cell patients on CTT immediately before transfusion and again 12 to 120 hours after transfusion. RESULTS: Comparable changes in Hb, hematocrit (Hct), reticulocyte count, and HbS with transfusion were observed in all subjects. Viscosity, Hct, and high‐shear HVR increased with transfusion while low‐shear HVR decreased significantly. CONCLUSION: Decreased low‐shear HVR suggests impaired oxygen transport to low‐flow regions and may explain why some complications of sickle cell anemia are ameliorated by CTT and others may be made worse.