Although sickle cells have increased intracellular viscosity, the viscosity of patient's blood is usually not increased, because of the low hematocrit typically found in such patients. When patients receive transfusions, their exercise capacity increases, but it is unclear whether the change is due primarily to increased hemoglobin concentration, or also reflects improved flow properties of the blood due to dilution of sickle cells with normal erythrocytes. To evaluate the relative importance of these two factors, submaximal exercise studies were performed before and after a series of transfusions. Exercise capacity improved and the patients were able to perform increased amounts of work at lower heart rates. Regression analyses indicated that the percent of sickle cells in patient's blood did not have a significant effect on exercise capacity after the transfusion-induced increase in hemoglobin concentration was taken into account. At the hematocrits typically found in patients, altered viscosity of blood due to the presence of sickle cells is much less important than hemoglobin concentration as a determinant of exercise capacity.
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