Rheological aspects of sickle cell anemia and related hemoglobinopathies

Text includes handwritten formulasTo evaluate the rheological behavior of certain hemoglobinopathies with respect to oxygen tension, the concept of optimum hematocrit was employed. The optimum hematocrit is defined as the hematocrit at which the erythrocyte flow rate is the greatest. It therefore represents the optimum oxygen carrying capacity of blood. Using a Weissenberg Rheogoniometer with cone-andplate geometry, viscosity measurements at different hematocrits and oxygen tensions were made on patients with the following diseases: normal, sickle cell, sickle cell trait, sickle cell hemoglobin C, homozygous hemoglobin C, sickle cell beta plus thalassemia, and sickle cell beta zero thalassemia. The rheological parameters obtained from a Casson's plot were the apparent yield stress and apparent high shear rate viscosity. These were utilized in the Casson's constitutive equation used to solve the one-dimensional equation of motion for steady state flow in a cylindrical vessel. From the solution of the equation of motion, the optimum hematocrit at each oxygen tension was determined. The optimum hematocrits for the normal, sickle cell trait, and homozygous hemoglobin C diseases were found to be independent of oxygen tension. Sickle cell hemoglobin C disease, sickle cell beta plus thalassemia, sickle cell beta zero thalassemia, and sickle cell disease all had oxygen dependent optimum hematocrits. The rheological severity was roughly that of the clinical severity. It was found that diseases which included hemoglobin C had abnormally low optimum hematocrits, while diseases which contained hemoglobin S, excluding sickle cell trait, had oxygen dependent optimum hematocrits. The results are discussed in terms of the Casson's yield stress and Casson's apparent high shear rate viscosity. The adequacy of oxygen transfer to the tissues was evaluated for normals, sickle cell disease, and the other hemoglobinopathies. Below oxygen tensions of approximately 6 mm Hg, at the same cardiac output, tissue oxygen transport appears reduced in sickle cell anemia due to the increased viscosity of the blood. The exact oxygen partial pressure at which oxygen delivery falls below normal depends on the MCHC , P^n, and blood viscosity