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During the intercrisis periods, patients homozygous for sickle cell anemia (SS) show clinical symptoms of severe impairment of oxygen transport mechanisms. We have determined respiratory lung function tests, arterial and venous blood gases and cardiocirculatory parameters in 39 SS patients (mean age 22 +/- 5 yr) at distance from any vaso-occlusive crisis or blood transfusion. The patient group was compared to subjects homozygous for HbA (AA) of the same ethnic origin. Determinations were made at rest and after a 5 min mild exercise period. The main alterations in oxygen transport parameters observed in SS patients were: 1) a moderate reduction in vital capacity and maximal ventilation (pure restrictive syndrome), 2) an arterial hypoxemia which worsened the already low oxygen content of blood due to anemia, and 3) a low arterio-venous oxygen saturation difference which is very surprising in anemic patients. The normal oxygen consumption rate was thus insured by a 70% increase of the cardiac output at rest. A particular abnormality found in SS patients was the high Po2 in mixed venous blood. The decreased affinity of blood for oxygen resulting from the polymerization of HbS in the erythrocytes led to an almost normal venous blood unsaturation and thus a decreased release of oxygen from this anemic blood. These results indicate that peripheral blood flow was permanently increased in SS patients. This may be in part at the origin of the arterial hypoxemia induced by an increased pulmonary blood shunting. Sickle cell anemia is more severe than other anemias of comparable intensity as, due to the sickling process, these patients appear to be at the upper limit of physiological compensatory mechanisms usually encountered in chronic anemia.