Sickle cell anemia is a condition caused by a genetic mutation in the HBB gene leading to the formation of abnormal hemoglobin, called sickle hemoglobin or hemoglobin S.

Hemoglobin is a protein found in red blood cells that binds oxygen. Hemoglobin S has reduced oxygen-binding capacity.

Hypoxia in sickle cell anemia

Sickle cell anemia patients frequently experience hypoxia, or low oxygen levels, in tissues due to the reduced oxygen-carrying capacity of hemoglobin S. Hypoxia can lead to various complications in sickle cell anemia patients.

For example, hypoxia is associated with pulmonary hypertension. It is thought that hypoxia may cause pulmonary hypertension via hemolysis, or the rupturing of red blood cells. Because hypoxia promotes a sickle shape in the red blood cells, it makes them more fragile and prone to rupture.

This sets free hemoglobin that reacts with a molecule called nitric oxide, thereby depleting it from the blood. Nitric oxide is needed for the relaxation of blood vessels, and therefore its depletion from the blood may contribute to pulmonary hypertension.

Hypoxia is also associated with vaso-occlusive crisis (VOC), where aggregating sickle-shaped red blood cells block the small blood vessels. When VOC occurs in the blood vessels of the lung, it can lead to acute chest syndrome (ACS), a condition where oxygen concentrations in the blood drop below a critical level. Recurrent ACS also increases the risk of stroke.

Oxygen therapy for sickle cell anemia

The supply of extra oxygen with oxygen therapy may be beneficial for some patients with sickle cell anemia. But the use of oxygen therapy in sickle cell anemia is controversial because high levels of oxygen are known to suppress the formation of new red blood cells.

This can worsen the anemia that is seen in these patients. Oxygen therapy is, therefore, only recommended when oxygen levels drop below a critical threshold.

Oxygen therapy in clinical trials for sickle cell anemia

A Phase 3 randomized clinical trial (NCT02813850) is investigating whether oxygen therapy can reduce the incidence of VOC in pregnant women with sickle cell anemia. The trial is currently recruiting patients at the Hôpital Necker Enfants-Malades in Paris.

Another clinical trial (NCT03412045) at the University of Nebraska aims to assess the effectiveness of oxygen therapy in sickle cell crisis. The primary outcome measure of the trial will be the reduction of pain. The trial is not yet recruiting patients but is planned to start in March 2019.

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