Pulmonary Hypertension, Hypoxia and Sickle Cell Disease

The study will look at the risk factors for pulmonary hypertension (high blood pressure in the lungs) in children and adolescents with sickle cell anemia (SCA) and examine the role of hypoxia (oxygen shortage) in the disease. In patients with SCA, red blood cells become sickle-shaped and tend to form clumps that get stuck in blood vessels, blocking blood flow to the limbs and organs. Blocked blood vessels can cause pain, serious infections, and organ damage. Many patients with SCA also develop pulmonary hypertension. Children and adolescents with SCA or Chuvash polycythemia (another blood disorder that carries an increased risk for pulmonary hypertension) may be eligible for this study. Participants undergo the following procedures at the beginning (baseline) and end of the study: * History, physical examination and blood tests . * Echocardiography (ultrasound study of heart function). * Transcranial doppler (brain ultrasound study to measure brain blood flow). * Lung function tests. * 6-minute walk (measure of the distance covered in 6 minutes of walking). In addition, patients are followed by telephone or by clinic visits every 6 months for a review of their medical history and medications. A physical examination is also done at 12 months.