Coloured 3D computed tomography (CT) scan of an embolism (blood clot, tube shaped) in a patient's pulmonary artery (green, upper centre). Smaller clots may cause shortness of breath, dizziness and chest pain. Larger clots can be fatal, as the blood is quickly starved of oxygen. Thrombolytic drugs can be used to break up a clot, and anti-coagulant drugs may help prevent the formation of others.