Pulmonary embolism is one of the most common causes of death in hospitalized people who must remain in bed for a long time. A pulmonary embolism usually occurs when a blood clot in a vein breaks off, travels through the blood stream, and lodges in the lung. In the United States, more than 600,000 people have a pulmonary embolism each year, and more than 60,000 of them die. Most of those who die do so within 30 to 60 minutes after symptoms begin.