A pulmonary embolism (PE) is a blockage in one of the blood vessels (arteries) in the lungs - usually due to a blood clot. A PE can be in an artery in the center of the lung or one near the edge of the lung. The clot can be large or small and there can be more than one clot. If there are severe symptoms, which occur with a large clot near the center of the lung, this is known as a massive PE, and is very serious.

In almost all cases, the cause is a blood clot (thrombus) that has originally formed in a deep vein (known as a DVT). This clot travels through the circulation and eventually gets stuck in one of the blood vessels in the lung. The thrombus that has broken away is now called an embolus (and can therefore cause an embolism).