Pulmonary embolism Dr Trisha Macnair A potentially fatal blood clot that blocks an artery to the lung.

In this article

What is pulmonary embolism? Symptoms

Causes and risk factors Treatment and recovery

Pulmonary embolism (PE) is a blockage of one of the arteries in the lung by a blood clot. The symptoms can be so vague it may be confused with many other conditions, and PEs are an important cause of sudden unexpected deaths.

Microscopic thrombi, or clots, are continually being made in the bloodstream. Normally these are broken down quickly, but under certain conditions they grow to form a larger clot that blocks a vein. This is called deep vein thrombosis (DVT).

Conditions that trigger a thrombosis include:

• Slow or turbulent flow of blood through the veins (venostasis).
• Abnormality of the clotting system of the blood, which makes it more prone to forming clots or less able to break them down.
• Inflammation of the blood vessel wall.

DVTs usually form in the veins of the lower leg or pelvis, but can start anywhere in the body. Pieces may break loose and travel through the bloodstream to the lung, where they block the pulmonary blood vessels and stop blood travelling through the lung to collect oxygen. This is a pulmonary embolism (PE), and if a major blood vessels is blocked the result is dangerously low levels of oxygen in the blood and tissues, and a heavy strain on the heart which is trying to pump against a blocked vessel.

It can be difficult to spot a thrombosis. DVT in the calf veins, for example, has usually spread above the knee before a diagnosis is made.

While tests show that almost everyone who develops DVT in the upper leg has a PE, only about half have symptoms. In many cases the clots are small, blocking only the smaller pulmonary vessels. About ten per cent of people with an untreated DVT will have a PE large enough to cause symptoms.

Symptoms of a PE may appear suddenly and include:

• Shortness of breath.
• Coughing up blood.
• Low blood pressure.
• Painful, swollen red leg (indicating DVT).

It's estimated that PEs are responsible for ten per cent of all deaths in hospital.

The condition is particularly common among the seriously ill or immobilised. More than ten per cent of people who rest in bed for a week develop PE, and it also occurs in 33 per cent of those in intensive care.

There are many factors that can increase a person's risk of a PE:

• Having an operation, especially orthopaedic surgery on the legs or gynaecological surgery (this makes clots more likely to form in the deep veins of the leg or pelvis).
• Major trauma.
• Serious burns.
• Cancer.
• Immobility, including prolonged periods of sitting still (during travel, for example).
• Pregnancy.
• Paralysis.
• Heart failure.
• Inflammatory bowel disease.
• Clotting disorders and thrombophilia.

Tests to confirm a PE include a lung scan called a ventilation-perfusion or V/Q scan (isotope lung scan), a pulmonary angiogram, electrocardiogram (ECG), ultrasound scan, CT or MRI scans, and a special blood test called a D-dimer test.

Treatment of a PE must be prompt: ten per cent of patients die within the first hour.

The specific treatment depends on the likelihood that an embolism has occurred, the severity of symptoms and whether there's existing heart or lung disease.

Anticoagulant drugs such as heparin or warfarin are used to stop clots in the lung growing any larger, allowing the body to break them down, and to prevent new clots forming. They don't dissolve the existing clot, but reduce the threat of another PE developing. It may be necessary to take these drugs for several months, and their effects have to be closely monitored.

Large embolisms may be treated with drugs called thrombolytics, which dissolve the clot. Sometimes surgery is needed to remove the clot.

There are ways of cutting down your risk of developing DVT or a PE including :

• Stop or cut down smoking.
• Avoid being overweight.
• Avoid taking the contraceptive pill and hormone replacement therapy (HRT).
• Take regular exercise.
• When travelling, exercise at regular intervals, keep well hydrated and wear compression stockings.

This article was last medically reviewed by Dr Trisha Macnair in July 2009.