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Pulmonary embolism

Page last reviewed: 13/07/2011

A pulmonary embolism is a blockage in one of the blood vessels in the lungs. It happens when part, or all, of a blood clot blocks the blood supply to your lungs. A pulmonary embolism is a serious, potentially life-threatening, condition.

Types of blood clot

Blood clots that develop in a vein are known as venous thrombosis. Pulmonary embolism is a form of venous thromboembolism (VTE), which means a blood clot in the vein.

Most cases of pulmonary embolism develop when part, or all, of the blood clot travels in the bloodstream from the deep veins in your leg and up into your lungs.

Having deep vein thrombosis (DVT) puts you at greater risk of developing a pulmonary embolism. DVT occurs when a blood clot forms in one of the veins in your leg.

Who is affected by pulmonary embolism?

Every year, one in every 1,000 people develops DVT, and around one in 10 people with untreated DVT will develop a pulmonary embolism.

Page last reviewed: 13/07/2011

If you have a pulmonary embolism, your symptoms will depend on its size. Symptoms may include:

  • breathlessness, which may come on suddenly, or gradually,
  • chest pain, which may be worse when you breathe in, and
  • sudden collapse.

You should see your GP as soon as possible if you have any of the above symptoms.

Page last reviewed: 13/07/2011

A pulmonary embolism usually occurs when part, or all, of a blood clot travels from the veins in your legs, up your body, and into your lungs. This often stems from deep vein thrombosis (DVT). 

The factors that increase your risk of developing a pulmonary embolism include:

  • getting older, 
  • having a previous blood clot,
  • having a family member with a blood clot,
  • cancer and its treatments,
  • the combined contraceptive pill (COC), or hormone replacement therapy (HRT),
  • pregnancy,
  • smoking,
  • obesity, and
  • having an operation on the lower half of your body.

However, the three main causes of a blood clot are:

  • slow blood flow,
  • blood that clots too easily, and
  • blood vessel damage.

These are described in more detail below.


When you are inactive, your blood tends to collect in the lower parts of your body, particularly in your lower legs. This is usually nothing to worry about because when you start to move, your blood flow will increase and blood will begin to move evenly around your body.

However, if you are immobile (do not move) for a long period of time, your blood flow can slow down considerably. Some common examples of situations where immobility might occur include:

  • following an illness, such as a stroke,
  • following an injury,
  • after an operation, or
  • while travelling on a long plane, train, or car journey.

 A slow blood flow increases the risk of a blood clot forming.

Blood vessel damage

If the wall of a blood vessel is damaged, it may become narrowed, or blocked, which can lead to a blood clot forming.

Blood vessels can be damaged by injuries such as broken bones, or severe muscle damage. Sometimes, blood vessel damage that occurs during surgery can cause a blood clot, particularly in operations that are carried out on the lower half of your body.

Conditions such as vasculitis (inflammation of the vein wall), and some forms of medication, such as chemotherapy, can also lead to blood vessel damage.

Medical and genetic conditions

Your risk of developing a pulmonary embolism is increased if you have a condition that causes your blood to clot more easily than normal. Conditions that increase the tendency for your blood to clot include:


  • cancer (chemotherapy and radiotherapy can make it worse),
  • heart failure
  • thrombophilia (a genetic condition), and Hughes syndrome
  •  (when your immune system attacks a particular fat in your body).

How the blood clots

Your blood contains cells called platelets and proteins, known as clotting factors, which together make the blood clot.

When a blood vessel is cut, the platelets and the clotting factors in your blood form a solid clot at the site of the wound. This clot acts as a plug to stop the wound bleeding.

Normally the blood only clots when a blood vessel is damaged and bleeds, for example when you cut yourself. Sometimes the blood may start to clot even when a blood vessel has not been damaged. If this happens, a blood clot can form within a vein or artery (thrombosis).

Page last reviewed: 13/07/2011

Around 50 per cent of people with pulmonary embolism develop the condition while they are already in hospital. If your doctor suspects that you have a pulmonary embolism, you may have to have the following tests.

D-dimer test

A D-dimer test is a special blood test that is used to detect pieces of a blood clot that have been broken down and are loose in your blood stream.

Computerised tomography pulmonary angiography (CTPA)

A computerised tomography pulmonary angiography (CTPA) is where dye is injected into the blood vessels of your lungs and a computerised tomography (CT) scan is taken. A CT scan works by taking X-rays to create a highly detailed image of the inside of your body.

If there is a pulmonary embolism in one of your lungs, it will show up on the CT scan as a gap in your blood supply.

Ventilation (perfusion) scan

A ventilation scan measures the amount of air and the blood flow in your lungs. If there is a part of your lung that has air in it, but no blood supply, it may be the result of a pulmonary embolism.

Page last reviewed: 13/07/2011

If you have a pulmonary embolism, you will need to take a medicine called an anticoagulant.

Anticoagulation (thinning of the blood)

Anticoagulant medicines prevent the blood clot from getting bigger. They can also help to stop part of the blood clot from breaking off and becoming lodged in another part of your bloodstream (an embolism).

Although they are often referred to as blood-thinning medicines, anticoagulants do not thin the blood. They alter chemicals within it, which prevents clots from forming so easily.

There are two types of anticoagulants that are used to treat pulmonary embolism. They are:

  • heparin, and
  • warafin.

Heparin is usually prescribed first because it works immediately to prevent further clotting. You may also need to take warfarin at a later date in order to prevent another blood clot from forming.


Heparin is available in two different forms:

  • standard heparin, and
  • low molecular weight heparin (LMWH).

Both forms of heparin are given by injection. Standard heparin is given intravenously (IV) which means it is injected into one of your veins. Low molecular weight heparin is given sub cutaneously (under the skin).

Standard heparin is also fed into the vein through an IV line, and must be given in hospital. As a dose of standard heparin can work differently from person to person, the dosage must be carefully monitored and adjusted where necessary. You may need to stay in hospital for between 5-10 days during which time you will have frequent blood tests to ensure that you receive the correct dosage.

LMWH works differently from standard heparin. It contains small molecules, which means that its effects are more reliable. A dose of LMWH can be given as one injection and you will not have to stay in hospital to be monitored.

Both standard heparin and LMWH can cause side effects that include:

  • a skin rash,
  • headaches,
  • an upset stomach,
  • bleeding,
  • a general feeling of being unwell, and
  • weakening of the bones (if taken for a long time).

In rare cases, heparin can also cause an extreme reaction that makes existing blood clots worse, and causes new clots to develop. This reaction, and weakening of your bones, is less likely to occur if you are taking LMWH.

In most cases, you will be given LMWH because it is easier to use and causes fewer side effects.


Warfarin is taken in tablet form. You may need to take it for up to six months after your initial heparin treatment in order to prevent further blood clots occurring. In some cases, warfarin may need to be taken for life.

As with standard heparin, the effects of warfarin vary from person to person so you will need to be closely monitored and have frequent blood tests to ensure that you are taking the right dosage.

When you first start taking warfarin, 2-3 blood tests a week may be necessary until your regular dose is decided. After this, you may only need to have a blood test every four weeks, at an anticoagulant outpatients' clinic.

The effectiveness of warfarin can be affected by your diet, any other medicines that you are taking, and by how well your liver is working. If you are taking it:

  • keep your diet consistent,
  • limit the amount of alcohol that you drink (no more than the recommended limit of 3-4 units a day for men, and 2-3 units a day for women),
  • take your dose of warfarin at the same time every day,
  • do not take any other medicine without checking with your GP, pharmacist, or anticoagulant specialist, and
  • do not take herbal medicines.

For pregnant women, heparin injections are given for the full length of treatment, instead of warfarin tablets.

Page last reviewed: 13/07/2011

If you are admitted to hospital, your healthcare team will assess your risk of developing a blood clot while you are there.

Risk factors

You are at risk of developing pulmonary embolism if you are having surgery, particularly if it is to the lower half of your body.

Severe injuries, or surgery itself, may damage your blood vessels, and being inactive while in hospital can cause your blood flow to reduce. These factors can make developing a blood clot more likely.

You may be at increased risk of developing blood clots if you:

  • are 40 years of age, or over,
  • have a general anaesthetic, or surgery on your abdomen, hips, or legs,
  • have had a previous blood clot,
  • have family members who have had a blood clot,
  • are overweight, or obese,
  • have ever had cancer, or
  • have problems with your heart, lungs, bowel, or joints.

Preventative measures

If any of the above applies to you, your healthcare team may recommend that you take an anticoagulant medicine while you are in hospital, and for a while after you go home. See
Treatment for more information about anticoagulant medicines.

You may also need to wear compression stockings and use compression devices while in hospital and for some time afterwards.

Compression stockings are worn around your feet, lower legs, and thighs, and fit tightly to encourage your blood to flow more quickly around your body. Compression devices are inflatable and work in the same way, inflating at regular intervals to squeeze your legs and encourage blood flow.

Once you are well enough, it is important that you try to move around, or do leg exercises as
soon as possible. You should also avoid taking long journeys for four weeks after you come out of hospital.

Smoking and diet

You can reduce your risk of developing a pulmonary embolism by:

  • not smoking,
  • eating a healthy, balanced diet (low in fat and sugar and high in fruit and vegetables),
  • getting regular exercise, and
  • maintaining a healthy weight (and losing weight if you are obese).

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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