A pulmonary embolism occurs when the artery that carries blood to the lungs (pulmonary artery) becomes blocked.

The blockage is usually a blood clot, although it can be a fat droplet, an air bubble or amniotic fluid (fluid that surrounds unborn babies).

If the blockage is caused by a blood clot, it probably will have come from one of the deep veins in your legs and is known as deep vein thrombosis (DVT). 

Three of the main reasons why blood clots develop are described below.

Inactivity 

If you are inactive, blood tends to collect in the lower parts of your body, particularly in your lower legs. This is not usually a problem because when you start to move, your blood flow increases and blood begins to move evenly around your body.

However, if you are immobile for a long period of time, the flow of blood around your body can slow down considerably. You are likely to be immobile:

• after a debilitating illness, such as a stroke
• after an injury or operation
• when travelling on a long journey by plane, train or car

If your blood flow slows down because of a prolonged period of inactivity, your risk of a blood clot forming increases.

Blood vessel damage

If a blood vessel is damaged, the inside of the blood vessel can become narrowed or blocked. This can result in a blood clot forming.

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

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

Blood that clots too easily

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 likelihood of your blood clotting include:

• cancer – cancer treatments, such as chemotherapy and radiotherapy, can also make your blood clot more easily
• heart failure
• thrombophilia – an inherited condition that makes your blood more prone to clotting 
• Hughes syndrome – a condition where the blood becomes abnormally sticky, increasing its tendency to clot 

Other factors

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

• your age – people aged 60 or over have an increased risk 
• having a previous blood clot
• having a family member who has had a blood clot
• being overweight or obese
• being pregnant – your risk is also higher for up to six weeks after giving birth
• smoking
• taking the combined oral contraceptive pill or hormone replacement therapy (HRT)

Your chances of developing a blood clot are very small if you are taking the pill or HRT, and your GP will usually assess your individual risk before prescribing either medication.

Diagnosing a pulmonary embolism can be difficult because the signs and symptoms vary between individuals and are common to many other conditions.

Around half of all people who develop a pulmonary embolism do so while they are in hospital. 

The condition may be suspected if:

• you have one or more associated risk factors, such as being over 60 years of age or having previously had a blood clot
• you have a blood clot in one of your legs, known as deep vein thrombosis (DVT)
• there is no other likely explanation for your symptoms

It is important that pulmonary embolisms are diagnosed correctly because treating them is not always easy and the treatments used can cause side effects.

Tests

There are a number of tests that may be carried out to help determine if you have a pulmonary embolism or rule out other causes of your symptoms.

For example, you may have a chest X-ray or tests to check how well your lungs are working. You may also have some of the more specialised tests discussed below.

Blood tests

Blood tests can be carried out to detect a number of signs of pulmonary embolism. One of the main tests looks for a substance called D-dimer.

D-dimer is a protein found in the blood after a blood clot has broken down. A D-dimer test can be used to help diagnose blood clotting abnormalities such as thrombosis (where a blood clot develops in a blood vessel).

If a blood test reveals high levels of D-dimer, it suggests that pieces of blood clot are loose in your bloodstream and may have become lodged in your pulmonary artery.

Computerised tomography pulmonary angiography

During a computerised tomography pulmonary angiography (CTPA), you are injected with a special dye before a computerised tomography (CT) scan is carried out. This dye makes it easier to see the blood vessels in your lungs during the scan.

A CT scan involves taking a series of X-rays to create a very detailed image of the inside of your body. If there is a pulmonary embolism in one of your lungs, it may show up as a gap in your blood supply during this scan.

Ventilation-perfusion scan

A ventilation and perfusion scan is used to examine the flow of air and blood in your lungs.

Before the scan, you will be asked to inhale a tasteless, odourless and slightly radioactive gas through a mouthpiece. This gas helps highlight the air flow in your lungs during the scan.

You will also be given an injection containing a small amount of radioactive material to highlight the blood vessels in your lungs during the scan.

If the scan shows parts of your lungs have air in them but no blood supply, it may be the result of a pulmonary embolism.

This blockage – usually a blood clot – is potentially life threatening, as it can prevent blood from reaching your lungs.

Signs and symptoms

The symptoms of a pulmonary embolism can sometimes be difficult to recognise because they can vary between individuals. However, the main symptoms include:

• chest pain – a sharp, stabbing pain that may be worse when you breathe in
• shortness of breath – this can come on suddenly or develop gradually
• coughing – this is usually dry, but may include coughing up blood or mucus that contains blood
• feeling faint, dizzy or passing out

You should visit your GP as soon as possible if you have a combination of these symptoms.

If your symptoms are particularly severe, dial 999 immediately and ask for an ambulance.

Read more about the symptoms of a pulmonary embolism and diagnosing a pulmonary embolism.

Why it happens

A pulmonary embolism is often caused by a blood clot travelling up from one of the deep veins in your legs to your heart and lungs.

A blood clot in one of the deep veins of the legs is known as deep vein thrombosis (DVT). DVT can occur for no apparent reason, but it often develops after long periods of inactivity, such as during a long-haul flight or if you are ill in hospital.

DVT can also occur during pregnancy, as a result of some medical conditions, such as cancer or heart failure, or if the wall of a blood vessel becomes damaged.

Read more about the causes of a pulmonary embolism.

How pulmonary embolisms are treated

Pulmonary embolisms are treated with anticoagulant medicines. These stop the blood clot getting bigger while your body slowly reabsorbs it, and reduce your risk of further clots developing.

If you are diagnosed with a pulmonary embolism, you will normally be given regular anticoagulant injections for about five days to begin with. You will also be prescribed an anticoagulant tablet called warfarin to take for several months.

As part of your treatment, you will need to have regular blood tests to check that the dose of warfarin you are receiving is correct. If the dose is too high, you may experience bleeding, and if it is too low, you may have further blood clots.

Keeping mobile will also help you maintain good blood circulation and prevent further blood clots forming.

Read more about treating a pulmonary embolism.

Preventing a pulmonary embolism

There are a number of methods that may be recommended to prevent a pulmonary embolism if you are at risk of developing blood clots.

These include:

• taking anticoagulant tablets, such as warfarin
• wearing compression stockings or using compression devices
• avoiding prolonged periods of inactivity
• a healthy lifestyle – such as stopping smoking (if you smoke) and eating a healthy, balanced diet that is low in fat and includes plenty of fruit and vegetables

Read more about preventing a pulmonary embolism.

If you are at risk of developing blood clots, there are a number of things that may be recommended to help prevent a pulmonary embolism.

Anticoagulants

If you are admitted to hospital to have surgery, your care team may recommend that you take an anticoagulant such as warfarin during your stay in hospital and when you go home.

This medication alters chemicals in your blood so that clots don’t form easily.

Read more about anticoagulants.

Compression stockings

It may also be recommended that you wear compression stockings and use compression devices while you are in hospital and after you go home.

Compression stockings fit tightly around your lower legs and encourage your blood to flow more quickly around your body.

Compression devices are inflatable and work in a similar way, expanding at regular intervals to squeeze your legs and encourage the flow of blood.

Increasing mobility

Pulmonary embolisms can sometimes occur when the blood flow slows down during a prolonged period of inactivity, such as during recovery from surgery or long-distance travel.

It is therefore very important that you increase your mobility as soon as possible after surgery by moving around or doing leg exercises. You should also avoid taking long journeys for four weeks after you come out of hospital.

If you are at risk of developing blood clots, consult your GP before embarking on long-distance travel. They can offer advice about ways to reduce the chances of a clot developing during your journey, such as:

• perform simple leg exercises, such as regularly flexing your ankles
• take occasional short walks when possible
• take advantage of refuelling stopovers, where it may be possible to get out and walk about
• wear elastic compression stockings

Read more about preventing blood clots when you travel.

Smoking, diet and exercise

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

• not smoking (if you smoke)
• eating a healthy, balanced diet that is low in fat, with plenty of fruit and vegetables
• getting regular exercise – at least 150 minutes a week
• maintaining a healthy weight and losing weight if you are overweight or obese

The Map of Medicine is used by doctors throughout the NHS to determine the best treatment options for their patients. NHS Choices offers everyone in England exclusive and free access to this cutting-edge internet resource, which lets you see exactly what your doctor sees.

The information in the Map has been approved by the UK’s leading clinical experts, is based on the best available clinical evidence, and is continually updated. To take advantage of this unique resource go to:

Map of Medicine: Pulmonary embolism

Signs and symptoms of a pulmonary embolism (a blood clot in the artery that carries blood to the lungs) include chest pain, shortness of breath and coughing.

It can be difficult to recognise the signs and symptoms of a pulmonary embolism because they can vary between individuals. Small clots may cause no noticeable symptoms.

Symptoms of a pulmonary embolism can include:

• chest or upper back pain – a sharp, stabbing pain that may be worse when breathing in
• shortness of breath – this may come on suddenly or develop gradually
• coughing – this is usually dry, but may include coughing up blood or mucus that contains blood
• feeling lightheaded or dizzy
• fainting

Many pulmonary embolisms are caused by a blood clot in the leg, known as a deep vein thrombosis (DVT), travelling towards the lungs. Some people with a pulmonary embolism therefore also have symptoms of DVT, such as pain, redness and swelling in one leg.

Seeking medical help

You should visit your GP as soon as possible if you experience any combination of the above symptoms. If it is not possible for you to visit your GP, you can call NHS 111 or contact your local out-of-hours service for advice.

You should dial 999 for an ambulance immediately if your symptoms are severe.

The main treatment for a pulmonary embolism is a type of medication called an anticoagulant, which stops your blood clotting easily.

The anticoagulant will prevent the clot getting larger while your body slowly absorbs it. It also reduces your risk of further clots developing.

In some cases, other treatments may be needed to remove or break up the clot. This can be done with medication called thrombolytics or, less commonly, surgery.

About half of all pulmonary embolisms occur in hospital. If you are not already in hospital, you will be admitted so you can receive treatment. If necessary, you will be given oxygen through a mouthpiece to help you breathe more comfortably.

Anticoagulants

Anticoagulants are often referred to as blood-thinning medicines, although they do not actually thin the blood. Instead, they alter chemicals in the blood to prevent clots forming easily.

The main anticoagulants used to treat pulmonary embolisms are low molecular weight heparin and warfarin.

Low molecular weight heparin is given as an injection. Regular injections of this medication are usually used as the initial treatment for a pulmonary embolism because they start working immediately.

Warfarin comes in tablet form, which you will usually take soon after the initial treatment with low molecular weight heparin. Warfarin takes longer to start working than heparin injections, but as it’s more convenient to take, it’s usually recommended for a longer period after you stop having these injections.

These medications are discussed in more detail below.

Low molecular weight heparin

There are two different types of heparin injection that may be used to treat a pulmonary embolism – standard (unfractioned) heparin and low molecular weight heparin (LMWH).

Most people diagnosed with a pulmonary embolism will initially require injections of LMWH for at least five days. After this initial treatment finishes, you will usually continue taking warfarin only.

Unfractioned heparin is rarely used nowadays because this type of heparin can work differently from person to person, which means you need to stay in hospital during treatment so your dose can be carefully monitored and adjusted if necessary.

LMWH works slightly differently from unfractioned heparin. It contains smaller molecules, which means that its effects are more predictable. LMWH can be given as regular injections without the need to stay in hospital to be monitored.

LMWH can cause side effects, including:

• a high temperature (fever)
• headaches 
• bleeding problems, such as rectal bleeding, blood in your urine or having prolonged nosebleeds)

Serious side effects are less likely to occur if you are taking LMWH, which is another reason why these medications will be recommended in most cases instead of unfractioned heparin.

Warfarin

If you are diagnosed with a pulmonary embolism, you will usually start taking warfarin tablets when you have the initial injections of LMWH.

Treatment with warfarin will usually be recommended for at least three months, although some people need to take it for longer than this. Occasionally, warfarin may need to be taken for the rest of your life.

Like unfractioned heparin, the effects of warfarin vary from person to person, so you will need to be closely monitored and have regular blood tests to ensure you are taking the right dosage. These tests can usually be carried out on an outpatient basis, which means you won’t need to stay in hospital.

You may need to have two or three blood tests a week when you first start taking warfarin until the correct dose is determined. After this, you may only need to have a blood test about once a month.

There are several factors that can alter the effectiveness of warfarin, including your diet, other medications you are taking and how well your liver is working.

While taking warfarin, you should therefore: 

• try to keep your diet consistent
• limit your consumption of alcohol and do not drink more than the recommended amounts (three to four units a day for men and two to three units a day for women)
• take your medication at the same time each day
• not take any other medicine without first checking with your GP, pharmacist or anticoagulant specialist
• avoid taking herbal medicines

As with heparin and fondaparinux, warfarin can cause a wide range of side effects, including:

• bleeding problems
• headaches
• diarrhoea
• nausea and vomiting 
• jaundice (yellowing of the skin and whites of the eyes)

Read more about the side effects of warfarin.  

Special considerations

There are some situations where your anticoagulant treatment may be different from normal.

For example, if you are pregnant, you will be given regular LMWH injections instead of warfarin tablets for the full length of your pregnancy. This is recommended because taking warfarin tablets while you are pregnant could harm your baby.

If you have cancer, you will usually be given LMWH injections instead of warfarin tablets for six months, or until the cancer is cured. This is recommended because evidence suggests that regular injections are more effective than warfarin tablets in these cases.

Removing the blockage

In more severe cases, treatment may be needed to remove the blockage.

This is usually done using injections of a type of medication called a thrombolytic, such as alteplase, which breaks up the blood clot.

Occasionally, a surgical procedure called an embolectomy may be used to treat a pulmonary embolism. This is where a surgeon makes a cut in the pulmonary artery and the blockage is sucked out.

However, this is a major operation so it is usually only recommended in particularly severe cases or if other treatments are unsuitable.