What Is a Pulmonary Embolism? Symptoms, CT Scans and Treatment Explained

What actually happens during a pulmonary embolism?

Blood normally travels from the right side of the heart into the pulmonary arteries, where it reaches the lungs.

If a blood clot travels through the venous circulation, it can pass through the right side of the heart and become lodged in a pulmonary artery.

This obstructs blood flow through part of the lung circulation.

The consequences depend on:

• —how large the clot burden is
• —which arteries are affected
• —the health of the heart and lungs
• —whether the right side of the heart is placed under strain

Where does the clot come from?

Most pulmonary emboli arise from a deep-vein thrombosis.

DVT usually develops in the deep veins of:

• —the calf
• —the thigh
• —the pelvis

Part of the clot can break away and travel to the lungs.

This is why DVT and pulmonary embolism are grouped together under the term venous thromboembolism — VTE.

What are the symptoms of pulmonary embolism?

Symptoms vary.

Common symptoms include:

Sudden breathlessness

This may occur without warning.

Chest pain

PE often causes sharp chest pain that becomes worse when taking a deep breath. This is known as pleuritic pain.

Coughing up blood

This is called haemoptysis.

Fast heart rate

The body's response to reduced oxygenation and circulatory stress may increase the pulse.

Dizziness or collapse

A large PE can significantly affect circulation.

Symptoms of DVT

Some people also have:

• —one-sided leg swelling
• —calf pain
• —redness
• —tenderness

Not everybody with PE has all of these symptoms.

When is it an emergency?

A pulmonary embolism can be life-threatening.

According to current NHS guidance, call 999 or attend A&E urgently if someone has features such as:

• —severe difficulty breathing
• —chest or upper-back pain
• —a very fast heartbeat
• —collapse or fainting

Who is at risk?

A blood clot is more likely to develop when one or more components of normal clot prevention are disturbed.

Risk factors can include:

• —recent surgery
• —prolonged immobility
• —hospital admission
• —major trauma
• —active cancer
• —previous DVT or PE
• —pregnancy and the period after birth
• —oestrogen-containing hormonal treatment
• —certain inherited or acquired clotting disorders

Sometimes PE occurs without an obvious major trigger. This is termed an unprovoked pulmonary embolism.

Does flying cause pulmonary embolism?

Long journeys involving prolonged immobility can increase venous-thrombosis risk, particularly when other risk factors are present.

However, most people who take a flight will not develop a pulmonary embolism.

Individual risk depends on the wider clinical context.

How do doctors decide whether PE is likely?

Symptoms such as breathlessness and chest pain have many possible causes.

Doctors therefore combine:

• —medical history
• —examination
• —vital signs
• —ECG
• —chest X-ray where appropriate
• —clinical prediction tools
• —blood tests
• —imaging

Current NICE guidance recommends using the two-level PE Wells score when pulmonary embolism is suspected.^[1]

What is the Wells score?

The Wells score combines clinical factors associated with PE.

These include factors such as:

• —signs of DVT
• —heart rate above 100
• —recent surgery or immobilisation
• —previous DVT/PE
• —coughing up blood
• —malignancy
• —whether PE is considered more likely than an alternative diagnosis

Under the NICE two-level system:^[1]

• —More than 4 points: PE likely
• —4 points or less: PE unlikely

It is a clinical decision tool — not a self-diagnosis calculator.

What is a D-dimer?

D-dimer is a blood test that detects products produced when blood clots are broken down.

A normal D-dimer can help rule out PE in appropriately selected people with a low clinical probability.

However:

A raised D-dimer does not mean you definitely have a blood clot.

D-dimer may be elevated for many reasons, including:

• —infection
• —inflammation
• —cancer
• —recent surgery
• —pregnancy
• —increasing age

NICE therefore uses D-dimer within a clinical diagnostic pathway rather than in isolation.^[1]

For people over 50, NICE advises considering an age-adjusted D-dimer threshold.^[1]

What is a CT pulmonary angiogram?

A CT pulmonary angiogram — CTPA — is a specialised contrast-enhanced CT scan designed to show the pulmonary arteries.

It is the principal imaging test used for suspected PE in many patients.

Contrast containing iodine is injected through a cannula in the arm.

Images are then acquired as the contrast passes through the pulmonary arteries.

A clot appears as a filling defect inside the contrast-filled artery.

What does a pulmonary embolism look like on CT?

Normally, pulmonary arteries fill brightly with contrast.

A pulmonary embolus creates an area where contrast does not completely fill the vessel.

Radiologists assess:

• —which arteries contain clot
• —how extensive the clot burden is
• —whether there are signs of right-heart strain
• —whether there are other findings that might explain the symptoms

Modern CTPA is central to PE diagnosis and can also provide prognostic information in selected cases.^[3]

Does everybody suspected of PE need CT?

No.

Imaging is guided by clinical probability.

Under current NICE recommendations:^[1]

PE likely

If the Wells score is more than 4, CT pulmonary angiography should generally be offered promptly where suitable.^[1]

PE unlikely

If the Wells score is 4 or less, D-dimer testing is usually performed first. A negative result may avoid unnecessary imaging. A positive D-dimer may lead to CTPA.^[1]

This approach avoids performing CT unnecessarily in everyone with chest pain or breathlessness.

What if I cannot have CT contrast?

CTPA may not be suitable in some situations, including selected people with:

• —severe renal impairment
• —significant contrast allergy
• —situations where radiation exposure is a particular concern

NICE identifies ventilation/perfusion imaging — V/Q SPECT or V/Q scanning — as an alternative in appropriate patients.^[1]

What is a V/Q scan?

A ventilation/perfusion scan compares:

• —airflow into the lungs
• —blood flow through the lungs

A mismatch between ventilation and perfusion can suggest pulmonary embolism.

It is particularly useful in selected patients when CTPA is unsuitable.

Can a chest X-ray diagnose PE?

Usually not.

A chest X-ray may be normal in pulmonary embolism.

Its role is often to look for other explanations for symptoms, such as:

• —pneumonia
• —pneumothorax
• —other lung abnormalities

NICE recommends chest X-ray as part of the initial assessment when PE is suspected.^[1]

How is pulmonary embolism treated?

The main treatment is anticoagulation.

These medicines reduce the blood's tendency to form further clots and allow the body's natural processes to gradually deal with the existing clot.

The choice of anticoagulant depends on factors including:

• —kidney function
• —cancer
• —pregnancy
• —other medications
• —bleeding risk
• —other medical conditions

Treatment decisions should be made by the treating clinical team.

Does anticoagulation dissolve the clot instantly?

No.

Anticoagulants are sometimes referred to as “blood thinners”, but they do not simply dissolve the clot immediately.

Their main role is to prevent extension and new clot formation while the body's natural fibrinolytic processes gradually remodel the existing thrombus.

What about thrombolysis?

Thrombolytic drugs actively break down clot.

Because they can cause serious bleeding, they are not routinely given to every patient with PE.

They are generally reserved for selected high-risk situations, particularly when the pulmonary embolism is causing significant haemodynamic instability.

Other advanced treatments may be considered in specialist centres.

Does everyone with PE need to stay in hospital?

No.

Current NICE guidance says that selected low-risk patients can be considered for outpatient treatment using validated risk-stratification tools and an appropriate follow-up plan.^[1]

Other patients require hospital admission.

The decision depends on clinical stability, clot severity, comorbidities, bleeding risk and practical circumstances.

How long does treatment last?

The duration of anticoagulation depends on why the clot occurred and the risk of recurrence versus bleeding.

Factors include whether the PE was:

• —provoked by a temporary risk factor
• —unprovoked
• —related to active cancer
• —recurrent

The appropriate duration should be discussed with the treating clinician.

Can pulmonary embolism happen again?

Yes.

People who have experienced VTE have a higher risk of recurrence than someone who has never had one.

The recurrence risk depends on the original cause and individual factors.

This is one reason deciding when to stop anticoagulation requires careful clinical assessment.

What happens to the clot afterwards?

In many people, the clot gradually resolves or becomes incorporated into the vessel wall.

Symptoms improve as blood flow and heart strain recover.

However, some people experience persistent breathlessness or exercise limitation afterwards.

A small minority can develop chronic thromboembolic pulmonary hypertension, where persistent obstruction contributes to abnormally high pressure in the pulmonary circulation.

Persistent or worsening symptoms after PE deserve clinical review.

Should everyone with PE have thrombophilia testing?

No.

NICE does not recommend routine hereditary thrombophilia testing for everyone who experiences a venous thromboembolism.^[1]

Testing may be useful in selected circumstances.

This is particularly dependent on:

• —whether the clot was provoked or unprovoked
• —family history
• —whether anticoagulation might be stopped

The key message

Pulmonary embolism is important because it can be life-threatening — but modern diagnostic pathways and effective anticoagulant treatment mean many patients recover well.

Rapid recognition matters.

Sudden breathlessness, pleuritic chest pain and coughing up blood should not simply be ignored.