How does a DVT cause a pulmonary embolism? 

A deep vein thrombosis (DVT) causes a pulmonary embolism (PE) through a process called embolisation, where a blood clot formed in a deep vein—usually in the leg—breaks loose and travels through the bloodstream. The clot moves through the heart and becomes lodged in the pulmonary arteries of the lungs, blocking vital blood flow and oxygen exchange. 

What We will cover in This Article 

• The physiological link between deep veins and the pulmonary system 

• The process of embolisation: how a clot dislodges and travels 

• The role of the heart in pumping the clot into the lungs 

• Why certain types of DVT are more likely to cause a PE 

• Risk factors that accelerate the movement of a blood clot 

• Medical interventions to prevent a DVT from becoming an embolism 

The Path from Leg to Lung 

The human circulatory system is a closed loop where veins return deoxygenated blood from the extremities to the heart. When a clot forms in a deep vein (DVT), it is often attached to the vein wall. However, this attachment can be fragile. If a piece of the clot breaks off, it is carried by the venous blood flow upward toward the torso. 

Because the veins get progressively wider as they move toward the heart, the clot can travel easily through the inferior vena cava and into the right side of the heart. The heart then pumps this blood and the clot directly into the pulmonary arteries. Unlike the veins it just travelled through, the pulmonary arteries in the lungs branch out and become increasingly narrow. The clot eventually reaches a vessel too small to pass through, causing an immediate blockage or ’embolism’. 

Factors That Cause a Clot to Break Loose 

Not every DVT will result in a pulmonary embolism, but certain triggers can increase the risk of a clot dislodging from the vein wall. 

Factor	Mechanism	Risk Level
Sudden Physical Activity	Muscle contractions can squeeze the vein and ‘pop’ the clot loose.	High (especially after long rest)
Clot Age	Newer clots are more ‘gel-like’ and less securely attached than older ones.	Very High
Clot Size and Location	Clots in the thigh (proximal DVT) have a larger volume and higher flow rate.	Higher than calf clots
Changes in Blood Pressure	Rapid changes in pressure can shear the clot away from the vessel wall.	Moderate

According to the National Institute for Health and Care Excellence (NICE) [NG158], the most dangerous period for a PE is shortly after a DVT has formed, before it has had a chance to become ‘organised’ or scarred into the vein wall. 

Proximal vs. Distal DVT Risks 

Medical professionals categorise DVTs based on their location, which significantly influences the likelihood of them causing a pulmonary embolism. 

• Distal DVT (Below the Knee): These clots are located in the smaller veins of the calf. While they can still cause a PE, the risk is lower because the clots are generally smaller and the blood flow is less forceful. 

• Proximal DVT (Above the Knee): These occur in the popliteal, femoral, or iliac veins. These veins are much larger, and a clot breaking loose from here is significantly more likely to cause a life-threatening blockage in the lungs. 

Clinical Evidence and Authority 

Research published in the British Journal of Haematology highlights that approximately 50% of patients with an untreated proximal DVT will develop a symptomatic or asymptomatic pulmonary embolism. This is why NHS protocols prioritise the rapid use of anticoagulants. These medications do not dissolve the clot but stop it from growing and help it ‘stick’ more firmly to the vein wall while the body begins its natural breakdown process. 

‘The transition from DVT to PE is often silent, which is why we treat suspected leg clots with the same urgency as a suspected lung clot,’ noted a clinical review from the Royal College of Physicians in 2017. 

To Summarise 

A DVT causes a pulmonary embolism by acting as the ‘source’ of a travelling clot. When a piece of the thrombus breaks away from the leg vein, it follows the natural path of blood return through the heart and into the lungs, where it becomes stuck. The risk is highest with clots located above the knee and during the initial days after a clot forms. 

If you experience severe, sudden, or worsening symptoms, call 999 immediately. This is critical if you have a known leg clot and suddenly develop shortness of breath or sharp chest pain. 

Authority Snapshot 

This article is based on the pathophysiology of venous thromboembolism as defined by NHS England and NICE [NG158]. The content outlines the mechanical process of clot migration and the clinical distinction between different types of venous thrombosis. Dr. Rebecca Fernandez is a UK-trained physician with an MBBS and experience in general surgery, cardiology, internal medicine, gynecology, intensive care, and emergency medicine. She has managed critically ill patients, stabilised acute trauma cases, and provided comprehensive inpatient and outpatient care. In psychiatry, Dr. Fernandez has worked with psychotic, mood, anxiety, and substance use disorders, applying evidence-based approaches such as CBT, ACT, and mindfulness-based therapies. Her skills span patient assessment, treatment planning, and the integration of digital health solutions to support mental well-being.