Why Do Blood Clots Form? DVT & PE Risk Explained
Slow blood flow, a greater tendency to clot and vessel-lining injury can combine to cause DVT. Dr Andrew Choong explains Virchow's triad, common risk factors, sensible prevention discussions and symptoms that need urgent medical assessment.
Why does a blood clot develop inside a vein when there has been no visible cut or injury? In this second video of my Blood Clot Awareness Month series, I explain three processes that help answer that question: slow blood flow, a greater tendency to clot, and injury to the vessel lining.
Together, these are known as Virchow's triad. One or more can contribute to deep vein thrombosis, or DVT. Understanding them helps explain why clot risk often changes around an operation, illness, pregnancy or a period of reduced mobility.
What happens when blood flow slows?
Movement helps blood return from the legs towards the heart. When the calf muscles contract, they support that return flow through the veins. Long periods without normal movement can allow blood to pool and increase the opportunity for a clot to form.
This can happen during bed rest, recovery from major surgery or a long journey. The level of risk varies: a person with a previous DVT, active cancer or several other risk factors may need a different prevention plan from someone without those factors.
Prolonged sitting is therefore part of the explanation, rather than a complete explanation on its own. An otherwise ordinary journey may become more relevant when it follows an operation or takes place during a period of illness.
When is blood more likely to clot?
Some conditions shift the body's clotting balance towards clot formation. Active cancer, pregnancy and the period after childbirth are examples. Certain hormone medicines and inherited clotting disorders can also contribute.
Previous DVT or pulmonary embolism is particularly relevant to a risk assessment. Family history can provide useful context, although it does not automatically mean someone has an inherited disorder or needs a blood test for one.
If you take a prescribed medicine that may affect clot risk, discuss it with your clinician. Stopping treatment without advice can create other problems; prevention decisions need to account for both the reason for the medicine and your individual circumstances.
How does injury to a vessel lining contribute?
The inside surface of a blood vessel normally helps blood flow smoothly. Surgery, trauma and inflammation can disturb that lining and activate the clotting response.
After an operation, several parts of the triad can overlap: there may be tissue or vessel injury, less movement than usual and a temporary change in clotting activity. That is why clot prevention is considered as part of surgical care, with measures chosen for the procedure and the patient.
A clot can also develop without a clearly identifiable trigger. The absence of recent travel or surgery does not rule out DVT.
What can help reduce the risk?
Prevention should match the situation. Useful discussions include when it is safe to mobilise after surgery, how to avoid prolonged immobility during travel, and whether a particular illness or previous clot calls for additional measures.
For some people, clinicians recommend anticoagulant medication or suitable compression as part of a prevention plan. These are not measures that everyone should start independently. Bleeding risk and other medical conditions affect what is appropriate.
Keeping normally hydrated is sensible general care, but drinking water is not a proven substitute for movement or an individual clot-prevention plan. If you have a previous clot or several risk factors, seek advice before a long journey or planned operation.
Which symptoms should not be ignored?
DVT may cause new swelling, pain, warmth or a colour change in one leg. These symptoms need urgent assessment because an examination alone cannot reliably establish the cause.
A clot can travel from a deep vein to the lungs and cause pulmonary embolism. Sudden breathlessness, chest pain, coughing up blood or fainting require emergency assessment. For these possible pulmonary embolism symptoms, call 995 in Singapore; do not wait for a routine appointment.
You can read more about deep vein thrombosis and its assessment. For a planned discussion of vascular symptoms or clot risk, contact my Singapore clinic at Mount Elizabeth Novena Specialist Centre, #09-22, 38 Irrawaddy Road, Singapore 329563.
This article provides general education and does not replace an individual medical assessment.