Thromboembolism (Blood Clots)

How clots form

Three factors, known as Virchow's triad, promote clotting:

1. slow or stagnant blood flow: immobility, long travel, bed rest, atrial fibrillation
2. damage to the vessel wall: surgery, injury, catheters, atherosclerosis
3. blood that clots too easily (hypercoagulability): cancer, pregnancy, estrogen, inherited conditions, inflammation

Venous thromboembolism

Deep vein thrombosis

A clot in a deep vein, usually in the calf or thigh. See deep vein thrombosis.

Symptoms:

• swelling of one leg
• pain or tenderness, often in the calf
• warmth and redness or discoloration
• sometimes no symptoms

Pulmonary embolism

A piece of clot travels to the lungs. See pulmonary embolism.

Symptoms:

• sudden shortness of breath
• sharp chest pain, worse with breathing
• fast heart rate
• coughing blood
• lightheadedness or fainting

PE is a leading preventable cause of death in hospital patients.

Arterial thromboembolism

Clots in arteries usually come from the heart (especially with atrial fibrillation, recent heart attack or heart valve disease) or from atherosclerotic plaques in large arteries.

• Stroke: a clot blocks a brain artery. See stroke.
• Acute limb ischemia: a peripheral arterial embolism blocks an artery in the arm or leg, causing the "6 Ps": pain, pallor, pulselessness, paresthesia (tingling), paralysis and poikilothermia (coldness).
• Mesenteric ischemia: blocked gut arteries cause severe abdominal pain.
• Kidney and spleen infarcts: flank or left upper abdominal pain.

Risk factors

Diagnosis

• clinical scores (Wells score) estimate likelihood
• D-dimer blood test: a normal result in a low-risk person helps rule out VTE
• compression ultrasound of the leg for DVT
• CT pulmonary angiogram for PE; V/Q scan when CT is unsuitable
• ECG and echocardiogram to look for a heart source of arterial clots
• CT angiography or catheter angiography for arterial occlusions
• thrombophilia testing in selected people

Treatment

Anticoagulants

• Direct oral anticoagulants (apixaban, rivaroxaban, edoxaban, dabigatran) are first-line for most VTE.
• Heparins (low-molecular-weight heparin) are used in pregnancy, some cancers and initially in hospital.
• Warfarin for some patients, such as those with antiphospholipid syndrome or mechanical heart valves.

Treatment usually lasts at least 3 months. People with unprovoked clots or ongoing risk factors may continue indefinitely.

Advanced treatment

• thrombolysis (clot-dissolving drugs) for massive PE with shock, and for stroke within time windows
• catheter-directed therapy or thrombectomy for selected PE, DVT, stroke and limb ischemia
• surgical embolectomy for acute limb ischemia
• IVC filters when anticoagulation is not possible

After a clot

• compression stockings may ease symptoms of post-thrombotic syndrome
• regular follow-up on anticoagulants
• long-term anticoagulation for atrial fibrillation to prevent stroke

Complications

• death from PE or major stroke
• post-thrombotic syndrome: chronic leg swelling, pain and ulcers
• chronic thromboembolic pulmonary hypertension
• limb loss after arterial occlusion
• bleeding from anticoagulants

Prevention

• in hospital: blood thinners, compression devices and early mobilization after surgery and during illness
• travel: walk every 1 to 2 hours, flex ankles, stay hydrated; compression stockings for high-risk travelers
• lifestyle: stay active, maintain a healthy weight, stop smoking
• hormones: discuss clot risk before starting estrogen-containing contraception or hormone therapy
• atrial fibrillation: anticoagulation when recommended

Frequently asked questions

What is the difference between thrombosis and thromboembolism?

Thrombosis is the formation of a clot in a blood vessel. Thromboembolism is when that clot, or a piece of it, travels through the bloodstream and blocks another vessel, such as a leg clot traveling to the lungs as a pulmonary embolism.

What are the warning signs of a blood clot?

A leg clot causes swelling, pain, warmth and redness in one leg. A lung clot causes sudden breathlessness, chest pain and a fast heart rate. An arterial clot causes stroke symptoms or a suddenly cold, pale and painful limb. All need urgent care.

Can a peripheral artery embolism cause death?

Yes. A clot suddenly blocking an artery to a leg or arm (acute limb ischemia) can cause tissue death within hours. Without fast treatment, it can lead to amputation, and the combination of muscle breakdown, kidney failure and underlying heart disease can be fatal. It needs emergency care.

How long do you need blood thinners after a clot?

Most people take anticoagulants for at least 3 months. If the clot was triggered by a temporary risk such as surgery, treatment may stop after 3 to 6 months. Unprovoked or recurrent clots, or ongoing risks like cancer, may need long-term treatment.

How can I prevent blood clots when traveling?

On trips longer than 4 hours, get up and walk every 1 to 2 hours, flex and stretch your calves, stay hydrated and avoid excess alcohol. People at higher risk may be advised to wear compression stockings or, rarely, take an anticoagulant.