Updated 22 May 2015


Thrombus is a process that allows the blood to clot.



Blood clotting is a natural process that allows the blood to thicken and form a clot of blood cells and fibrin to stop the bleeding when a blood vessel is severed. If the body did not have the ability to clot blood, then people would bleed to death after even a minor cut.

However, when a thrombus (blood clot) forms in an artery, blocking the blood flow to the heart muscle or the brain, a heart attack or stroke can follow. Or when blood lingers in one of the heart’s chambers (due to certain heart conditions), a piece of the blood clot can be pumped from the heart, travel through the bloodstream, and lodge in either an organ or an artery, cutting off the blood supply from that point. This is called an embolism.

There are many other conditions related to blood clots, for example:

  • Coronary thrombosis is a blood clot in coronary arteries leading to a heart attack
  • Deep vein thrombosis is a blood clot in the leg veins
  • Pulmonary embolism is a blood clot in the lungs
  • Retinal vein occlusion is a blood clot in a vein of the eye


Three primary factors can predispose a person to the formation of a blood clot (thrombus):
  • Roughness or scar tissue along the endothelial lining of a blood vessel. This endothelial injury on its own can lead to blood clotting in the heart chambers (due to damage to the inner lining after a heart attack or to infection in the heart valves) or in the arteries. Blood clots can form in arteries where the endothelial layer of the vessel is damaged by atherosclerotic plaque, or where it is injured by trauma or inflammation. The injury may be caused by hypertension, turbulent flow over damaged heart valves or bacterial endotoxins. Elevated blood cholesterol or homocysteine levels, radiation and cigarette smoke can cause this damage. All endothelial damage will lead to the exposure of underlying collagen and this will be the first step in platelet aggregation and formation of a blood clot.
  • Pockets of sluggish blood flow and turbulent blood. Turbulence causes endothelial injury and also leads to countercurrents and pockets of sluggish flow, leading to the build up of clots. This can happen at sites of artherosclerotic plaques, sites of abnormal arterial dilations (aneurysms), after a previous heart attack (causing damage to the endothelial lining and to the heart muscle, leading to sluggish flow) and due to mitral valve damage (after rheumatic heart disease – a common problem in South Africa). Atrial fibrillation can also lead to sluggish flow. Certain blood disorders such as a high red cell count (as in Hemochromatosis) and sickle cell anaemia can thicken the blood and lead to slower blood flow.
  • Hyper-sticky blood is uncommnon, but mainly related to genetic blood clotting disorders (including the factor V or Leiden mutation, which can be an indicator of risk for Deep Vein Thrombosis).

Who is at risk?

Injury to the inner walls of the blood vessels and sluggish blood flow are associated with certain conditions. People with the following conditions are more at risk to develop blood clots:
  • Atherosclerosis
  • Trauma to a blood vessel due to an accident, surgery, fracture or burn
  • Recent heart attack or stroke
  • Heart valve disease
  • Heart failure
  • Infection
  • Auto-immune disorders like lupus (SLE) or rheumatoid arthritis)
  • Bleeding disorders like haemophilia
  • Inflammatory bowel disease
  • Certain cancers
  • History of varicose veins or other vascular diseases
  • Pregnancy
  • High blood pressure
  • Elevated platelet levels
  • Atrial fibrillation
  • Genetic factors such as inherited tendency for deep vein thrombosis

In addition, the following lifestyle risk factors can increase the chances of developing a blood clot. These risk factors include:

  • Smoking
  • Obesity (more than 10 kg overweight)
  • Lack of exercise
  • Bedrest more than four days
  • Use of the Pill
  • High dosages of oestrogen
  • Sitting in one position for a prolonged time (as in an aeroplane)

What are the symptoms?

The symptoms of a blood clot depend on where the blood clot is located in the body. New Page 1

Where Blood Clot is Lodged

Possible Symptoms

Lung (Pulmonary embolism)

Sharp chest pain, rapid heart rate, blood-tinged coughing, shortness of breath and low grade fever.

Arm or Leg

(eg Deep vein thrombosis)

A blood clot in an artery can cause sudden pain, swelling and a slight blue colouration. If it is in a vein, then it could cause swelling and tenderness.

Brain (Stroke)

Visual disturbances, weakness, seizure, speech impairment, leading to a stroke or a transient ischaemic event. Artherosclerotic plaques in the carotid artery can lead to blood clots lodging in the carotid artery or deep in the brain.

Heart (Heart attack)

Chest pain could lead to a heart attack. Blood clots forming in the heart can also travel to any organ or artery in the body. Conditions that can lead to blood clots forming in the heart include heart valve disease, previous heart attack, atrial fibrillation and heart failure.


Severe abdominal pain, vomiting and/or diarrhoea.

How are blood clots diagnosed?

Diagnosis of a blood clot begins with a complete medical history, physical examination and other sophisticated tests.

How can blood clots be treated?

The treatments for a blood clot will depend on whether the clot has formed in a vein or artery, the location and size of the clot, and the patient’s general health.

1. If it has formed in an artery and has caused a heart attack or stroke, then clot busters (eg streptokinase) may be used for the first two days to dissolve the blood clot. They must be given within three hours of the heart attack or stroke. More invasive procedures may be indicated, where a catheter is inserted and delivers thrombolytic enzymes directly to the blood clot, or the catheter will be used to treat the blood clot by angioplasty or a stent procedure. Higher concentrations of enzymes can be used. This medication is followed by treatment with anti-clotting medication (eg heparin or warfarin) to prevent more blood clots from forming.

2. If a blood clot forms in a vein, you face the risk that the blood clot can travel to the lung and cause a life-threatening pulmonary embolism. In this case blood clots are usually treated with a combination of painkilling medication, anti-clotting medication and elevation/bandaging of the affected area to reduce swelling.

In extreme situations, surgery may be required to remove a clot before it starts to travel to the lungs.

Can blood clots be prevented?

Prevention is important, particularly in people prone to blood clots.

The risk group for blood clots in arteries includes:

  • People with mitral valve damage
  • People with an enlarged heart
  • People with heart failure

The best prevention in this group is: regular exercise, effective treatment of other heart-related medical conditions, and anti-clotting medication. This can be an aspirin daily.

The risk group for developing blood clots in a vein include:

  • People with varicose veins
  • Obese people
  • People with chronic heart failure
  • Or people with habits or work that require them to stay in one position for long periods of time as with air travel (Venous thrombosis related to air travel is known as Economy Class Syndrome).

The best prevention in this group is regular exercise, movement or stretching, maintaining optimal weight, and not smoking, drinking enough fluids and avoiding alcohol (especially during a flight). People should avoid sitting with crossed legs and wearing tight garments below the waist.

To reduce the risk of blood clot formation after surgery (particularly knee, shoulder or hip surgery) during the recovery period, your doctor may prescribe aspirin or other anti-clotting medication to prevent blood clots.

Doing calf and leg exercises before surgery and as soon as possible after surgery and during a flight can be helpful in blood clot prevention.

(Reviewed by Dr Sean Latouf, Cardiologist, Christiaan Barnard Memorial Hospital)


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