First Light Healthcare


With Dr Alex John


What is a blood clot and how do they form?

Sometimes, our bodies’ clotting system goes wrong and a blood clot (thrombus) develops in a blood vessel when it shouldn’t. This can happen if…

  1. The surface of a blood vessel becomes rough or disturbed (such as after an operation or bone fracture). Studies have shown that 25-50% of surgical patients develop a blood clot after an operation.


  1. The blood is moving very slowly, when it is more likely to clot (e.g., during lack of movement on a long-haul flight or long car ride).


  1. The components of the blood are different due to medication taken (such as the oral contraceptive pill), genetic variation (blood clots can be hereditary) and with certain cancer types.


Pieces of these clots can break off and travel around the body with the blood. These moving clots are called emboli and usually only stop moving when they get to a narrow part in the vascular system. Emboli moving in the right side of the vascular system usually get lodged in the lungs (causing a pulmonary embolism or PE); while emboli in the left side of the vascular system usually lodge in the brain and kidneys.

A pulmonary embolism cannot cause a stroke because it occurs in the right side of the vascular system. However, all types of clots and emboli have the potential to be fatal.


How do I know if I have Deep Vein Thrombosis (DVT) or Pulmonary embolism (PE)?

While 65% of below-the-knee DVTs are asymptomatic, the remaining occurrences present with pain, swelling and redness in the leg.

Pulmonary embolism sometimes causes a sharp pain in the chest that is worse when you breathe in. A chest x-ray will not reveal whether you have suffered a pulmonary embolism, instead a specialist scan is required.


What is the chance of developing Deep Vein Thrombosis (DVT) when flying?

It is quite low for the general population, estimates range from: 1 in 10000 to 1 in 40000.


What can I do to reduce the chance of developing Deep Vein Thrombosis (DVT) or Pulmonary embolism (PE) when flying?

It is advisable to keep hydrated by drinking lots of water and get up regularly to move and stretch your limbs. There is no evidence that prophylactic aspirin helps.

If you are at risk of developing Deep Vein Thrombosis (one factor), wearing stockings (flight socks) can help prevent the pooling of blood. However, if you are at high risk for developing blood clots (more than one factor), then your GP may recommend medication that can be taken orally or be given as an injection.


How are Pulmonary Embolisms (PEs) and Deep Vein Thrombosis (DVTs) treated?

They are usually treated with blood thinners administered over several months. Occasionally, minor clots do not require anticoagulation. Sometimes people require medication long term to prevent further clot formation. Rarely, filters are placed in the body to collect dangerous clots.



Ian Wilkinson et al, 2017. The Oxford Handbook of Clinical Medicine. Oxford: Oxford University Press.


Arthur Guyton and John Hall, 1984. Textbook of Medical Physiology. London: W.B. Saunders Company.


By Cape Byron Medical Centre