How Atrial Fibrillation Causes Blood Clots

Atrial Fibrillation

How Atrial Fibrillation Causes Blood Clots

Atrial Fibrillation is an irregular heart rhythm caused by chaotic cell activity in the top chambers of the heart. In the absence of an accessory pathway (heart abnormality like WPW) it poses two main issues. Firstly the patient can feel unwell due to the affect Atrial Fibrillation has on blood circulation around the body. Some people may feel light headed or faint and others may feel very short of breath. Another potentially more dangerous risk is the formation of blood clots in the Atrium.
Blood clots are dangerous because they have the ability to block a blood vessel (in a similar way to when food blocks a kitchen sink). When this happens blood is unable to reach the tissue that particular vessel usually supplies, this can lead to damage and possibly death of tissue. This can be particularly dangerous if the tissue in question is an organ such as the brain or heart.

How do Blood Clots Form in Atrial Fibrillation?

As a general rule, if blood is stationary for too long it can clot – a formation of fibrin, clotting factors and platelets grouping together, similar to an early scab you may get if you cut your skin. To me blood clotting factors and fibrin in blood plasma are like children at a water park. Imagine a group of children in some water rapids, when the water is moving quickly they are unable to join together to make a human raft because they are being buffeted by the powerful currents and waves. If the rapids suddenly slows then the children will all be able to grab on to one another and group together.

We are made familiar with this concept when taking a long haul flight. People are encouraged to move about and stretch their legs to discourage Deep Vein Thrombosis (DVT). This is where the blood cells in the veins ‘pool’ (remain in one place) and as a result are given the opportunity to group together and produce a clot. If a clot formed in a Vein breaks free then it will end up in the Lungs. If the clot is fairly small it will be safely and harmlessly filtered out, if the clot is larger, you may develop a Pulmonary Embolism.
Arterial blood normally moves around the body pretty quickly (we all know what happens if you severe an artery – excuse the visuals) and for this reason blood clots forming as a result of slow moving arterial blood aren’t very common. Unfortunately Atrial Fibrillation affects the flow of blood and creates an environment that encourages the formation of clots especially in the LEFT ATRIUM. This is made even more problematic because the LEFT ATRIUM is before the brain when it comes to the bloods movement. If a clot is formed in the Left Atrium and then breaks free, it could very

well end up heading to your brain before the lungs can filter it out.

If a very small clot ends up in a blood vessel in the brain then it can cause a TIA or Transient Ischaemic Attack, larger clots can cause Stroke, Severe Stroke and Death.
In summary, we are generally protected from clots forming in this way by our bodies keeping the blood moving but Atrial Fibrillation is a game changer.

Changes in Flow in Atrial Fibrillation.

Atrial contraction pushes
blood out of the chamber


In a normal heart rhythm, your left Atrium contracts in a synchronous manor and squeezes together like a pump ejecting the blood from the cavity. Imagine filling a balloon with water and then letting go of the end, the balloon contracts and forces the water out.  When the Atrium is fibrillating this squeezing of the chamber is not occurring and blood is not being routinely and systematically ejected. This allows the blood to pool in the chamber and grants the platelets the opportunity to cling to one another or small imperfections in the heart wall, potentially developing in to a clot.

Lower Pressure in Pulmonary System

Image courtesy of
Blood is under different pressures at along different parts of the circulatory system. The pressure in the left Atrium is usually lower than most of the Systemic Circulatory system. Where pressure is lower blood is more likely to slow down or pool. Think of two cans of beer, by shaking one up you increase the pressure in it. Now open it up and see how there is much more ‘movement’ of liquid in the can you have shaken up just by changing the pressure in the ‘chamber’. In Atrial Fibrillation the pressure inside the chamber is diminished and this acts as another factor allowing the blood to be stationary.
The Left Atrial Appendage
Cave Like Structure at 3 o’clock


Within the left Atrium is the Left Atrial Appendage, A structure that helps the heart deal with higher atrial pressures that can be caused by the strong muscles of the Left Ventricle contracting. This appendage provides relative stasis owing to its shape and the trabeculations within it is thought to play a major role.
Look at the video below, the river is flowing furiously downstream, however due to its structure and eddy occurs where water and structures can get stuck. It is theorised that a similar process can occur at the strial appendage.
Remember, stationary blood can clot, if the clot then breaks free it is moving around your blood stream and can do some damage. Some clots will remain in your Atrium harmless until they dislodge. This is why before a DC Cardioversion – a process where your hearts rhythm is returned to normal, patients often have a Trans Oesophageal Echo. A test that looks to see if any clots have been formed, if there is a clot in place then it would be unsafe to return the person to a natural rhythm before the clot is broken down.
Clotting in Atrial Fibrillation is why it is important for people to be on an Anitcoagulant (such as Warfarin). These are medications that help to prevent the formation of blood clots and in turn reduce the risk of stroke. More novel treatments include the ‘closure’ of the left Atrial Appendage using a small PLUG. I will feature this in a future post.
Right time to pack for my mid week break!
Thanks for reading I hope this has cleared up the need for preventative medication in Atrial Fibrillation.
Cardiac Technician

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