How Atrial Fibrillation causes a stroke

The Heart pumps blood around the body so its easy to understand that a blood clot that forms and detaches from inside your heart could pass into a brain vessel and thus cause damage.  What perhaps is less easy to understand is HOW a blood clot forms in the heart and WHY? In this short article I have described the workings of the chambers of the heart and how this can sometimes lead to blood clot formation.

The whole process is shown very nicely in this animation:

Normally, the left heart contracts to pump blood into the arteries around the body:

1/ Pulmonary veins carry blood into the left atrium

Four large pipes (pulmonary veins) carry oxygenated blood from the lung circulation into the left atrium or ‘receiving chamber' situated at the back of the heart.

2/ Atrial contraction primes the left ventricle

Normally, prior to every heart beat, the left atrial muscles contract to squeeze blood through the mitral valve priming the main pumping chamber of the heart (the left ventricle).

This creates a strong and steady heart beat with a regular ‘pulse’. The priming action is believed to increase efficiency of the heart pumping action- similar to the turbo charger on a high performance engine.

3/ The electro myocardial signals contraction of the left ventricle

As the left ventricle fills, the electrical signal that initiates each heart beat (situated in the sinoatrial node or natural pacemaker) sweeps through the atrioventricular node and down the conducting system (know as the His bundles) - causing the ventricles to contract, ejecting the blood from the heart into the aorta which feed the whole body- which can be felt as a pulse in the neck or wrist

What stops blood sticking normally?

  1. The walls of the left atrium are normally very smooth and slippery.
  2. When in normal rhythm, the atrial muscles contract before each heart beat, and this movement helps prevent blood 'sticking'.
  3. Healthy left atrial lining (endothelium) can secrete anticoagulant proteins which help inhibit clot formation.

As people get older the muscles of the walls of the atria are more likely to become electrically unstable 

Instead of beating just once for every heart beat or pulse, the atria can go into an abnormal rhythm of their own where they quiver at roughly 600 cycles/ min, known as ‘atrial fibrillation’ where the usual movement of the atrial wall is lost and no longer functions to prime the ventricle.

Ageing, high blood pressure, high alcohol intake or heart disease all increase the chances if this occurring either intermittently or permanently.

Atrial fibrillation a condition that affects one in four of us- and by the age of 70, over 10% of us will have it!

During periods of atrial fibrillation the fibrillation causes quivering (roughly 600 cycles per minute) of the atrial muscles instead of contraction and in time the endothelial lining loses some of its anticoagulant properties. This creates the environment for clots to form- especially in the recess of the left atrial appendage.

During atrial fibrillation the heart beats erratically and often faster- which sometimes causing symptoms

The pumping of the various heart chambers is activated by tiny electrical signals passing through their walls so that during periods of atrial fibrillation the ventricles pump at an erratic and sometimes fast rate. As well becoming less efficient by losing the priming action of the atria, the heart beat and the pulse becomes faster and is no longer  regular (like a clock) but instead becomes irregular or erratic.

Because a number of factors determine the rate at which the heart pumps during atrial fibrillation, many people will have either no or minimal symptoms, but over half will notice a faster heart beat (sometimes called palpitation) , breathlessness or reduced energy.

During AF >24 hours stasis can cause a blood clot

During periods of continuous atrial fibrillation lasting more than a day, the lack of motion in the walls of the left atrium creates stasis where blood is no longer moving across surface, especially in the region of its recess known as the left atrial appendage. Because stagnant blood promotes coagulation - where blood proteins interact together to form a gel, this can eventually form a blood clot on the inside surface of the heart.

Occasionally a blood clot can detach and embolise into the circulation causing a stroke or heart attack

Although usually adherent to the atrial wall, a blood clot can sometimes detach itself and subsequently carried by the blood stream through the heart and into the circulation - a process known as embolism. An arterial embolism is a dangerous condition because the blood clot can travel into an artery which tapers- and thus becomes blocked, thus causing a loss of oxygen and energy to a vital part.  If carried to the brain artery this block can cause a stroke, to the heart a heart attack’, but other scenarios include limb and bowel ischaemia 

Related stories

Calculating your personal risk of stroke in atrial fibrillation
I have been diagnosed with Atrial Fibrillation. What Now?
Heart or Brain attack resembles an Aircrash
More information about atrial fibrillation 
See HASTE Patient LIBRARY for more Information on AF 



Written by

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Edward Leatham is a Consultant Cardiologist in Surrey and a Trustee of Haste and Haste Academy.

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