Why Do Blood Clots Form in Atrial Fibrillation?

Blood clots are the dreaded complication of the abnormal heart rhythm aka, Atrial Fibrillation.  By itself the rhythm rarely is fatal.

What makes it a serious heart condition is the fact that blood clots commonly form if the rhythm is not treated properly. 

Atrial Fibrillation is an irregular heart rhythm arising from the upper chambers of the heart called the Atria.   Normally, the atria act as priming chambers for the pumping of the heart.  When functioning normally, the atria contract at the normal rate of 60-90 beats a minute and work in sync with the lower chambers to fill the lower chambers. 

When Atrial Fibrillation develops this normal synced contraction patter goes afoul.
The upper chambers no longer have a well-developed contraction process.  They shake and quiver chaotically at rates up to 600 per minute. 

Blood flow in the upper chambers stagnates.  Rather than moving forward quickly, the blood swirls about and often moves much like a slow moving cloud of smoke.

Indeed this is what cardiologists have named this slow flow.  We call it “smoke” or the more precise medical term is “Spontaneous Echo Contrast.”  The red blood cells are moving so slowly that they can actually be visualized by the ultrasound machine on a Trans-esophagel Echocardiogram (TEE).  (See the attached video file)

One area of the heart that is particularly likely to form a clot is called the Left Atrial Appendage.  This is a small pouch that is an embryonic remnant found in the Left Atrium (also one in the right Atrium). 

Blood often tends to swirl, clump, and clot in this chamber. Indeed, it is commonly the source of a blood clot that forms a stroke. 

Warfarin/Coumadin® are medications the make the blood thin and help protect against blood clot formation in Atrial Fibrillation

Who is likely to form a blood clot?

Cardiologists assign risk using something called the C.H.A.D.S.  score.  If you have any history of congestive heart failure, have high blood pressure (hypertension), age greater than 75 years old, diabetes, or if you have a history of previous stroke, transient ischemic attack, or have had an embolism- you are at increased risk

•    C- Congestive heart failure (any history)- 1pt.
•    H- Hypertension (prior history)- 1 pt.
•    A- Age > 75 years- 1 pt.
•    D- Diabetes- 1 pt.
•    S- Secondary prevention in patients with ischemic stroke, TIA, or systemic embolic event- 2 pts

The risk of an abnormal CHADS score in Atrial Fibrillation is the following:

•    0 Score:  low risk for ischemic stroke or peripheral embolization. Can be managed with aspirin.
•    1-2 are at intermediate risk (1.5-2.5 % risk of stroke per year) If prior ischemic stroke, TIA, or systemic embolization to be at high risk recommended to be on warfarin/coumadin®