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How Long Does It Take for an A-Fib Clot to Form? The ASSERT Clinical Trial

Background: Of A-Fib stroke patients, 23% die and 44% suffer significant neurologic damage. This compares to only an 8% mortality rate from other causes of stroke.

How Long Does It Take for a Clot to Form? Some doctors say it only takes around 5 minutes for an A-Fib clot to form and cause a stroke that kills you.

This is generally not accepted thinking among Cardiologists and Electrophysiologists (EPs). The ASSERT clinical trial gives us some insights.

How Do Clots Form and Cause Strokes?

Clots aren’t formed instantaneously. It takes a while for blood to pool and form a clot of significant size. If you have a ten-minute attack of A-Fib, for example, it’s unlikely a clot/stroke will develop.

When someone is in A-Fib, blood is not being effectively pumped out of the left atrium. There are spots where blood can pool such in as the Left Atrial Appendage (LAA). This pooled blood can form a clot.

When the left atrium again beats normally, it can push this clot downstream into the left ventricle and into the bloodstream. From there, the clot can travel into the brain causing an ischemic (blocking) stroke.

Patients in permanent A-Fib are at higher risk of clots and stroke. But not in just a few minutes.

(Another risk of A-Fib is a hemorrhagic stroke when a blood vessel bursts, causing bleeding in the brain.)

ASSERT stands for “Asymptomatic Atrial Fibrillation and Stroke Evaluation in Pacemaker Patients and the Atrial Fibrillation Reduction Atrial Pacing Trial”.

The ASSERT Study

The ASSERT clinical trial is a fascinating study with data collected by pacemakers and defibrillators (ICDs). Researchers looked at pacemaker patients and their risks of developing Silent A-Fib and stroke. Their primary question was: Do Pacemakers Prevent A-Fib?

A secondary benefit of the study is the A-Fib patient data collected. In particular, when and how long it takes for A-Fib patients to develop a serious risk of stroke.

The study gives us insight into when and how long it takes for A-Fib patients to develop a serious risk of stroke.

Study Description: The ASSERT study enrolled 2,580 patients, 65 years of age or older, with hypertension and no history of A-Fib, in whom a pacemaker or defibrillator (ICD) had recently been installed.

The pacemaker and ICD devices were programmed to detect silent A-Fib (i.e., Subclinical Atrial Tachycardia [SCAF]) when the heart rate reached 190 beats or more per minute lasting more than 6 minutes. The devices were checked at a clinical visit 3 months later. These patients were then followed up for around 2.5 years.

How Long in Silent A-Fib to Significantly Increase Clot/Stroke Risk

In the ASSERT study they found that it took more than 17.72 hours to significantly increase the annual stroke risk. The results of all patients are divided into four quartiles:

Duration Quartile: Time in Silent A-FibAnnual Stroke Risk
≥ 0.86 Hours1.23 %
0.87-3.63 Hours0 %
3.64-17.72 Hours1.18 %
˃ 17.72 Hours4.89 %

Researchers found the annual stroke risks for patients with Silent A-Fib for less that 17.72 hours were similar to the stroke risk for healthy people (which is considered to be 1%).

The ASSERT study basically said that it takes around 24 hours of silent A-Fib to develop a serious clot/risk of stroke (on average 3.1%).

Contrary Interpretation: In a later analysis of the same ASSERT study by Van Gelder (2017), patients with lengths of Subclinical Atrial Tachycardia (SCAF) from 6hrs to 24hrs were not significantly different from patients without SCAF.

Similar Trial Results: The TRENDS study, a prospective, observational study, also used implanted devices and found similar results as the ASSERT study.

Do Pacemakers Work to Prevent A-Fib?

The primary question of the ASSERT study was: Do Pacemakers Prevent A-Fib?
Finding: Pacemakers (continuous overdrive pacing) “does not prevent clinical atrial fibrillation.”

Editor’s Comments

Editor's Comments about Cecelia's A-Fib storyShorter Episodes of A-Fib Not Generally Dangerous: Despite studies such and ASSERT and TRENDS, we still need many more studies on how long it takes for a clot/stroke to form. Probably the most useful data to date does come from the ASSERT study where it took around 24 hours of silent A-Fib before clot/stroke risk was significantly increased.
People with shorter episodes of A-Fib or silent A-Fib, such as may occur after a successful catheter ablation, may not need to be on anticoagulants at all. Remember that anticoagulants are high risk drugs that shouldn’t be taken unless there is a real risk of stroke.

The general consensus is that A-Fib clots/strokes take around 24 hours to develop. In a popular article in Bottom Line Health, Dr. Antonio Gotto, cardiovascular disease specialist at Weill Cornell Medical College in New York City, says it takes one day for a clot to form.

Resources for this article

• Healey, J.S. et al. Subclinical Atrial Fibrillation and the Risk of Stroke. The New England Journal of Medicine 2012; 366:120-129. DOI: 10.1056/NEJMoa1105575

• Glotzer, T. V. et al. The Relationship Between Daily Atrial Tachyarrhythmia Burden From Implantable Device Diagnostics and Stroke Risk―The TRENDS Study. Circulation: Arrhythmia and Electrophysiology, August 4, 2009. 2009;2:474-480. doi: 10.1161/CIRCEP.109.849638

• Gotto, Jr., Antonio M. Bottom Line Health, Vol 26, November 2012, p. 4.

• Van Gelder, I.C. et al. Duration of device-detected subclinical atrial fibrillation and occurrence of stroke in ASSERT. European Heart Journal, Volume 38, Issue 17. 1 May 2017, Pages 1339-1344,

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