Picking up where we left off last week, we now turn to the most serious aspect of atrial fibrillation—that it can cause a stroke. And, as I indicated previously, strokes occur in people with atrial fibrillation whether they are aware of the atrial fibrillation or not. In fact, when looking at people older than 55 who have had a stroke with no readily discernible cause, subsequent monitoring reveals atrial fibrillation in 10-20% of them, making it important to screen these people for atrial fibrillation.
Why does atrial fibrillation cause strokes? When the atria are fibrillating, they aren’t actually pumping, and blood can sit in nooks and crannies within these chambers and form clots due to stasis (lack of movement). One large cavity that accounts for 90% of the clots that we find in atrial fibrillation is called the left atrial appendage—more on that later. If a clot dislodges from within the heart, it travels into the circulation and can potentially go to one of the brain vessels and block it off. Then that part of the brain loses its blood supply and dies—this is what we call a stroke.
While atrial fibrillation raises the risk of stroke about five-fold, other factors affect a person’s absolute risk. A history of any of the following—congestive heart failure, hypertension, diabetes, stroke, vascular disease—increases the risk. In addition, getting older does, too, and so does being female (though not as much as the other factors). Doctors have created a scoring system called the CHA2DS2-VASc Score to determine who needs to be treated to prevent strokes. Each of the letters stands for one of the risk factors.
We learned several years ago that aspirin—while beneficial for preventing some types of strokes—has minimal impact on lowering stroke risk from atrial fibrillation. At the same time, we discovered that warfarin (often used as the brand Coumadin) lowers the risk by 70%! However, warfarin is difficult to use, as we have to monitor a blood test called the INR anywhere from weekly to monthly, and the drug interacts with multiple medications, as well as foods, making for instability in its dosing.
Luckily, we have four drugs that have been available during the last decade—apixaban, dabigatran, rivaroxaban, and edoxaban—that don’t require blood tests, as their blood levels are quite stable. They are generally safer and more effective than warfarin, so we preferentially use them. Their downside is that they are more expensive, all of them still being on patent—their brand names are Eliquis®, Pradaxa®, Xarelto® and Savaysa®. And the downside to all anticoagulants (the category of medications that these drugs are part of) is that they increase the risk of bleeding. However, in patients who have an elevated stroke risk, the bleeding risk is small in comparison.
For people who cannot tolerate an anticoagulant, there are new devices that are comparably effective. These devices (including Watchman®, Lariat®, ArtiClip®, and Amulet®) take advantage of the fact that most of the stroke risk from atrial fibrillation comes from clot in the left atrial appendage. In various ways, these devices seal off the connection between the left atrial appendage and the rest of the left atrium, preventing clot there from entering the general circulation.
Since we have the ability to drastically reduce the risk of stroke from atrial fibrillation, it’s imperative that we find out who is having it! We do so by monitoring people who have had strokes or have palpitations that suggest atrial fibrillation. There are monitors that are worn for 24-48 hours, as well as those that can be worn for 1-4 weeks. We also have small implantable monitors that are placed just under the skin overlying the heart and can monitor a person for 3 years or more.
So, if you have had a stroke or feel your heart beating irregularly at times, see a cardiologist to determine if you should be screened for atrial fibrillation. It’s a worthwhile investment of your time.
Greg Koshkarian, MD, FACC