Where do most stroke-causing clots come from? Top cardiologist reveals a 'tiny pouch' in heart

Stroke is a leading cause of death and disability worldwide, with many cases stemming from blood clots that travel to the brain. In 2024, stroke statistics indicate that it remains a leading cause of death and disability, with estimates of 12 million new cases and over 6.5 million deaths annually worldwide. In fact, according to the National Institutes of Health, in 2025, nearly 12 million new strokes occur annually, with one in four adults over 25 expected to have a stroke in their lifetime.

A heart attack is a heart attack caused by blocked blood flow to the heart. Now, unlike a heart attack, a stroke is a brain attack, caused by interrupted blood flow to the brain. In the case of a stroke, a significant number of these clots originate in a small, often overlooked part of the heart, a secret ‘tiny pouch’. This tiny pouch, located in the upper left chamber of the heart, can become a silent source of danger, especially in individuals with atrial fibrillation (AFib), a common heart rhythm disorder.

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The danger hidden in the ‘tiny pouch’

When people think of the heart, they rarely picture small architectural “nooks” or “pouches.” Yet inside the left upper chamber of the heart (the left atrium) lies a tiny pouch called the left atrial appendage (LAA). This appendage is like a little ear-shaped sac that extends from the atrium wall.

Medical research and cardiologists now agree that in patients with atrial fibrillation (AFib) — a common irregular heart rhythm — this pouch becomes a danger zone. Because the heart’s upper chambers don’t contract efficiently in AFib, blood can stagnate or pool in the appendage. Over time, this stagnant blood may clot.

Once a clot forms there, it can break free, travel through the bloodstream, and block arteries in the brain — causing an ischemic stroke. In fact, cardiology studies estimate that up to 90% of stroke-causing clots in patients with non-valvular AFib originate in the left atrial appendage.

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Renowned cardiologist Dmitry Yaranov, MD, who goes by the username of “heart_transplant_doc” on Instagram, agrees.

In an Instagram video, he says, “Your heart has a tiny pouch called the left atrial appendage—kind of like a little pocket off the left atrium. And if you have atrial fibrillation, that pouch is where >90% of clots that cause strokes are born.”


Why this tiny pouch can cause havoc

To understand why the LAA becomes such a high-risk spot, consider three key factors:

Blood flow and pooling: In normal hearts, the atria squeeze with each beat, pushing blood forward. But with AFib, the contraction is weak or chaotic. As a result, in the appendage, blood doesn’t get cleared fully. This slow or stagnant flow gives clots a chance to form.

Anatomy and shape: The LAA is narrow and folded, with crevices that can trap blood. Some people have more complex or tortuous appendage shapes, which may worsen stagnation and clot risk. Research using imaging and 3D models shows that LAA shape correlates with stroke risk.

Lack of other drainage path: Because the appendage is a dead-end sac, it doesn’t have an easy outlet. Clots that form there are more likely to escape into the circulation when the heart changes pressure or flow conditions.

Because of these features, the LAA becomes a potent source of embolic clots in AFib patients.



How to manage the threat

Knowing the LAA’s role has helped in finding ways to reduce stroke risk, especially in the vulnerable population, by targeting this pouch directly.

Left Atrial Appendage Closure (LAAC): This procedure involves implanting a device or using a clamp to seal off or close the opening of the LAA. The goal is to prevent the clots formed there from escaping. Devices like WATCHMAN, Amulet, and WaveCrest act like internal plugs or filters. Once sealed, any clot that forms in the LAA is trapped and prevented from traveling to the brain.

WATCHMAN Device: One of the best-known and widely used options, the WATCHMAN is a mesh-style implant placed inside the LAA via a catheter (through a vein in the leg). After placement, tissue grows over it, sealing off the pouch.

According to Yaranov, “the logic is simple: If we close the pouch… we block the stroke.

As per him, a device called the Watchman could be used effectively for the resolution. What does it do? Yaranov says, it’s “a small umbrella-like implant that goes into that pocket and seals it shut—permanently.”

Studies suggest it can reduce stroke risk comparable to blood thinners — and sometimes let patients stop using long-term anticoagulants.

There are surgical or hybrid approaches as well. In patients already undergoing open-heart surgery, surgeons may clip, remove, or sew shut the LAA at the same time. This approach offers stroke protection without an added invasive procedure later. These strategies are especially valuable for patients with AFib who cannot take long-term blood-thinning medicine due to bleeding risks.

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