Atrial fibrillation is a common heart condition that happens when the top two chambers of the heart, the atria, beat too fast and with an irregular rhythm (fibrillation). This condition can decrease the heart’s pumping capacity, which can cause blood cells to pool and stick together, forming clots in a small pouch on the heart called the left atrial appendage. If a clot escapes from the appendage and gets into your arteries, it may block the blood supply to your brain and cause a stroke. Atrial fibrillation is associated with a 3-5 times increased risk of stroke.

The purpose of the LAAOS-4 study is to determine if closure of the left atrial appendage using a closure device called the WATCHMAN FLX™, in addition to taking oral anticoagulant medications, is more effective at reducing strokes and blood clots in your body, than taking oral anticoagulant medications on their own.

The primary efficacy outcome of LAAOS-4 is ischemic stroke or systemic embolism. Secondary efficacy outcomes include all-cause stroke, systemic embolism or transient ischemic attack, change in MoCA score between visits, new disabling ischemic stroke with mRS >2 measured at 90 days post-stroke, CV mortality and all-cause mortality.

The LAAOS-4 study will enroll participants from North America and Europe. After enrollment, a patient will be randomized to either the intervention arm (receive the WATCHMAN FLX™ device) or the control arm (stay on oral anticoagulants). Follow-up visits will occur at 6-month intervals until the end of the study.

Study Type

Interventional - Drug

Study Design

Multicentre, prospective, open-label, randomized controlled trial with blinded assessment of endpoints (PROBE)

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NO. of Sites


NO. of Participants


Study Period

2023 - 2029



Boston Scientific Corporation

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