A left atrial appendage closure is a procedure that closes off the opening of the left atrial appendage. The left atrial appendage is a part of the heart next to the upper chamber, or atrium, on the left side. If someone has a heart problem, such as atrial fibrillation, blood can pool in the left atrial appendage and form a clot. If the left atrial appendage is closed off, blood can’t pool there. This makes someone less likely to have a blood clot that can travel to the brain and cause a stroke.