The left atrial appendage (LAA) is a small, ear-shaped pouch extending from the left atrium, one of the upper chambers of the heart. Its presence and specific characteristics hold importance for heart health.
Understanding the Left Atrial Appendage
The LAA is an outpouching of the muscular wall of the left atrium, situated anteriorly in a groove between the left atrium and left ventricle. It typically lies over the superior part of the left ventricle or the pulmonary trunk, close to the left circumflex artery and pulmonary veins. Inside, the LAA’s walls are lined with muscular ridges known as pectinate muscles, giving it a rough, uneven texture, unlike the smooth interior of the main left atrial chamber. Its shape varies significantly among individuals, with common forms including the “chicken wing,” “cactus,” and “windsock” morphologies. On average, the LAA measures approximately 46 millimeters in length and can hold about 9 milliliters of blood.
Its Normal Role in the Heart
In a healthy heart, the LAA participates in regulating blood volume and pressure. It serves as a reservoir, holding a small amount of blood, particularly when pressure increases in the left atrium, such as during left ventricular contraction. The LAA also contributes to neurohormonal regulation by secreting atrial natriuretic peptide (ANP). This hormone is released when the LAA’s walls stretch due to increased blood volume. ANP promotes the excretion of sodium and water by the kidneys and relaxes blood vessels, helping to lower blood pressure and manage blood volume.
When Function Goes Wrong: Clot Formation
The LAA’s unique structure, particularly its multiple lobes and muscular ridges, can become problematic when the heart’s rhythm is irregular, such as in atrial fibrillation (AF). In AF, the left atrium does not contract effectively, leading to a loss of coordinated blood flow within the LAA. This disruption causes blood to stagnate and pool in the LAA’s complex crevices, creating an environment conducive to the formation of blood clots, or thrombi. Over 90% of blood clots that form in the atria of patients with non-rheumatic atrial fibrillation originate within the LAA. A blood flow velocity within the LAA of less than 20 centimeters per second is strongly linked to clot formation.
Why Understanding Its Function Matters
Understanding the LAA’s function, both normal and abnormal, carries implications for cardiovascular health. The primary concern with LAA-related clot formation is the risk of stroke. If a blood clot forms within the LAA, it can dislodge and travel to the brain, blocking blood flow and causing an ischemic stroke. As the LAA is the most common site for clot formation in atrial fibrillation patients, understanding its role is important for guiding medical approaches to reduce stroke risk.