What is the clinical significance of the left atrial appendage in atrial fibrillation?

Clinical Significance of the Left Atrial Appendage in Atrial Fibrillation

The left atrial appendage (LAA) is the main site of thrombus formation in patients with atrial fibrillation, responsible for approximately 90% of cardioembolic strokes in non-valvular AF, making it a critical therapeutic target for stroke prevention. 1

Anatomical and Physiological Significance

• The LAA is a finger-like extension originating from the main body of the left atrium with complex and variable morphology that predisposes to blood stasis, especially during atrial fibrillation 2
• It functions physiologically as a decompression chamber during left ventricular systole and other periods of elevated left atrial pressure 3
• The LAA has developmental, ultrastructural, and physiological characteristics distinct from the left atrium proper, including higher distensibility and a high concentration of atrial natriuretic factor granules 3

Thromboembolic Risk Assessment

• LAA flow velocity (LAAV) serves as a quantitative surrogate parameter for thromboembolic risk, with velocities below 55 cm/s significantly increasing the risk of thrombus formation regardless of underlying rhythm 4
• LAA size (>34 cm³), morphology (particularly cauliflower shape), number of lobes (≥3), and low-velocity flow are considered risk factors for thrombus formation 1
• Transesophageal echocardiography (TOE) is the most frequently used diagnostic tool for detecting LAA thrombi with sensitivity of 93-100% and specificity of 99% 1
• Advanced imaging with contrast-enhanced cardiac CT and CMR also provide highly diagnostic (99%) detection of LAA thrombi 1

Therapeutic Implications in AF Management

LAA Occlusion/Closure

• Surgical closure of the LAA is recommended as an adjunct to oral anticoagulation in patients with AF undergoing cardiac surgery to prevent ischemic stroke and thromboembolism (Class I, Level B recommendation) 1
• Surgical closure should be considered as an adjunct to oral anticoagulation in patients with AF undergoing endoscopic or hybrid AF ablation (Class IIa, Level C recommendation) 1
• Stand-alone endoscopic surgical closure of the LAA may be considered in patients with AF who have contraindications for long-term anticoagulant treatment (Class IIb, Level C recommendation) 1
• Percutaneous LAA occlusion devices (such as WATCHMAN and Amplatzer Cardiac Plug) may provide an alternative to OAC for AF patients at high risk for stroke with contraindications for chronic OAC 1

Limitations and Considerations

• Not all strokes in AF patients are cardioembolic or due to AF, and the LAA is not the only left atrial region where thrombi can originate 1
• There may still be a need for antithrombotic therapy in AF patients even after removal or closure of the LAA 1
• Procedural risks of LAA occlusion include stroke, major bleeding, device-related thrombus, pericardial effusion, vascular complications, and death 1
• Device-related thrombi occur with an incidence of 1.7%-7.2% and are associated with higher risk of ischemic stroke 1
• Peri-device leaks (0-5 mm) are present in approximately 25% of cases after LAA closure 1

Clinical Implications Beyond Thromboembolic Risk

• LAA structure and function are key elements for understanding multiple clinical conditions beyond AF, including cardioembolism, heart failure, and mitral valve diseases 1
• LA enlargement and remodeling have become cornerstone diagnostic and prognostic markers in various cardiovascular conditions 1
• Obliteration or amputation of the LAA may help reduce thromboembolism risk but could result in undesirable physiological consequences such as reduced atrial compliance and decreased capacity for ANF secretion 3

Imaging Modalities for LAA Assessment

• Transesophageal echocardiography remains the gold standard for LAA thrombus detection 1
• Ultrasound contrast agents improve the diagnostic accuracy of TOE for detecting thrombi 1
• Cardiac CT and CMR with dedicated protocols are highly accurate alternatives for LAA thrombus detection 1
• 3D imaging techniques provide enhanced visualization of LAA anatomy and are particularly useful for pre-procedural planning of LAA closure 1

The clinical significance of the LAA in AF extends beyond being a site of thrombus formation to include important physiological functions and implications for various therapeutic approaches in stroke prevention.