Background: 
In atrial fibrillation (AF), thrombus formation typically occurs within the left atrial appendage (LAA) and is less common in the left atrial cavity (LAC)1-3. Based on transesophageal echocardiography (TEE), the prevalence of LAA thrombus in AF patients in the absence of anticoagulation is 7.5%, with a 1.8% prevalence of LAC thrombus4, though extra-appendage thrombus appears to be a rare finding in non-valvular atrial fibrillation5. LAA/LAC thrombus may be associated with embolization to the brain, causing stroke. The LAA has distinct structural, anatomic, and physiologic properties that permit the structure to serve its role as a decompression chamber during periods of elevated left atrial pressure. The volume, orifice size, length, shape, and lobulation of the LAA may vary markedly between individual6, 7.
The pathogenesis of thrombus formation in the LAA in the setting of non-valvular AF (NVAF) is not fully understood. Investigators have examined structural and functional characteristics of the LAA and how they contribute to LAA thrombus formation and cardioembolic stroke (CES) risk. Reduced blood inflow and outflow velocities at the LAA orifice based upon Doppler transesophageal echocardiography (TEE) measurements have predicted LAA thrombus risk8. More recent studies seeking to correlate descriptive LAA morphologies with thromboembolic risk have reported lower risks of embolic events with Chicken-Wing LAA morphology7-9.
The LAA anatomic determinants that predict thromboembolic risk in NVAF have not been completely elucidated. This study aimed to compare the LAA orifice size, maximal depth, and peak exit velocities between AF patients with LAA thrombus and high-risk AF patients without thrombus. Identifying novel LAA anatomic determinants of thrombus formation may help stratify AF thromboembolic risk in the future.