The increased left atrial pressure in patients with MS gradually produces left atrial dilation. Such atrial enlargement can lead to the onset of atrial fibrillation and also to thromboembolic complications if a clot forms in the atrium or appendage due to low velocity blood flow. Treatment may include anticoagulation with IV heparin or oral coumadin, pharmacologic rate control, and pharmacologic or electrical cardioversion for hemodynamically significant or acute onset atrial fibrillation. In patients scheduled for cardioversion, TEE may be performed first to rule out the presence of LA thrombus.8