Overt intravascular thrombus formation is rare with heart failure in most species other than domestic cats and humans. Cats frequently form a thrombus in a severely enlarged left atrium, most commonly in the left auricle. This thrombus then commonly breaks loose and flows into the systemic circulation. Although thrombus formation in cats occurs most commonly when the left atrium is large because of cardiomyopathy , it can also occur without left atrial enlargement, sometimes even in an otherwise healthy cat. In a cat with severe left atrial enlargement, the primary reason for development of the thrombus is assumed to be blood flow stasis. Red cells aggregate when blood flow decreases below a critical velocity. When the left atrium enlarges, blood flow velocity decreases (assuming no mitral regurgitation ). The area of lowest velocity is the left auricle. In some cats, red cell aggregation can be visualized as so-called spontaneous contrast, or "smoke," within the left atrium. In some cats, both a thrombus and spontaneous contrast can be noted. In a cat with a thromboembolism, the left atrial thrombus has already detached and is gone by the time an echocardiogram is done.