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.
The site where the thromboembolism lodges after it has broken loose from its site of attachment in the left atrium depends on the size of the thrombus and on blood flow patterns. A very small thrombus can gain entry to a coronary artery and cause myocardial infarction. A midsized thrombus can exit into a branch coming off the aorta, such as the brachiocephalic trunk (and then most commonly to the right subclavian) or an intestinal branch. Most thrombi are large and end up at the terminal aorta, causing acute cessation of blood flow to the hind limbs. The result is pulselessness (no femoral pulse), pallor (pale or purple foot pads), poikilothermy (decreased rectal temperature and cold hind limbs), and, initially, extreme pain. The gastrocnemius muscles tend to be very firm. Cats often can move the legs above the stifles, and the tail is commonly unaffected. In some cats, only one hind limb is affected.
Diagnosis of arterial thromboembolism is usually based on clinical signs, physical examination findings (eg, lack of femoral pulses), and Doppler blood flow readings of the hind limbs. Ultrasonography can also be used to identify the thromboembolism in the terminal aorta. In cats, misdiagnosis of neurologic abnormalities as an arterial thromboembolism is common. In some cats the clot will lyse on its own over time (1–72 hours). However, some of these cats have residual problems (eg, dry gangrene). Numerous treatments, including surgical removal, administration of thrombolytic agents, and rheolytic thrombectomy, have been tried but are largely unsuccessful when compared with waiting for the thromboembolism to lyse on its own. Euthanasia is common. Clopidogrel (18.75 mg/day per cat) has been shown to be effective at preventing recurrence of these events in cats, so it is assumed to be effective at preventing first occurrence also; however, by no means is it 100% effective. Aspirin is believed to be ineffective unless combined with clopidogrel. Factor Xa inhibitors, such as apixaban and rivaroxaban, are currently being studied.
