The annual rate of prosthetic valve thrombosis with me- chanical valves ranges from 0.1% to 5.7%. Higher rates of mechanical valve thrombosis are seen for some specific valve types, within the first 3 months after valve implanta- tion and, for mechanical valves implanted in the mitral or tricuspid position compared with the aortic position.8 Recommendation-Specific Supportive Text 1. Mechanical prosthetic valve thrombosis is diag- nosed by an abnormally elevated velocity or gra- dient across the prosthesis, with either limited leaflet motion or attached mobile densities con- sistent with thrombus, or both. Prosthetic valve obstruction is usually defined as a mean trans- valvular gradient increase >50% (or an increase >10 mmHg across an aortic prosthesis) compared with baseline, after exclusion of other causes, such as a high-output state. When mechani- cal valve thrombosis is suspected, imaging and Doppler data from TTE provide information on valve function, estimated pulmonary pressures, and LV size and systolic function.9 Leaflet motion should be visualized with CT or TEE (particularly for a mitral prosthesis) or fluoroscopy (for an aor- Prolonged periods of obser- vation under fluoroscopy or TEE may be required to diagnose intermittent obstruction. The pres- ence and quantification of thrombus and pannus should be evaluated by either TEE or CT.6,7,10 12 CLINICAL STATEMENTS AND GUIDELINES Recommendation for Intervention for Mechanical Prosthetic Valve Thrombosis Referenced studies that support the recommendation are summarized in Online Data Supplement 38. COR LOE Recommendation For patients with a thrombosed left-sided mechanical prosthetic heart valve who present with symptoms of valve obstruction, urgent initial treatment with either slow-infusion, low- dose fibrinolytic therapy or emergency surgery Synopsis Patients presenting with a thrombosed mechanical valve require urgent therapy. 