ABSTRACT
Patients with valvular heart disease and in particular patients with prosthetic heart valves are at increased risk of systemic thromboembolism. In the absence of antithrombotic therapy, systemic embolism and stroke have been reported in between 5% and 50% of patients, depending upon the valve site, the type of valve replacement, and the presence of comorbid conditions [1, 2]. With the use of anticoagulants, the rate of systemic embolism has been reduced to 1-3% per year [3]. However, the intensity of anticoagulation to effectively reduce the risk of thromboembolic complications must be balanced against the risk of bleeding, which in turn is related to the level of anticoagulation used and to individual patient risk factors [4]. Therefore, risk factors that increase the incidence of systemic embolism as well as risk factors that increase the risk of bleeding complications must be considered when defining the need for starting antithrombotic therapy and the intensity of anticoagulation in patients with valvular heart disease and prosthetic heart valves. These factors include age, smoking, hypertension, diabetes, hyperlipidemia, type and severity of valve lesion, presence of atrial fibrillation, heart failure or low cardiac output, size of the left atrium (over 50 mm on echocardiography), previous thromboembolism, previous major bleeding, events or clinical conditions at high risk for bleeding, and abnormalities of the coagulation system including hepatic failure [5]. Furthermore, the type, number, and location of prostheses implanted must be considered. Mechanical prostheses are more thrombogenic than bioprostheses or homografts, and hence patients with mechanical valves require lifelong anticoagulant therapy. Moreover, the intensity of treatment varies according to the type of mechanical prosthesis implanted. First generation mechanical valves, namely the StarrEdwards caged ball valve and Bjork-Shiley standard valves, have a high thromboembolic risk [6]; single tilting disc valves have an intermediate thromboembolic risk; and the newer (second and third generation) bileaflet valves have low thromboembolic risks [7]. Finally, thromboembolic events are commoner with prosthetic mitral valves than aortic valves and in patients with double replacement valves compared with those with single prostheses [8]. Table 7.1 gives the different types of prosthetic valves and their thrombogenicity.
