NEXT PAGEMore than 60,000 valve replacements are performed annually in the United States. Prosthetic heart valves may be either mechanical or bioprosthetic. Mechanical valves which are composed mostly of metal or carbon alloys, are classified according to their structure as caged-ball, single-tilting-disk, or bileaflet-tilting-disk valves. Bioprosthesis may be heterografts, which are composed of bovine tissue mounted on a metal support, or homografts, which are preserved human aortic valves.
Prosthetic valves differ from each other with regard to several characteristics such as durability, thrombogenecity, and hemodynamic profile. With rare exceptions, mechanical valves are very durable, most lasting at least 20 to 30 years. Mechanical valves are also thrombogenic which require the patient to undergo long-term anticoagulant therapy. Once implanted, each valve has its own hemodynamic profile and effective orifice area. The heterograft bioprosthesis and caged-ball mechanical valve have the smallest effective orifice areas, whereas the homograft bioprosthesis has the largest.
Based on these characteristics, mechanical valves are preferred with patients who are young or have a life expectancy of more than 10 to 15 years, or who require long-term anticoagulant therapy for other reasons (e.g., atrial fibrillation). Bioprosthetic valves are preferred in patients who are elderly or have a life expectancy of less than 10 to 15 years, or who cannot take long-term anticoagulant therapy.
Valve Thrombosis occurs in patients with bioprosthetic valves as well as those with mechanical valves who are receiving adequate anticoagulant therapy. It may be manifested clinically as pulmonary congestion, poor peripheral perfusion, or systemic embolization. The mortality rate associated with surgical therapy for valve obstruction is approximately 15%. Valve replacement is preferred to thromboectomy because it has a lower incidence of recurrent thrombosis.
Embolization. The risk of embolization is increased with mitral-valve prosthesis, caged-ball valves, and multiple prosthetic valves. Other variables which increase the risk are atrial fibrillation, an age of over 70 years, and depressed left ventricular systolic function.
Structural Failure of Bioprosthetic Valves
Most patients whose valves fail have severe regurgitation due to a tear
or rupture of one or more of the valve cusps which have become calcified and
rigid. The incidence of bioprosthetic valve failure is particularly high in
patients less than 40 years old and in those with mitral prosthesis. These patients
usually note the gradual onset of dyspnea and other symptoms of heart failure.
Endocarditis. Prosthetic-valve infection occurs at some time in 3 to 6 percent of patients. Early endocarditis (occurring less that 60 days after valve replacement) usually results from perioperative bacteremia arising from skin or wound infections or contaminated intravascular devices. Late endocarditis (occurring more than 60 after valve replacement) is usually caused by the organisms responsible for native valve endocarditis.
NEXT PAGE