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Chapter 13: THE ECHOCARDIOGRAM PROSTHETIC CARDIAC VALVES
Echocardiography is a critically important tool in the evaluation and serial follow-up of mechanical and bioprosthetic valves.372 Unfortunately, the increased echo reflectivity of prosthetic valves (especially the mechanical models) causes extensive distal shadowing and reverberations that markedly limit the utility of transthoracic 2D echocardiography (Figs. 13-91 and 13-92). TTE imaging may detect partial ring dehiscence manifest as abnormal "rocking" motion of a prosthetic valve. TTE may also identify reduced movement of the valve disks or leaflets and may occasionally visualize adherent thrombi, tissue ingrowth, and vegetations.373-375 Leaflet thickening, detachment, and flail motion also may be visualized for bioprosthetic valves.
Figure 13-91: Apical two-chamber view of a mechanical prosthetic valve (mitral position) during systole. The left atrium is completely obscured by ultrasonic shadowing (arrows). LV = left ventricle.
Doppler interrogation is the cornerstone of the echocardiographic assessment of prosthetic valvular stenosis and regurgitation.376-379 Color-flow imaging can document the presence, direction, and size of the forward flow stream. Color-flow Doppler can also detect regurgitant flow jets, but like 2D imaging, is limited by acoustic shadowing distal to the prosthesis. Doppler color jets due to prosthetic AR can be readily visualized from the transthoracic apical view, but jets produced by prosthetic mitral and tricuspid regurgitation are often obscured.380,381 Therefore, although detection of prosthetic regurgitation by transthoracic Doppler is usually feasible, quantitation is often difficult. A small flow signal shortly after valve closure may be observed frequently with prosthetic valves and is likely related to the blood caught behind the occluder as it closes.382
Doppler flow velocities and gradients (calculated by the Bernoulli equation) through normal prosthetic valves vary depending upon the type, position, and diameter of the prosthesis.376-379 The velocities and gradients across prosthetic valves are flow-dependent as well383 and therefore related to LV function. Given these variables, it is not surprising that a wide range of transvalvular gradients exists for normally functioning prosthetic valves. Nevertheless, "normal" ranges have been reported for various valve types and can be used as a guide to recognize malfunction. High prosthetic valvular gradients due to increased flow volume rather than stenosis can be recognized by high flow velocity across the remaining native valves, a short pressure half-time for mitral prostheses, and a short ejection time for aortic prostheses. With aortic valve prostheses, peak systolic Doppler velocities may indicate higher systolic pressure gradients than those actually found during cardiac catheterization.384,385 This problem may be more prevalent with StarrEdwards (ball-in-cage) and St. Jude (bileaflet tilting disks) valves than with Medtronic-Hall (single tilting disk) and bioprosthetic valves. The inaccuracies with Starr-Edwards and St. Jude valves are probably due to the presence of multiple flow channels (with various orifice areas) and the phenomenon of flow recovery.385,386 Because of these variabilities, an echocardiographic examination is warranted following prosthetic valve implantation to establish its baseline Doppler characteristics.387 As opposed to peak gradients, mean transvalvular gradients calculated by Doppler correlate reasonably well with direct catheter measurements.
TEE has dramatically changed the diagnostic approach to prosthetic valve dysfunction,380,381 and is especially useful for assessing mitral prostheses, as it overcomes the problem of left atrial shadowing and reverberation Fig. 13-93). TEE is extremely accurate in the detection of prosthetic regurgitation and impaired movement of the valve occluder, and it is the diagnostic procedure of choice in most cases of suspected prosthetic valve endocarditis.388-390 Small thrombi, tissue ingrowth, infected or sterile vegetations, and even sutures in the sewing ring usually can be readily visualized. The enhanced sensitivity of TEE requires operator experience and judgment, as nearly all mechanical prostheses exhibit a normal small amount of regurgitation, which should not be misinterpreted as pathologic.382 TEE may also visualize thin fibrinous strands sometimes attached to prosthetic valves; these structures appear to be a potential source of cardiogenic embolization.391,392 The technique is quite accurate in the diagnosis of prosthetic valve thrombosis, a potentially fatal medical emergency, and can assist clinical decision making in this disorder.393-395
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