The challenge of assessing heart valve prostheses by Doppler echocardiography.

The assessment of prosthetic valve function remains challenging. Echocardiography has become the key diagnostic tool not only because of its noninvasive nature and wide availability but also because of limitations inherent in alternative diagnostic techniques. Invasive evaluation is limited particularly in mechanical valves that cannot be crossed with a catheter, and in patients with both aortic and mitral valve replacements, full hemodynamic assessment would even require left ventricular puncture. Although fluoroscopy and more recently computed tomography allow the visualization of mechanical valves and the motion of their occluders, the evaluation of prosthetic valves typically relies on Doppler echocardiography. Although Doppler echocardiography has become an ideal noninvasive technique for the evaluation of native heart valves and their function, the assessment of prosthetic valves has remained more difficult. Although the evaluation of secondary effects on heart chambers, ventricular function, and pulmonary circulation can in general be provided accurately, the evaluation of prostheses themselves has major limitations. The assessment of valve morphology as well as valvular and perivalvular regurgitation is complicated by artifacts and shadowing caused by the prosthetic material. This is particularly the case when transthoracic echocardiography is used, but it also remains a major limitation for transesophageal echocardiography. Although the latter may provide important information on bioprosthetic valve function by visualizing leaflet morphology and motion, and on mechanical valve function in mitral prostheses in which it frequently allows the diagnostic evaluation of occluder motion, prosthetic valve function is primarily assessed using Doppler echocardiography (mainly using transthoracic echocardiography). As for native valves, Doppler echocardiography can be used to measure transvalvular velocities and gradients and to calculate effective orifice areas in patients with valve prostheses. However, major differences and some limitations must be considered when interpreting Doppler measures of transprosthetic flow velocities and orifice areas. First, transvalvular velocities cannot be used to accurately calculate the pressure drop across certain valve types, because of the rather complex flow velocity profiles in mechanical valves. This is particularly the case in bileaflet prostheses, in which flow contraction causes a low-pressure field followed by significant pressure recovery within the central orifice and flow channel between the two leaflets. This phenomenon results in high central velocities, which are detected