Evaluation of prosthetic valve function is an important aspect of follow up of patients after valve replacement. Clinical parameters would include the careful documentation of symptoms and physical findings. Patients are instructed to listen to the valve sounds while in a silent room in case of mechanical prosthesis and to report if the sounds suddenly decrease in intensity. A decrease in intensity of valve sounds may indicate thrombus formation in the mechanical prosthesis. Normal auscultatory events in case of mechanical prosthesis would include opening and closing sounds with a metallic character as well as flow murmurs as the prosthetic valve is inherently stenotic.
Patient with slowly progressive prosthetic valve obstruction or severe paravalvular leak may present with progressive exertional dyspnoea while presentation with acute pulmonary edema may be the situation in case of prosthetic valve thrombosis. Embolic episodes can also occur with prosthetic valve thrombosis.
Echocardiography, both transthoracic and transesophageal, are the important modalities of prosthetic valve evaluation. Image intensifier fluoroscopy when available, is useful in visualising the movement of the valve discs or balls if they are radio-opaque. Abnormal movements of the valve stents or cage suggest partial dehiscence of the prosthesis due to slipping of sutures and may be an ominous sign.
Though echocardiography is an important modality for evaluation of prosthetic valve, dense shadows in case of mechanical prosthesis can make evaluation of structures and flows behind the valve difficult. Similarly small thrombi or vegetations attached to the valves may also be missed. Evaluation of the left atrial side of the prosthesis and mitral regurgitation necessitates transesophageal echocardiography in case of prosthetic mitral valve. Aortic regurgitation can be picked up well with transthoracic echocardiogram as the left ventricle is not within the shadow region. Presence of spontaneous echo contrast in the left atrium should also make one suspect mitral prosthetic thrombosis.
The valve gradients across the prosthetic valves are usually higher than normal even for a normally functioning prosthetic valve. A gradient above 10 mm Hg across the mitral and peak gradient more than 40 mm Hg across the aortic valve are significant. Highly sensitive Doppler equipment can detect even the normal closure back flow or seating regurgitation with normally functioning prosthetic valves. Pathologic regurgitation on the other hand is of high velocity with intense aliasing and often along the wall of the left atrium in case of mitral valve (Coanda effect). It is mentioned that sometimes a paravalvular leak due to suture dehiscence may be laminar and missed. Rocking abnormal movement of the valve ring or stent will be an indicator in such cases.
Prosthetic valve evaluation should begin in the postoperative period with careful documentation of findings including the exact location from which good images are obtained. This is essential to detect changes with time which might indicate prosthetic valve dysfunction. Intraoperative echo if available, will be quite useful.
In a chronic case of valve obstruction, differentiating between thrombus and pannus is important. Pannus is more often seen in the aortic position and thrombus more in the mitral position. Thrombus is a much more common cause of obstruction than pannus. Pannus is highly echogenic compared to a thrombus. Thrombus is less echogenic and may be mobile. Associated spontaneous echo contrast is another indication of thrombus.
Detecting vegetations over a prosthetic valve is another difficult task requiring familiarity with the normal echoes of the particular valve. Transesophageal echocardiography (TEE) is useful in this regard because of the higher resolution and the ability to image the atrial side of the mitral prosthesis better. TEE is also useful in evaluating valve ring abscess and pseudoaneurysm.
In case of bioprosthetic valves, thickening, calcification or perforation may be noted. Sometimes a flail leaflet may also be picked up on echo.
3D echocardiography is a promising tool in the evaluation of prosthetic valves.