Tissue valves, or bioprostheses

 Homograft valves

 Mechanical valves

Bioprosthetic Valves

Tissue valves are composed of three biologic leaflets with an anatomic structure similar to that of the native aortic valve. With stented prosthetic valves, the leaflets (typically porcine), or pericardium (usually bovine or equine) shaped to mimic normal leaflets, are mounted on a cloth-covered rigid support that functions as the crown-shaped aortic annulus with a raised “stent” at each of the three commissures (Fig. 13-3). Variations in the support structure and leaflet types abound in commercially available valves; some include anticalcification treatments. Current generation stented tissue valves include the Edwards Magna, St. Jude Trifecta and the Medtronic Mosaic valve. Older examples of stented heterografts include Carpentier-Edwards porcine valves, Hancock porcine valves, and Ionescu-Shiley bovine pericardial valves. “Stentless” tissue valves also have been developed that use a flexible cuff of fabric or tissue, instead of rigid stents, to support the valve leaflets. Stentless valves often are implanted as part of a composite tissue valve and aortic root, for example, the Medtronic freestyle valve and root.

In the past, bioprosthetic valves were only implanted surgically using cardiopulmonary bypass to support the circulation while the valve is implanted. Newer approaches to aortic and pulmonic valve implantation include a transapical surgical approach and a nonsurgical transcatheter approach. The bioprosthetic valves used for catheter implantation are mounted on a compressible stent (see Suggested Readings 20 to 24).

Homograft Valves

Homograft valves are cryopreserved human aortic or pulmonic valves harvested at autopsy. Typically, the valve and great vessel are preserved as a block, to be trimmed appropriately at the time of implantation in the aortic or pulmonic position. While the fluid dynamics of a homograft are similar to those of a native valve, flow velocities are slightly higher and valve areas slightly smaller than for a normal native valve because of the space occupied by the homograft annulus in the patient’s outflow tract. Because of late severe tissue calcification with homograft valves, the approach usually is reserved for adults with complex aortic root abscesses.

Mechanical Valves

A variety of mechanical valves currently are available. In addition, several other types of valves, which were implanted in the past, are still in situ in some patients. The two basic types of currently implanted mechanical valves are:

In the past, ball-cage mechanical valves also were used and may still occasionally be encountered. With a ball-cage valve, a spherical occluder is contained by a metal “cage” when the valve is open and fills the orifice in the closed position.

Valved Conduits

Valved conduits are used in congenital heart surgery and in ascending aortic repairs when both a new passageway for blood flow and a valve are needed. The conduit may be biologic (e.g., a homograft) or artificial (e.g., Gore-Tex or Dacron) material. A conduit may incorporate either a stented tissue or a mechanical valve with fluid dynamics similar to those for a valve implanted in the native annulus. The stentless valve and root prosthesis also is used in this situation.

 Structural failure

 Thromboembolic complications

 Endocarditis

Primary Structural Failure

Failure of a bioprosthetic valve to open or close properly (mechanical failure) usually is the result of slowly progressive tissue degeneration with fibrocalcific changes of the leaflets, which results in increased resistance to opening (stenosis) or failure to coapt during valve closure (regurgitation). Typically, failure of tissue valves occurs 10 or more years after valve implantation. Acute bioprosthetic valve stenosis is rare. Acute bioprosthetic regurgitation can occur with a leaflet tear, usually adjacent to a region of calcification.

Failure of a mechanical valve can occur because of faulty design or wear and tear of the prosthetic material resulting in disk escape or incomplete valve closure. However, these complications were seen only with older generation valves (which may still be present in a few patients). Current generation mechanical valves are reliable and very durable. More often, mechanical valve stenosis or regurgitation is due to thrombus formation or pannus ingrowth around the valve, impairing disk excursion or closure.

With both bioprosthetic and mechanical valves, paravalvular regurgitation can occur around the sewing ring because of loss of suture material postoperatively; this is most often related to fibrocalcific disease in the valve annulus. The new onset of paravalvular regurgitation late after surgery raises the possibility of an infectious process (endocarditis) resulting in valve dehiscence.

Thromboembolic Complications

Prosthetic valves, particularly mechanical valves, are prone to thrombus formation which may result in systemic embolic events or valve dysfunction. Echocardiographic evaluation for prosthetic valve thrombus is limited, except with very large masses, because of shadowing and reverberations. In addition, clinical events may be associated with clots smaller than the limits of clinical ultrasound resolution. Thus, echocardiography cannot exclude the possibility of thrombus on a prosthetic valve; in patients with embolic events, the prosthetic valve itself is potential source of embolus.

Endocarditis

Infection of a valve prosthesis is a serious clinical problem, so suspected endocarditis is a frequent indication for echocardiography in patients with prosthetic valves. Endocarditis on a bioprosthesis may result in vegetations similar to those seen on a native valve. However, with a mechanical valve, the infection often is paravalvular, and no discrete vegetation may be present.

Bioprosthetic Valves

Aortic homografts appear similar to native aortic valves except for some increased thickness in the left ventricular (LV) outflow tract and the ascending aorta at the proximal and distal suture sites. Typically, the homograft is implanted using the mini-root technique with the homograft replacing a segment of the native aorta. This approach necessitates reimplantation of the coronary arteries. In the past, the aortic homograft sometimes was positioned inside the patient’s native aorta with appropriate trimming to maintain patency of the coronary ostia. In patients with endocarditis, the attached anterior mitral leaflet of the homograft may be used to patch a ventricular septal defect or abscess cavity. The echocardiographic appearance of a homograft is very similar to that of a native aortic valve, except for the associated surgical changes. Standard parasternal long- and short-axis image planes provide optimal visualization of valve leaflet anatomy and motion.

Stented tissue prosthetic valves have a trileaflet structure similar to that of a native aortic valve. An M-mode recording through the leaflets shows the typical “boxlike” opening in systole (for the aortic position) or diastole (for the mitral position) as is seen with a normal native aortic valve. However, with conventional valve designs, the echogenic sewing ring and struts may limit visualization of the leaflets with the specific ultrasound appearance of the supporting structures depending on the specific model (Fig. 13-7). Because there is marked variability in surgeons’ valve preferences, it is helpful to look online at photographs of the valves most commonly encountered at your institution. Stentless bioprosthetic valves have an echocardiographic appearance very similar to a native aortic valve, other than increased echogenicity in the aortic root in the early postoperative period. This valve is best identified by reviewing the chart or asking the patient about any cardiac surgical procedures before beginning the study. Percutaneous valves (Fig. 13-8) appear similar to a native valve with increased paraannular thickness due to the supporting expanded stent.

Improved images of prosthetic tissue valves can be obtained from a TEE approach, particularly for valves in the mitral position, because the ultrasound beam has a perpendicular orientation to the leaflets with no intervening structures from this approach. With aortic valve prostheses, TEE imaging is less rewarding because the posterior part of the sewing ring shadows the valve leaflets. When images of the leaflets themselves are suboptimal, Doppler data can provide valuable information.

The longevity of bioprosthesis valves typically is limited by slowly progressive tissue failure with fibrocalcific changes resulting in leaflet deformity (leading to regurgitation), increased stiffness (leading to stenosis), or both. Echocardiographically, increased echogenicity and irregularity of the leaflets may be noted, although images of the leaflets often are suboptimal because of shadowing and reverberation.

Mechanical Valves

Ultrasound imaging of mechanical valves from a transthoracic echocardiographic (TTE) approach is frustrating because of severe reverberations and acoustic shadowing. While imaging may provide clues as to the type of valve prostheses (e.g., “low-profile” bileaflet or tilting-disk valve versus “high-profile” ball-cage valve), obviously it is simpler to ascertain the exact valve type and size from the patient’s medical record or valve identification card. Assessing motion of the valve occluder often is difficult. For example, the leading edge of a tilting-disk valve results in a strong reverberation across the image obscuring motion of the disk itself. In addition, an oblique image plane often is obtained relative to the prosthetic valve because orientation of the prosthesis within the annulus is not standard. With a tomographic plane perpendicular to the open bileaflet valve, the two leaflets can be identified clearly; this is an image plane that is best identified on multiplane TEE imaging or using 3D volumetric imaging (Fig. 13-9).

Technical limitations make the identification of prosthetic valve endocarditis or thrombosis problematic because the abnormalities may be obscured by reverberations or hidden by acoustic shadowing. TEE imaging can be helpful in identifying thrombus or infected vegetations on the atrial side of a mitral prosthesis because the TEE approach avoids “masking” of the LA by the prosthetic valve from TTE parasternal and apical windows. In a patient with a mechanical aortic valve, the subaortic region can be evaluated well from a TTE approach from parasternal and apical windows. In this situation, TEE images are less helpful because of shadowing of the outflow tract by the posterior aspect of the prosthesis.

Microcavitation

An incidental finding in some patients with a mechanical prosthetic valve is the phenomenon of spontaneous contrast. This phenomenon is similar to the spontaneous LA contrast seen in patients with an enlarged LA and low-velocity flow, which has been reported to be associated with a high propensity for thrombus formation. However, with a prosthetic valve, there are only a few bright mobile echogenic particles that are seen downstream from the valve, even in the absence of a low-flow state. The presumed mechanism of prosthetic valve spontaneous contrast is microcavitation due to impact of the occluder against the sewing ring.

Valved Conduits

Ultrasound imaging of a bioprosthetic or mechanical valve in a conduit (e.g., right ventricular [RV] to pulmonary artery) may be difficult because of ultrasound attenuation by the conduit prosthetic material. Stenosis in a valved conduit can occur as a result of either stenosis of the valve prosthesis or fibrotic ingrowth along the length of the conduit. In addition, residual or progressive stenosis at the proximal or distal anastomosis site can occur. Imaging the narrowing in the conduit often is difficult, but a careful Doppler examination may allow detection of the abnormal flow velocities. CW Doppler is used to assess the maximum flow velocity, while pulsed Doppler or color flow imaging is used to localize the level of obstruction along the length of the conduit.

Prosthetic Valve “Clicks”

The motion of the occluder of a mechanical valve (or the tissue leaflets of a biologic valve) creates a brief, intense Doppler signal that appears as a dark narrow band of short duration on the spectral display (Fig. 13-10). Audibly, this signal is similar to the valve “click” appreciated on auscultation. However, unlike auscultation, usually both opening and closing valve clicks are seen on spectral Doppler analysis. The Doppler signals associated with valve opening and closing are similar to those seen with native valves but are of greater intensity. The motion of the occluder also may result in color flow artifacts, with color signals covering large areas of the image that are inconsistent from beat to beat.

Antegrade Flow Patterns and Velocities

Bioprosthetic valves have a flow profile similar to that of a native aortic valve, with three leaflets that open to a circular orifice (in systole in the aortic position or in diastole in the mitral position), providing laminar antegrade flow with a relatively blunt flow profile. In the mitral position, the orientation of the bioprosthesis results in the inflow stream being directed anteriorly and medially toward the ventricular septum in most patients instead of toward the ventricular apex, as is seen for normal native valves. This results in a reversed vortex of blood flow in mid-diastole, as seen in an apical four-chamber view.

Flow profiles of different mechanical valves vary substantially, and none is analogous to flow across a normal native valve. Bileaflet mechanical valves have complex fluid dynamics that affect the Doppler echocardiographic evaluation of these valves. With the leaflets open, there are two large lateral valve orifices with a small narrow central slit-like orifice. The flow velocity profile shows three peaks corresponding to these three orifices, with higher velocities in the center of each orifice. The local acceleration forces within the narrow central orifice result in localized high-pressure gradients in this region of the valve, which often are substantially higher than the overall pressure gradient across the valve (Fig. 13-11).

The fluid dynamics of a tilting-disk valve are characterized by two orifices in the open position, one larger than the other (major vs. minor), with an asymmetric flow profile as blood accelerates along the tilted surface of the open disk. Subtle variations in this flow pattern depend on the shape of the disk (convex vs. concave surface) in addition to the sewing ring design.

With a ball-cage valve in the open position, blood flows across the sewing ring and around the ball occluder on all sides. When the valve closes, a small amount of regurgitation is seen circumferentially around the ball as it seats in the sewing ring.

Prosthetic valve normal velocities, pressure gradients, and valve areas depend on the valve type, size, and position. However, compared to a normal native valve, all prosthetic valves are inherently stenotic to some extent. Specifically, the expected antegrade velocities and pressure gradients across a normally functioning prosthetic valve are higher than the corresponding values for a native valve. Similarly, the effective orifice area of a prosthetic valve is smaller than the orifice area of a normal native valve. While manufacturers have data on in vitro flow characteristics for each valve, in vivo echocardiographic data are sparse owing to the large number of valve types and sizes. Even in a large study, only a few patients have the same valve type, position, and size. In the available Doppler studies of normal prosthetic valves, data often are presented in various ways. Some studies report the mean ± 1 standard deviation for each variable; others include the range as well. Expected normal velocities, pressure gradients, and valve areas for several commonly seen prosthetic valves are shown in Tables 13-2 and 13-3.

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