Prosthetic 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. “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. In the past, bioprosthetic valves were implanted only surgically using cardiopulmonary bypass to support the circulation while the valve was implanted. Now, specially designed bioprosthetic valves can be implanted by a transcatheter approach with the tissue leaflets mounted on a compressible stent ( Fig. 13.4 ).

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. Whereas the fluid dynamics of a homograft are similar to those of a native valve, flow velocities are slightly higher and valve areas are 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

Various 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:

• ▪

  A bileaflet valve in which two semicircular disks hinge open to form two large lateral orifices and a smaller central orifice ( Figs. 13.5 and 13.6 )

  
  

• ▪

  A tilting-disk valve in which a single circular disk opens at an angle to the annulus plane, being constrained in its motion by a smaller “cage,” a central strut, or a slanted slot in the valve ring

In the past, ball-cage mechanical valves also were used and are still occasionally 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. Conduits are constructed from biologic (e.g., a homograft) or artificial (e.g., various types of fabric) material, either with a stented tissue or a mechanical valve. Fluid dynamics similar to those for a valve implanted in the native annulus. A stentless valve in root prosthesis also can be used in this situation.

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, a process that 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. 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, thus 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 with consequent 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 a 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

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 limit visualization of the leaflets, with the specific ultrasound appearance of the supporting structures depending on the specific model ( Fig. 13.8 ). When examining a patient with an unfamiliar valve type, a quick look online at valve photographs can be helpful. Stentless bioprosthetic valves have an echocardiographic appearance very similar to that of 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.

Transcatheter valves (see Fig. 13.4 ) in the aortic or pulmonic position appear similar to normal native valves, with three thin valve leaflets. However, increased echogenicity or thickness of the para-annular region is evidence. Some transcatheter aortic valves have a longer supporting cage that extends into the left ventricular (LV) outflow tract or ascending aorta. Transcatheter mitral valves have a supporting cylindrical cage that extends into the LV chamber, with a variable appearance depending on specific valve type.

Aortic homografts appear similar to native aortic valves except for some increased thickness in the 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 is an option for patching 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.

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 are typical, although images of the leaflets often are suboptimal because of shadowing and reverberation.