Diseased heart valves can be treated in several ways. These include: 1) treatment with medication; 2) surgical repair; 3) surgical replacement. If the surgeon chooses to replace a natural heart valve, the first step is to remove the diseased natural valve (excise the valve) and then implant a prosthetic heart valve in its place. Prosthetic valves used to replace the heart’s diseased natural valves come in different sizes to fit the patient and are made from a variety of materials.
There are two broad categories of heart valves that are classified according to the type of material:
bioprosthetic or tissue valves made primarily from animal tissue [i.e., a pig’s aortic valve, a cow’s pericardium (sac surrounding its heart) or human valves from cadavers]
mechanical valves constructed from synthetic material
When possible, it is often preferable to surgically repair the patient’s valve rather than to replace it with a prosthetic device. Valve repair usually involves the surgeon modifying the tissue or underlying structures of the mitral or tricuspid valves.
Valve repair can be performed with or without implantation of an annuloplasty ring/band, a device that provides support so that the patient’s natural leaflets close completely and function normally.
There are a wide variety of tissue valves. They are defined below:
- Autograft/Autologous tissue:
- autograft – valve that has been moved from one position to another position within the same individual (i.e., transfer of the pulmonary valve to the aortic position)
- autologous tissue – constructed from patient’s own tissue
- Homograft (or Allograft) -human valves or tissue taken from cadavers
- Heterograft (or Xenograft) – valves or tissue taken from animals (i.e., pig or cow) and preserved
A heterograft is a type of biological valve made with animal tissue. A porcine valve is made from the pig’s aortic heart valve and is either sewn onto a flexible or semi-flexible frame to make a “stented” valve, or the natural aortic root is left intact to function as the frame to make a “stentless” valve. Another type of tissue valve is a pericardial valve. Pericardial valves traditionally contain leaflets made from bovine (cow) pericardium (the sac surrounding the heart) and are sewn onto a flexible or semi-flexible frame. porcine (pig) stented valve porcine (pig) stentless valve bovine (cow) pericardial stented valve Figure 3-2: Tissue Valves
There are three basic types of mechanical valves. A bileaflet valve has two semicircular discs (referred to as leaflets) that are mounted on hinges within a housing and open and close simultaneously. A tilting disc valve has a housing with a single circular disc that tilts open and closed. The disc is typically supported by “struts”. The ball and cage design has a spherical ball that moves within a cage.
Criteria and Selection of Heart Valves
Selection of a Heart Valve
The choice between mechanical and tissue valves is typically dependent upon an individual assessment of the benefits and risks of each valve and the lifestyle, age and medical condition of each patient.
The choice of implanting a tissue or mechanical valve is often based on patient age. However, there is no clear agreement on the exact age cut-off where a tissue valve may be preferable to a mechanical valve.
The preoperative condition of the patient is extremely important in determining which type of valve is to be used. Very sick patients who have other diseases may be good candidates for tissue valves. Their other problems may include coronary artery disease or kidney problems that often require other surgeries, during which anticlotting medication cannot be taken.
There are many variables that the surgeon/cardiologist must evaluate when determining which valve option is best for the patient.
Advantages of Tissue Valves
- Lower risk of forming blood clots
- No long-term anticlotting medication needed in most cases, which may result in a better quality of life for the patient (i.e., no daily medication and reduced periodic lab tests)
Disadvantages of Tissue Valves
- Potential risk of needing another operation to replace the valve
Advantages of Mechanical Valves
- Potentially lasts forever
- Blood flow is good
Disadvantages of Mechanical Valves
- Require long-term anticlotting medication to minimize the risk of forming blood clots
- May have dietary and activity constraints
- Valve clicking sound may disturb the patient