S. Waldman MECH 393

Design of Prosthetic Heart Valves

Heart valves prevent the backflow of blood which ensures the proper direction of blood flow through the circulatory system.

The heart has four valves: Two atrioventricular valves

(tricuspid and mitral) that prevent backflow into the artia

Two semilunar valves (pulmonary and aortic) that prevent backflow into the ventricles

S. Waldman MECH 393

Heart Valve Disease

There are numerous complications and diseases of the heart valves that prevent the proper flow of blood. Essentially, heart valve diseases fall into two categories, stenosis and incompetence.

A stenotic heart valve prevents the valve from opening fully, due to stiffened valve tissue. Thus, more work is required to push blood through the valve.

An incompetent valves cause inefficient blood circulation by  permitting backflow of blood in the heart.

S. Waldman MECH 393

Treatment Options

On a large scale, medication is the best alternative.

Although in some cases defective valves have to be replaced with a prosthetic valve in order for the patient to lead a normal life. An enormous amount of research and development has proven to be beneficial, as prosthetic heart valve technology has saved hundreds of thousands of lives.

The two main prosthetic valve designs are: Mechanical Heart Valves Bioprosthetic Heart Valves

S. Waldman MECH 393

Evolution of Mechanical Heart Valves

The first prosthetic heart valve was implanted in 1952 by Charles Hufnagel. The device was an acrylic ball valve inserted into the descending aorta. As the valve only prevented regurgitant flow from the lower body, cardiac work was only partially relieved and coronary flow was not improved. In addition, embolization and thrombosis of the valve frequently occurred, and the noise generated by the valve was disconcerting — reminiscent, according to some, of a ticking time bomb.

S. Waldman MECH 393

Evolution of Mechanical Heart Valves

In spite of its generally poor success, others recognized the importance of the approach which led to the development of over 30 different mechanical designs worldwide.

These valves have progressed from simple caged ball valves, to modern bileaflet valves. Heart valves are designed to fit the peculiar requirements of blood flow through the specific chambers of the heart, with emphasis on producing more central flow and reducing thrombosis.

S. Waldman MECH 393

Caged-Ball Design

In addition, the ball causes damage to blood cells due to collisions. These damaged blood cells release blood clotting agents, thereby requiring patients to take anticoagulants.

The caged ball design is one of the early designs, that uses a small ball that is held in place by a welded metal cage.

Although effective, caged-ball valves completely block central flow (blood to flow through the valve centre) and the heart must work harder to compensate for the momentum lost to the change of direction of the fluid.

S. Waldman MECH 393

Tilting Disc Design

In the mid-1960s, a new class of mechanical valves were designed that utilized a tilting disc to better mimic the natural patterns of blood flow. Tilting-disc valves have a floating polymer disc held in place by two welded struts. The tilting motion provides improved central flow while preventing backflow and also reduce mechanical damage to blood cells.

Although vastly superior to the caged-ball design, tilting discs valves have a tendency for the outlet struts to fracture as a result of fatigue.

S. Waldman MECH 393

Bileaflet Design

In 1979, a new mechanical heart valve was introduced: bileaflet valves. These valves consisted of two semicircular leaflets that pivot on hinges. The leaflets swing open completely, parallel to the direction of the blood flow. The result is the closest approximation to central flow achieved in a natural heart valve. For this reason, the bileaflet valve is the most popular of the modern designs.

The problem with these valves is that the leaflets do not close completely, which permits some backflow. Since backflow is a property of a defective valve, the bileaflet design is still not ideal.

S. Waldman MECH 393

Materials

Current designs use materials that do not induce clotting in the blood stream.

Most commonly used materials include: stainless steel alloys molybdenum alloys pyrolitic carbon for the valve housings and leaflets silicone, polytetrafluoroethylene (teflon®) polyester (Dacron®) for sewing rings

A new generation of mechanical valves made of materials with improved blood contact properties, better wear characteristics and resistance to infection are currently under development.

S. Waldman MECH 393

Advantages and Disadvantages

AdvantagesThe main advantage of mechanical valves is their high durability. Mechanical heart valves placed in young patients can typically last for their lifetime

DisadvantagesThe major problem with all mechanical valves is the increased risk of blood clotting. As a preventative measure, mechanical valve recipients must take anticoagulantants. These anticoagulants can cause birth defects in the first trimester of fetal development, rendering mechanical valves unsuitable for women of child-bearing age.

S. Waldman MECH 393

Future of Mechanical Heart Valves

To develop the next generation of mechanical heart valves, new age tools that are being used to improve valve design, which include:

accelerated wear testing advanced blood contact property testing computer assisted design and manufacturing coatings to reduce the chance of infection and improve healing

S. Waldman MECH 393

Bioprosthetic Heart Valves

Bioprosthetic heart valves are valves made from actual valve tissue (animal or human).

These valves hold many advantages over mechanical valves: design is closer to the natural valve better hemodynamics do not cause damage to blood cells patients do not require long-term

anticoagulants do not suffer from structural problems

(e.g. fatigue)

S. Waldman MECH 393

Bioprosthetic Heart Valves

Animal tissue valves are often referred to as xenograft valves. These valves are constructed from recovered heart tissue at the time of commercial meat processing. After fabrication, they are chemically crosslinked to limit degradation.

The most commonly used animal tissues are porcine aortic valves (explanted valve with an attached Dacron cloth sewing skirt) and bovine pericardial valves (sewn leaflets from pericardial tissue with an attached Dacron cloth sewing skirt). Bioprosthetic valves have good durability and usually last for 10-15 years.

The common cause of failure in these valves is due to tissue calcification. Calcification stiffens the valve tissue leading to the restriction of blood flow through the valve (stenosis) and/or generation of tears (from stress concentrations) in the valve leaflets.