ARTIFICIAL HEART VALVE

Typical Number in Hospital:

Cost Bands:

References: 4

All four valves in the heart can become diseased either restricting the flow (stenosis) or leaking backwards (insufficiency or regurgitation).

Over 200 000 mechanical and tissue valves have been implanted in humans and there are three main types. These are:

1. Ball and cage valves such as the Smeloff.

2. Hingeless tilting disc valves such as the Bjork-Shiley.

3. Tissue valves employing animal tissue such as the valve taken from a specially bred pig.

In each case the valve is mounted on a sewing ring made of metal or plastic sheathed in woven fabric (usually Dacron) which is sewn into the orifice created by excising the natural valve. No artificial valve is a perfect substitute for the natural valve and the design requires a trade off of various factors to achieve the optimum solution.

The valve may restrict the flow to some extent, and allow more regurgitation than is desirable, and so the largest possible valve is used in the space available. The use of synthetic materials causes damage to the blood and long-term anticoagulant therapy is always necessary. The life of the valve is limited by fatigue and corrosion and this may be exacerbated by poor materials and poor design leading to excessive stresses in the structure or turbulence in the blood.

Considerable effort has been expended in the testing of artificial valves to optimize these factors. They are usually tested in heart simulators, sometimes called pulse duplicators, which employ a Perspex model of the heart with the valves mounted in it and a pump which generates a pulsatile flow driving a viscous liquid with similar mechanical properties to real blood.

Tissue valves require special treatment to ensure sterility and minimal antigenic reaction from the body.