Mechanical heart valve replacements can save the lives of those they are implanted in, however like any man made technology they are far from perfect. All three types show a high degree of durability, but some have better performance characteristics than others.
Caged Ball Valve:
- Very simple design, only moving part is the ball.
- Poor hemodynamics, blood must flow around the ball making the heart work harder
- Higher rate of thromboembolism
Tilting Disc Valve:
- Improved hemodyanamics
- Lower rates of thromboembolism
In 1979 a new version of the tilting disc valve, the Bjork-Shiley convexo-concave model, shown here, was introduced. This was an attempt to improve blood flow characteristics by increasing the angle at which the leaflet opened. This valve encountered major structural problems. The struts tended to fracture causing the disc to be released into the patient’s bloodstream causing a major embolism. This valve was removed from the market in 1986.
- Most implanted valve today
- Most improved hemodynamics
- Still a risk for thrombosis
- Allow some backflow
The St. Jude Bi-leaflet shown at right is the most widely implanted valve.
In addition to mechanical heart valves, there is another class of replacement valves called tissue valves. These are made from human or animal tissue, and are an alternative to the mechanical variety. Both valve types have strengths and weaknesses.
As discussed above, the most prominent advantage of the mechanical valves is their durability, in a fully grown patient they have been known to last over 40 years. Tissue valves may need to be replaced one or more times in a patient’s life.
However due to the nature of the materials and the design hemodynamics, the mechanical valves are at a much higher risk for clotting and therefore embolism. They also have a higher risk of major bleeding. Because of the clotting risks patients with mechanical replacements must be placed on anti coagulant drug for the rest of their lives.
Another disadvantage of mechanical valves is that they cannot be implanted in patients who are still growing. In this case, a tissue valve transplant known as the Ross Procedure is a useful alternative. The Ross Procedure involves replacing the patient's aortic valve with his own pulmonary valve. The pulmonary valve is usually replaced with a graft from another donor. The translocated pulmonary valve then grows with the child. This is not a desirable surgery as this double valve replacement has higher surgical risks.