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Case Study - Mechanical Heart Valves
The Human heart
Click the picture to enlarge
A principal organ in the body, the heart is a powerful muscular pump without which a person could not survive. Over an average lifetime the heart will beat almost 3 billion times, working to pump blood to the extremes of the body and back. The heart contains two inlet and two outlet valves, each essential for maintaining the direction of flow. Oxygen-depleted blood enters a chamber on the right side of the heart, is then re-oxygenated via the lungs and fed back into the left side of the heart, where it is pumped back out into the body.
Without these valves to control the direction of flow the heart would have a much harder job pushing the blood into adjacent chambers.
The two inlet valves - the tricuspid and mitral - are known as the atrioventricular valves and the two outlets - pulmonary and aortic – the semilunar valves.
All the valves have three cusps except for the mitral valve, which has two. In general the cusps open to allow the blood to flow freely forward through the valve. When the flow slows down and finally stops, a pressure difference across the valve causes the cusps to close, preventing any back flow.
Cardiac valves, which consist of thin flaps of flexible, tough endothelium-covered fibrous tissue firmly attached at the base to fibrous valve rings, are subjected to considerable pressures. The pressures withstood by the valves on the left side are five times greater than for the right, the peak pressure being 100 to 120 mmHg. The mitral and aortic valves are therefore more prone to ageing and wear which is just one of the causes of heart valve disease and failure.
Valve disease and malfunction
Heart valves may become impaired for a variety of reasons. Essentially, people are either born with heart valve abnormalities or acquire them through disease or ageing. The damage can either be in the form of stenosis (tightening of the valve preventing forward flow of blood) or regurgitation in which the valve does not close properly, allowing back flow. With careful supervision and medication, people with heart disease can live comfortably. In some cases however, surgical repair or even replacement of the defective valve(s) is necessary. In the United States alone, approximately 50,000 heart valves are replaced a year.
Desirable characteristics of a replacement valve
Although prosthetic heart valves have developed considerably since their first designs in the 1950s, the perfect design that would satisfy all of the following desirable characteristics has yet to be made.
- Closure is prompt and complete
- Chemically inert and non-haemolytic
- Durable for extended intervals
- Easily and permanently inserted into an appropriate [physiologic] site
- Interface between prosthesis and patient heals appropriately
- Not annoying to the patient (noise-free)
Mechanical valve design and evolution
All mechanical valves require the patient to follow a rigorous [anticoagulant] therapy for the remainder of their life, due to the continual risk of thrombosis (blood clotting) leading to a possible embolism and death of the patient.
Through design, the haemodynamics (blood flow characteristics) of the valve can be improved, reducing the risk of blood cell damage and subsequent clotting.
Caged Ball Model
The first mechanical valve to be successfully implanted in a patient was developed by Hufnagel in 1952 and consisted of a caged ball design.
The main characteristic of this mechanical valve is the forced peripheral flow of the blood (i.e. it must flow around the outside of the ball) rather than a central flow that characterises the human valve. As a consequence, the heart must do extra work to compensate for the momentum lost in changing the direction of flow. In order for the ball to block the valve when the valve is shut, it has to be larger than the opening in the valve; therefore a relatively large ball must be used.
Finally, the ball itself damages the blood cells due to impacting on the ball’s surface. These damaged cells then release clotting agents, thus explaining the high thrombogenicity characteristic of this design.
Tilting Disc Model
Following on from this in the mid 60s a new design was introduced that substantially improved on the haemodynamics aspect off mechanical valves by using a tilting disc. The use of a tilting disc provided an increased central flow without allowing any back flow, thus reducing the occurrence of blood clotting.
The original Bjork-Shiley valve is one of the most successful single disc valves ever made. However, attempts to further improve on the haemodynamics of the valve by increasing the tilting angle of the disc and by modifying the disc shape proved to be a disaster. In 1986 it was removed from the market but by then an estimated 86,000 people world-wide had received the valve. The valve was subject to mechanical failure caused by the fracture of both legs of the outlet strut leading to the escape of the disc and to the death of the patient.
Introduced in 1979, and still the most widely-used, the bi-leaflet heart valve consists of two semicircular leaflets that pivot on hinges.
These leaflets swing open in a position nearly parallel to the flow thus enabling an essentially central flow. However when the leaflets are in the closed position they never quite meet, allowing some back flow to occur.
Today the valve is renowned for its excellent durability and good haemodynamics although as for all mechanical valves it does still require the patient to take anticoagulants. A comparison of the velocity distribution for the single leaflet and bi-leaflet valves is shown below.
Click the picture to enlarge
The turbulent flow and presence of stagnant flow (shown in blue) indicated in the velocity distribution for the single leaflet valve illustrates the high risk of blood clotting associated with this design.
This contrasts with the velocity distribution of the bi-leaflet valve showing smooth symmetric flow, with no areas of stagnation accounting for its low associated thrombogenicity in comparison to previous designs.
Material Selection and desirable properties
Another factor, other than the design which plays an important role in the reduction of the risk of blood clotting for patients with a mechanical valve, is the material used.
The material must be strong, durable, fatigue resistant and of course biocompatible. In the 1960s mechanical valves were suffering due to wear or thrombosis. However a material was discovered, used at the time to coat nuclear fuel particles, which had the right characteristics for heart valves. This material was pyrolytic carbon, which is the principal material still used for mechanical heart valves today.
Pyrolytic Carbon or PyC is resistant to wear, strong, durable, is highly resistant to blood clotting and causes little damage to blood cells. It owes its unusual mechanical properties and its biocompatibility to a unique microstructure. The structure resembles that of graphite but is subtly different. In graphite, planar hexagonal arrays of covalently bonded carbon atoms are stacked so that the atoms in every other layer are coincident. Each layer is bonded by weak Van-der-Waals forces. In pyrolytic carbon, on the other hand, the layers are stacked in a disorderly manor causing wrinkles and distortions to occur within each layer. These distortions are responsible for the increased ductility and durability of PyC.
Every time the heart beats a valve must open and close, therefore for a material to be successful in an artificial valve it must be highly fatigue resistant. Tests have shown that the strength of pyrolytic carbon is not affected by cyclic loading and further is not prone to cyclic stress-induced degradation. This means that it will not degrade over time, due to its own mechanical movement, making it an extremely suitable choice for heart valves.
Like a ceramic, PyC is brittle and therefore cracks, once generated, can propagate through the material causing failure of the heart valve, resulting generally in the death of the patient. Although it must be noted that unlike a pure ceramic, pyrolytic carbon is ductile making it more difficult for a crack to occur in the first place.
When a foreign material is placed into the human body, the immune system responds by coating it with layers of blood, however some materials such as PyC are not recognised as foreign and are thus called 'blood compatible' or biocompatible. A key indicator of how biocompatible a material is is the amount of blood cells (platelets) that will adhere to it. If platelet adhesion is high, there is a high risk of blood clotting occurring.
Tests have shown that Pyrolytic carbon, produces substantially less platelet aggregation than other materials tested (including titanium alloy, diamond like carbon, polycrystalline diamond) and thus as stated previously is highly biocompatible.
The search for new materials and surface coatings with the required properties of high fatigue resistance and biocompatibility superior to those of pyrolytic carbon continues but in the mean time, PyC is adequate resulting in only very few failures (approximately 50 failures in every million valves in service).
This article is based on a case study developed under the supervision of Dr. Irene Turner of the University of Bath.
Mechanical Heart Valve