Tissue Versus Metal Valves: Weighing The Factors

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Artificial valves were first used in heart valve replacement surgery over 30 years ago. The success of this surgery has legitimized replacement of diseased and/or damaged heart valves with artificial valves as a routine surgical procedure. Approximately 150,000 valves are replaced worldwide each year. While the demand for artificial valves in Europe and the United States has stabilized, world demand for these prostheses is increasing at the rate of about 10 percent per year.

There are two main types of artificial heart valves available: mechanical (metal) and tissue (pig) valves. However, there is frequently no consensus pertaining to the issue of the ideal type of valve device for most patients. The decision should be based on a thorough discussion between the surgeon and patient. Ideally, the patient should make the final decision. The purpose of this article is to provide an overview of the risks and benefits of these two types of valves, as well as indications for a preferred type of artificial valve in different patient groups.

Mechanical Valves
Mechanical artificial heart valves were first introduced in 1960. In general, the mechanical valve is made of some metallic alloy.

The benefits
The main advantage of this type of artificial heart valve is its excellent lifetime durability. Theoretically, the valve can last forever. In other words, if you have a mechanical valve, it is unlikely that you will require another surgery on the same valve. Avoiding another operation is very appealing to many, especially those that are young.

The drawbacks
However, the major drawback is the need for long-term anticoagulation with a "blood thinner". This medication goes by different names but the most common one is coumadin, which is a potent blood thinner and requires careful monitoring by a physician. Aspirin is also a blood thinner but it is not potent enough for mechanical heart valves. Taking a blood thinner is necessary to prevent the formation of blood clots on the valve. These blood clots can break off and cause a variety of problems such as a stroke and malfunction of the heart valve. As a result, a blood thinner is an absolute requirement after placement of an artificial mechanical valve.

But taking a blood thinner has its own complications. The major problem is a risk of bleeding. Studies have reported that the rate of anticoagulation-related bleeding complications in patients with mechanical valves is as high as 30-40% over 10 years versus 10-20% for patients with tissue valves. A bleeding complication can be minor, such as bleeding from a superficial cut, but it also could be major, such as bleeding in the brain or in the gastrointestinal tract.

Bioprosthetic Valves
Bioprosthetic valves are made from biological tissues, mostly from valves of pigs. After preserving the valve in a special preservative called glutaraldehyde, the tissue strength is increased and the chance of the patient's immune system rejecting this foreign material is diminished. These processed biological valves are then mounted on a stent (splint left inside body to help healing process) for implantation. There are other tissue valves as well and we will discuss them later on.

The benefits
Tissue valves have low risk of clot formation. The incidence of clot formation associated with tissue valves is no different from patients with mechanical valves taking adequate blood thinning medication. Therefore, most patients with tissue valves do not need to be on blood thinners. This is the major advantage of tissue valves. Blood thinner-related bleeding complications are much less frequent in patients with tissue valves than patients with mechanical valves.

The drawbacks
On the other hand, tissue valves have limited life spans. They don't last forever and if you have a tissue valve, you will need another operation in the future, especially if you are young. The main cause of valve tissue degeneration is calcium deposition with resultant hardening of the tissue known as calcification. This degenerative process increases with time. The failure rate at 3 years after implantation is miniscule, but increases to 5 to 10 percent at 6 to 7 years and reaching 30 percent by 10 years. In children and adults under 35 years of age, tissue degeneration is even faster. The reason for this marked increase in failure rate is not entirely clear. Nevertheless, the failure rate of tissue valves implanted in patients under 35 years old is about 45 percent at 10 years after the initial operation. The process of tissue degeneration tends to be gradual, which allows for early detection and monitoring, as well as re-operation for valve replacement when it becomes necessary.

Other tissue valves available
There are other tissue valves available today. Of course, the best valve is your own valve. Valve repair, rather than valve replacement, should be done whenever possible. One of the most popular tissue valves today is made from bovine pericardium - the lining of the sac around the heart of a cow. These bovine pericardial heart valves seem to last longer than porcine (pig) artificial heart valves. A human heart valve is also available. It is called a cryopreserved human heart valve and does not require blood thinner and has a long life span.

Ross procedure
For aortic valve replacement, some surgeons have also transplanted the patient's own pulmonary valve into the aortic position. This is called a Ross procedure and is more commonly performed in children for a different condition. In the adult, this operation requires someone who has fair amount of experience with the procedure. The downside of the Ross procedure is that the operation transforms a relatively straightforward operation into a complicated one. If the procedure fails, the patient may be left with two abnormal valves.

Choosing a Heart Valve: What are the Criteria?
The selection of an artificial heart valve should be based on patient characteristics including:

Let's take care of the easy ones. If one is already on a blood thinner such as coumadin, one should go with a mechanical heart valve. There is no sense in having a tissue valve if the patient has to take coumadin anyway. Aspirin does not count as a blood thinner here.

Associated medical problems
There are certain groups of individuals that are at high risk of early tissue valve breakdown. Examples include kidney failure and any conditions that lead to a high blood calcium level. These individuals probably should have mechanical heart valves.

Pregnancy
Women in their childbearing years should give serious consideration to tissue heart valves, even if the valves may degenerate at a more rapid pace. Pregnant women cannot take coumadin since blood thinner can cause defects in the developing embryo. As the possibility of future pregnancy can never be ruled out with certainty, it is best for women in this age group to receive tissue valves. After the childbearing years are over, a mechanical valve can then be implanted when signs of tissue valve degeneration begin to surface.

Age
Age is an important factor in valve selection because of the clear difference in valve durability with respect to time.

Many heart specialists recommend tissue valves for patients over 65-70 years of age. Older individuals may have more problems associated with blood thinners. However, one should also take into consideration the patient's perceived life expectancy. It is reasonable to select mechanical valves for older patients if they are otherwise in good health and expected to live for more than 10 years after the operation. Alternatively, patients who are younger than 65 with multiple medical problems and short life expectancies may want to consider tissue valves.

Young patients with longer life expectancies may give more serious consideration to mechanical valves since tissue valves begin to degenerate and fail after 5-10 years of implantation. But this is a complicated issue. The blood thinners that necessarily accompany mechanical valves make certain activities such as skiing high risk. Furthermore, the risk of a major bleeding complication each year is low but over a 10-year interval, it becomes quite significant. Finally, with the advancement of cardiac surgery and skill of heart surgeons, the risk of a re-operation is very low these days making tissue valves more attractive to young patients.

Risk or contraindication to anticoagulation
Contraindication to anticoagulation (patients who cannot easily tolerate blood thinners) strongly favors the selection of tissue valves since anticoagulant therapy is required with mechanical valves. This group of patients includes those with bleeding disorders, concomitant diseases with high risk of bleeding such as aneurysms, arteriovenous malformations, and gastrointestinal diseases, or those with lifestyles or occupations that predispose them to high risk of injury and bleeding. On the other hand, for patients who need long-term anticoagulation for disorders such as atrial fibrillation, mechanical prostheses are indicated.

Patient preference
Finally, patient preference needs to be seriously considered. Some patients prefer to take blood thinners to avoid the possibility of another operation in the future. Others fear bleeding to death after an accident while on anticoagulation. An informed choice of the patient is honored whenever possible. If the physician feels that the patient made the wrong choice, a referral to another physician for a second opinion may be requested.

Summary
For decades, mechanical and tissue valves have been successfully implanted in patients with valvular disease. However, each type of artificial valve has its advantages and drawbacks. Mechanical valves have lifetime durability but require long-term anticoagulation. This type of prosthesis is recommended for patients who have no problem tolerating blood thinners. Tissue valves do not require blood thinners but are less durable. Elderly patients and patients who cannot use blood thinners are the main recipients of tissue valves. Other selection criteria such as patient preference must also be considered. There are many factors to consider in choosing an artificial heart valve, and there is seldom a 'right answer'. Still, knowing the benefits and drawbacks of both the tissue and mechanical valves is the best starting point for making a smart decision.