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Artificial Organs Cardiovascular [valves] [assist] [total heart]

The human heart beats about 35 million [times] in order to pump millions of gallons of blood through an individual's circulatory system in just one year. The machinery of the heart and the circulatory system takes a tremendous amount of abuse, abuse which sometimes leads to dangerous wear and tear. Over 700,000 deaths a year, in the US, are attributed to heart failure. Many devices have been created to help people with heart problems. These devices range from artificial valves, to assist devices, to total heart replacement units.

Valves

Artificial heart valves are very difficult to make properly because of the tremendous performance requirements associated with such a crucial replacement. The materials must be extremely durable, the valve must resist extensive wear, it must be completely impervious when sealed, it should be easy to implant surgically, and there should be little or no tendency of blood to clot on the valve.

The first type of artificial heart valve, a development which was the result of the work Dr. Charles Hufnagel and Dr. Dwight Harken, was composed of a cage and a ball. Originally, the valves were made with bulky cages and hollow balls made of acrylic. More modern versions have cages made of titanium and balls made of silicone. Ball valves worked well because they did not wear out easily, however their hemodynamic (blood flow) functionality did not even remotely match that of a natural heart valve, and the shape and size of the cage made it difficult to implant and sometimes caused problems with other parts of the heart.

In an attempt to solve the problem of the size of the valve, lower-profile valves were introduced in which a disc was substituted for the ball. Although this decreased profile did make the more fit for implantation, the performance of the valve was not increased any, and artificial valves still not nearly as good as real ones.

In 1969, the Bjork-Shiley and the Lillehei-Kaster tilting disc valves increased the use of prosthetic heart valves tremendously. Tens of thousands of these valves were implanted in the United States alone. Unfortunately, attempts to improve the hemodynamics of the once successful valve led to disaster. Certain models developed strut fractures which often resulted in heart failure. Although most models of the Bjork-Shiley valve are extremely durable and have very low (nearly zero) rates of structural failure, all Bjork-Shily heart valves were removed from the US valve market in 1992. Modification on this so-called "leaflet" valve continued for years, some of which are still used today.

The next step in artificial heart valve technology was the integration of actual, living tissue into the design. Either porcine valves (from pig valve tissue) or bovine pericardial tissue (cow heart tissue) is sewn onto a metal wire stent. These types of implant have been quite successful and have excellent hemodynamics.

Click here* for extensive information on artificial heart valves.

Assist Devices

Assist devices are devices which do not replace the heart completely, but aid a weak or damaged heart in pumping blood through the circulatory system (most often as a temporary solution until a heart transplant can be performed. There following are two examples of current cardiac assist devices:

The HeartMate is made by Thermo* Cardiosystems.

The HeartMate LVAS is an implantable cardiac-assist device that takes over the pumping function of the natural heart. There are two versions of the HeartMate system: the air-driven version, which is powered by an external console, and the electric version, which is powered by batteries that can be worn discreetly to allow the patient complete mobility. The air-driven version, which is sold commercially in the U.S. and Europe, is used to sustain patients awaiting heart transplants. The electric version is sold commercially in Europe, both as a "bridge to transplant" and as a long-term alternative to transplant. The electric version is also being evaluated in the U.S. as a bridge to transplant and as a long-term alternative to medical therapy under clinical studies. Each HeartMate device is designed to assist the main pumping chamber, or left ventricle, of the natural heart, which is responsible for pumping oxygen-rich blood from the lungs throughout the body. The device, which is implanted below the diaphragm, is attached between the natural heart and the aorta (the main artery for feeding blood to the entire body), leaving the natural circulation undisturbed while providing all of the energy necessary to propel blood throughout the body. The air-driven HeartMate pump weighs about one and a half pounds, and is approximately four inches in diameter and less than two inches thick. The electric version is slightly heavier and thicker than the air-driven pump, due to the size and weight of the motor. Unlike a total artificial heart, the LVAS allows a patient's natural heart to be left in place where it can still perform certain biological functions such as regulating blood flow.

The Thoratec* pump is another heart assist device. It uses the previously described leaflet valves. This unit is very versatile. Blood can be taken from the left atrium or the left ventricle. It is then pumped into the aorta. Furthermore, right heart support can be achieved by installing the pump receive blood from the right atrium and pump it into the pulmonary artery. This is the only system that offers total circulatory support - left, right or biventricular. As of December 1, 1997, this system has been used in more than 879 patients and is currently being used in heart centers worldwide

Total Artificial Heart
(Stats. from Guy, 1998)

Cardiac failure is a tremendously common occurence. The Institute of Medicine estimates that, by the year 2010, 35,000 to 70,000 patients will be candidates for permanent cardiac replacement or support. As real hearts for transplantation are tremendously difficult to come by, the invention of total artificial hearts is one which is in tremendous demand. As of now, however, the FDA has approved only two such devices for humans in the United States: The Penn State and the Jarvik-7 (now CardioWest)pneumatic total artificial hearts. However, these hearts are nowhere near ready for permanant use, as the best transplant survivor lasted only 620 days after transplantation with a Jarvik-7.

In 1988, the National Heart, Lung, and Blood Institute (NHLBI)provided five-year contracts to companies to work on permenant, tether-free artificial hearts. Having ended the contracts without success in 1993, the NHLBI awarded further contracts to three of the orginal groups. In 1996, two of these groups, Penn State/3M and Texas Heart Institute* were selected to continue their studies. If experiments are successful, human studies should be ready to be performed by the year 2000. There are, of course, many other groups around the world which are trying to create a similar system, but none of these appear to be as close to human trials (Guy, 1998).