Artificial Heart and Heart Valve
An artificial heart is a manmade device that is intended to replace the heart muscle that pumps approximately 2,000 gal (7,571 L) of blood through the body each day. The heart muscle is composed of several chambers and the blood flow into and out of the chambers is controlled by a system of valves. Valve failure can lead to congestive heart failure, pulmonary edema, and other serious cardiovascular illnesses. Replacement of diseased or defective heart valves with artificial valves can be a solution to restore heart function.
Four one-way valves control the movement of blood into, through, and out of the human heart. All are designed to permit the flow of blood in one direction only and to prevent its backflow. Blood returning from the body to the heart enters the right atrium and from there passes through a valve called the tricuspid (or right atrioventricular) valve into the right ventricle. The right ventricle pumps the blood through another valve (the semilunar valve) to the lungs where carbon dioxide is removed and the blood is infused with fresh oxygen. Blood returns to the left atrium and is pumped into the left ventricle through the bicuspid or mitral (or left atrioventricular) valve. When the left ventricle contracts, it forces blood through the aortic semilunar valve into the aorta and on through the body.
Heart valves are constructed of a pair of flaps. The flaps are made of strong, thin, and fibrous material that is connected by strong fibers to muscles within the main heart muscle. This construction allows the flaps to remain shut against back pressure, thus allowing blood to flow only one way through them.
When the valves malfunction because of birth defects, deposits of cholesterol or calcium on the leaflets
of the valves, or because of a rheumatic disease, they do not close completely. Backflow of blood can result (a physician hears this as a heart murmur). Though a small volume of backflow is not harmful, a deteriorating valve that permits greater and greater amounts of blood to pass back through it can have serious consequences on the heart itself.
It is often necessary to replace the natural valve with a man-made artificial device. A one-way valve is a rather simple device to construct to function outside of the body. Inside the body, however, the valve must perform flawlessly and unceasingly for many years. The early mechanical valves, though adequate for controlling blood flow, tended to fracture after some years of use. The metal cage enclosing the valve broke under repeated and constant taps from the movable part of the valve. A fractured valve means a malfunctioning valve, possibly the formation of a blood clot and potential heart attack or stroke.
One modern design of an artificial valve consists of a ring by means of which the valve is sewn into place in the heart and some means of controlling the flow of blood. One artificial valve known as the ball-and-cage model has a three-pronged cage within which is a ball. The ball lifts to allow blood to pass through and is pressed down into the valvular opening to seal it and prevent backflow. Another design, called a disk-and-cage valve, has a similar action, except the ball is replaced by a flat disk that swivels back and forth to open and close the passage.
A more modern replacement valve uses the valve from a pig heart, treated to prevent rejection by the human immune system, mounted into a ring. The ring is sewn into the heart. The leaflets in the pig valve behave more like those in the human heart with less danger of breakage than occurs in the cage-type valves.
The use of artificial hearts as a replacement for an ailing human heart began in 1953. Then, the use of a heart-lung machine designed by a physician named John Gibbon in open-heart surgery demonstrated that an artificial device could, at least for a time, replace the real heart. In 1966, Michael DeBakey implanted a partial artificial heart. From 1982 to 1985, William DeVries carried out a series of implants of a device called the Jarvik-7 artificial heart. The first heart was implanted in Barney Clark, who lived for 112 days before dying of complications caused by the device. Mechanical and other problems ultimately stopped the use of the Jarvik-7. In 2000, the modified version of the Jarvik heart (the Jarvik 2000) was implanted. This was the first completely artificial heart to be installed.
In July of 2001, surgeons at Jewish Hospital in Louisville, Kentucky, implanted an artificial heart (the AbioCor® heart) in Robert Tools, a 59-year-old man whose own heart was damaged and failing. The AbioCor® heart was the first self-contained artificial heart to be implanted in a human. It is also the smallest artificial heart yet devised, being about the size of a softball.
Tools suffered a serious stroke and died in November, 2001. Nonetheless, his progress since the installation of the artificial heart was encouraging. As of May 2003, at least eight patients received AbioCor® hearts.
Guy, T.S. "Evolution and Current Status of the Total Artificial Heart. American Society for Artificial Internal Organs Journal 44 (1998): 28–32.
American Society for Artificial Internal Organs, PO Box C, Boca Raton, FL 33429. (561) 391–8589. [cited October, 19, 2002]. <http://www.asaio.org>.
"How Artificial Hearts Work." How Stuff Works [cited October 19, 2002]. <http://www.howstuffworks.com/artitificialheart.htm.>.
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