Artificial heart and heart valve
Artificial heart and heart valve
Valves: Control and construction
An artificial heart is an synthetically made device that is intended to replace the heart muscle that pumps approximately 2,000 gal (7,571l) of blood through the body each day. In 2004, around 100,000 people in the United States were candidates for heart transplants, with about 4,000 in critical need of them. However, fewer than 2,500 transplants are performed annually, primarily due to a lack of available donor organs. 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. Heart valves are flaps of tissue within the heart that open to allow blood to flow from one of the heart’s four chambers to the next and then close to prevent any blood from leaking back. 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.
Valves: Control and construction
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 synthetically made 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.
History
Medical researchers have long attempted to develop a mechanical pump to take over its job in the event of damage or disease. In 1935, French-born surgeon Alexis Carrel (1873–1944) and famed American aviator Charles Lindbergh (1902–1974), designed a perfusion pump that kept excised organs, including the heart, alive by circulating blood through them. News reports called this device an artificial or robot heart. 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 physician John Gibbon in open-heart surgery demonstrated that an artificial device could, at least for a time, replace the real heart.
The first total artificial heart (TAH) was implanted in 1957 in a dog at the Cleveland Clinic by Willem Kolff (1911–), a Dutch-born surgeon, and Tetsuzo Akutsu. Kolff later led a medical team at the University of Utah at Salt Lake City in developing the artificial heart. At the urging of another TAH pioneer, American cardiovascular surgeon Michael DeBakey (1908–), the United States government, through the National Institutes of Health, established an Artificial Heart Program in 1964 to develop both partial and total artificial heart devices. By 1966, DeBakey had designed and implanted a pneumatically driven component called a Left Ventricular Assist Device (LVAD) to serve the chamber of the heart that pumps blood out into the arteries. This was an important development, for the great majority of severe heart disease is caused by left ventricle failure.
In 1966, DeBakey implanted a partial artificial heart. The first human artificial heart implant was carried out by Denton Cooley (1920–) and his surgical team at the Texas Heart Institute in 1969. The pneumatically driven Dacron-lined plastic heart, designed by Argentine-born Domingo Liotta (1924–), was a temporary measure to keep a patient alive until a heart transplant could be performed.
From 1982 to 1985, American cardiothoracic surgeon William DeVries (1943–) 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.
KEY TERMS
Aorta— The major blood vessel leaving the heart and carrying blood to numerous other smaller vessels that branch off and deliver blood to the entire body.
Atrium (plural: atria)— One of two upper chambers of the heart. They are receiving units that hold blood to be pumped into the lower chambers, the ventricles.
Congestive heart failure— A heart condition in which the heart has enlarged because of back pressure against its pumping action. The heart will finally become so large and inefficient that it is unable to pump enough blood to maintain life.
Leaflets— Fibrous tissue flaps that open and close in the heart valves to allow the passage of blood. Ventricles —The two lower chambers of the heart; also the main pumping chambers.
As of May 2003, at least eight patients had received AbioCor® hearts. In 2004, the Syncardia CardioWest (CW-TAH) was developed by researchers from the University of Arizona. The CW-TAH is the first implantable artificial heart to be approved by the U.S. Food and Drug Administration (FDA). This device marked the first time a mechanical device became available on a broad basis to replace temporarily a weakened, dying heart. The new device replaces the lower half of the heart for desperately sick patients as they wait in hospitals for heart transplants.
As of November 2006, the person to have survived the longest with a CW-TAH artificial heart has lived for 602 days. So far, about 79% of patients who received the CardioWest Total Artificial Heart were able to sustain their life functions long enough to receive a heart transplant. In September 2006, another artificial heart was developed, the AbioCor artificial heart, by the AbioMed company.
During the early years of the 2000s, heart value replacement has become a common procedure. According to the U.S. National Institutes of Health, as of 2004, the success rate for heart valve surgery is very good. Only 2-5% of patients undergoing the procedure die because of it. For the majority of patients surviving the procedure, about 67% are still alive nine years after the surgery.
As of 2006, cardiovascular surgeons are hopeful that practical artificial hearts, ones that will keep patients alive for long periods of time so that people can resume normal lifestyles, will become available within the next fifty years.
See also Anatomy, comparative; Birth defects; Cholesterol; Electrocardiogram (ECG); Pacemaker.
Resources
BOOKS
Kaiser, Larry R., Irving L. Kron, and Thomas L. Spray, eds. Mastery of Cardiothoracic Surgery. Philadelphia, PA: Lippincott, Williams & Wilkins, 2007.
Del Nido, Pedro J., and Scott J. Swanson, eds. Sabiston & Spencer surgery of the Chest. Philadelphia, PA: Elsevier Saunders, 2005.
PERIODICALS
Guy, T.S. “Evolution and Current Status of the Total Artificial Heart.” American Society for Artificial Internal Organs Journal 44 (1998): 28–32.
OTHER
American Society for Artificial Internal Organs. “From
Discovery to Clinical Use.” <http://www.asaio.com/> (accessed November 28, 2006).
“How Artificial Hearts Work.” How Stuff Works. <http://health.howstuffworks.com/artificial-heart.htm> (accessed November 28, 2006).
Brian Hoyle