Professor of Surgery University of Texas and Surgeon The Texas Heart Institute Houston, Texas
Heart transplantation has remained the best available option for some patients with terminal heart failure since its introduction in 1967. Despite this encouraging fact, however, the survival rate after transplantation is limited, and there is a wide gap between the number of available donor hearts and the number of patients who need them. Because of these limitations, researchers are always looking for better ways to help patients dying of heart failure. These efforts include the development of mechanical circulatory devices such as the total artificial heart and the left ventricular assist device (LVAD).
Patients usually die of heart failure because the left ventricle, the heart's primary pumping chamber, does not function properly. Researchers therefore have directed their efforts at developing the LVAD, which takes over the function of the left ventricle. In the early 1970s, research efforts, supported by the Device and Technology Division of the
National Heart, Lung, and Blood Institute (NHLBI), were directed toward producing devices for long-term support (greater than two years).
The pumps were also used for short-term support in the hope that they would provide the necessary time for recovery of heart function. These pumps were pulsatile: The pump's action created a pulse similar to that of the natural heart and could draw blood from either the left ventricle or the left atrium and discharge it into the aorta.
In 1978, the Texas Heart Institute began using LVADs as bridges to transplantation. The devices were able to support patients during the time gap between imminent heart failure and heart transplantation. A similar device was implanted in 1984 at Stanford and again in 1986 at the Texas Heart Institute. Today, these large devices require an external connection to a battery pack. Despite this inconvenience, these devices have successfully supported many patients who otherwise would have died.
Over time, researchers observed that hearts that were supported by the LVAD and allowed to rest for longer time periods actually recovered some cardiac function; some patients have avoided transplant altogether. In turn, transplant surgeons have come to realize that long-term LVAD support improves the function of the body's other organs, leading to better outcomes when and if patients undergo transplantation. In the future, such devices may be implanted permanently in some patients as an alternative to cardiac transplantation.
Alternative designs for mechanical circulatory devices are also being investigated. These include continuous flow pumps, which are considerably smaller than other assist devices currently in use. There is no need for valves because the blood flow is continuous.
The first temporary continuous flow pump, the Hemopump®, was successfully used in 1988 at the Texas Heart Institute. Continuous flow pumps offer great promise of widespread application, and the first clinical trials of the newest version were scheduled to begin in 1999.
In some cases, the heart is so damaged by disease that adequate support can only be obtained with a total artificial heart. Earlier versions of the total artificial heart included two single pumps, one to replace the right ventricle and one to replace the left ventricle, and the power source was outside the body. The first totally artificial heart was implanted at the Texas Heart Institute in 1969, followed by an-
The Abiomed total artificial heart.
other implantation in 1982. Other implantations of permanent artificial hearts soon followed. These implantations were fraught with complications, and the devices and power consoles were large and cumbersome.
However, continued NHLBI funding from the mid-1980s enabled researchers to develop a smaller implantable device in which electrical power is transmitted across rather than through the skin. These pumps are quieter and appear to be less likely to create the blood clots that plagued recipients of the earlier devices. Because this newly developed totally artificial heart can be nearly, if not completely, implanted under the skin, the risk of infection should be lower. These artificial hearts are currently being tested at the Texas Heart Institute and at Penn State, Hershey. They are almost ready for initial clinical trials and should be ready to implant in patients by the early part of the twenty-first century.
Many advances have been made in the field of mechanical circulatory support, and future generations of these devices hold great promise. Although heart transplantation remains an alternative for a select group of patients, it is currently not available to the vast majority of patients who are dying from heart failure. In the next millennium, a long-term device should be widely available for the treatment of heart disease and terminal heart failure, hopefully sparing many of these patients from an untimely death.
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