The Future in Artificial Heart Technology

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Cardiothoracic Surgeon

Oxford Heart Centre John Radcliffe Hospital Oxford University Oxford, England

CURRENTLY, THERE ARE more than three million heart failure patients in the United States, with more than four-hundred thousand new cases every year. Treatment of heart attack has come a long way. Today, physicians are able to use clot-busting drugs and catheters to save thousands of lives. There is, however, an unfortunate consequence of this rapid advance. Some of these people who are saved, particularly patients with coronary artery disease, develop heart failure.

Because transplantation is limited, physicians have turned to mechanical hearts, a concept that has captured the imagination of cardiac surgeons, the public, and the media alike. Although there are no fully implantable mechanical hearts available, physicians do have mechanical treatment options for end-stage heart disease.

In fact, within the next ten years, a miniaturized blood pump is destined to become the treatment of choice to relieve symp-

^ i a toms and prolong life in older heart failure patients. Physicians have recently discovered that "resting" the heart with such a blood pump may promote recovery in some patients. This raises the possibility that circulatory support can be used as a therapy in conjunction with other treatments.

Left Ventricular Assist Devices

The totally artificial heart, which received much publicity in the 1970s and 1980s, was conceptually flawed. These unwieldy devices were acceptable as a bridge to transplantation (when meant to keep patients alive until they could receive a heart transplant) but were never a long-term solution. To be successful, an artificial heart must be more than a reliable blood pump; it must be forgettable. That was something the big artificial hearts could never be.

In recent years, however, physicians have realized that whole-heart replacement may not be necessary. After all, more than 90 percent of heart failure patients can be sustained with left ventricular support alone.

Currently, left ventricular assist devices are used mainly as a bridge to transplantation. The modern left ventricular assist device consists of a blood sac that is compressed by a pusherplate mechanism that is either electrically or air driven. Artificial heart valves direct the blood flow. This system mimics the human left ventricle by providing pulsatile blood flow while taking the burden of pumping off the patient's heart. This is a workable solu tion and saves many lives, but the serious problems of pump size, noise, driveline infection, and stroke remain. Nevertheless, some patients have had these devices in place for up to four years and enjoyed an acceptable quality of life. This has encouraged us to use the left ventricular assist device for long-term support in patients who are not eligible for transplantation.

The Next Generation of Assist Devices

The ideal treatment for chronic severe heart failure must be reliable, cost effective, easy to manage at home, and capable of providing adequate circulation. Keeping these goals in sight, another generation of heart assist devices called axial flow impeller pumps is on the horizon. These are compact, silent, nonpulsing blood pumps that provide up to eight quarts of flow per minute.

Among these is the thumb-sized Jarvik-2000, which fits within the failing left ventricle and pumps blood to the aorta. The impeller revolves at up to eighteen thousand rpm, moving blood so rapidly that the red cells remain undamaged. The controller and batteries are the size of a portable telephone and fit easily onto a normal belt.

Other ingenious blood pumps are under development, including some with magnetically suspended rotors. These fully implantable, miniature artificial hearts will greatly increase our ability to treat heart failure, although we do not know their reliability and complication rate.

A recent revelation has been the effect of chronic rest on the failing left ventricle. For years, we have known that prolonged bed-rest, which reduces heart function, results in improvement. However, the benefits are limited by the negative effects of inactivity on the limb muscles, blood vessel tone, and nervous system.

Ideally, patients would be able to exercise their bodies while their hearts rested. This is now possible with long-term implantable blood pumps. When we compare the heart muscle at the time of blood pump insertion to the muscle during transplantation, there is often a shift in heart muscle cells towards normal, both in shape and function. This discovery that recovering hearts were being removed at transplantation, coupled with the shortage in donors, led to the use of blood pumps to induce heart recovery.

Although the benefits of this therapy are obvious, there are certain requirements that must be met before this strategy has a chance for success. The first is a user-friendly blood pump for patients of all sizes. This must be simple to implant and remove, without the risk of infection, and easy to control. Second, it must be implanted before the heart degenerates to a point at which the heart failure cannot be reversed.

In our limited experience with this approach, certain factors are apparent that separate those with sustainable heart recovery from others who will slip back into heart failure. Patients with long-lasting recovery tend to be younger, have a shorter history of heart failure, show a more rapid improvement in heart performance, and require a shorter period of blood pump support.

The type of heart disease is also important to recovery. Coronary artery disease patients with large areas of dead muscle will not recover. However, young patients with viral infections involving the heart can often be supported with a blood pump until the inflammatory process completely resolves. Even those who require external cardiac massage and a heart-lung machine during blood pump insertion can be restored to nearly normal cardiac function.

A Look at the Future

The scope of mechanical heart failure therapy is developing rapidly as new blood pumps emerge from the bioengineering laboratories. These will eventually be used as often for heart failure as the pacemaker is for rhythm problems. The major issues are not ethical but economic. In the further future, new drugs, gene therapy, and tissue engineering with the patient's own heart muscle cells will be used to promote recovery of the patient's heart in conjunction with periods of mechanical circulatory support.

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