heart, artificial Since the 1960s there have been many attempts to develop implantable pumps to replace the function of the heart. These were initially evaluated in animals. Only in the past few years have the newer designs, refined in the light of experimental findings in animal trials, been used with reasonable success in humans. Improved materials as well as advances in electronics and mechanical engineering have also played a major part in making the artificial heart sufficiently safe and effective to allow limited clinical application.
In principle, the device consists of a rigid chamber, made of an inert material such as titanium, usually of hemispherical shape and about 7–8 cm in diameter, within which there is a moving polyurethane diaphragm which evacuates the contained blood. An inlet and an outlet valve ensure flow in one direction. Early models used an external pump to pneumatically displace the moving diaphragm. Recent designs have miniaturized electrical motors activating a pusher plate within the device, but connected to externally carried batteries by wire, or by a transcutaneous electrical energy transfer system. As the devices are not linked to any of the normal influences in the body which naturally control the output of the heart, there have to be control systems which modify the artificial pump's output and regulate the pressure of the blood flowing into the device.
Most causes of
heart failure, for which use of an artificial device might be contemplated, affect the left ventricular pumping chamber. It is therefore possible to use a mechanical pump which takes its input of blood from the diseased left ventricle and returns the blood at appropriate pressure to the aorta — thus acting as a
left ventricular assist device. It is this form of device which is presently showing most clinical success and has widest application.
For patients with both left and right ventricular failure, devices are available which have two parallel pumping chambers. This device is a true ‘artificial heart’, and is a mechanical alternative to a heart transplant.
The problems associated with artificial devices used to replace the heart are considerable. Clotting of blood within the device is a risk. Clots can immobilize the artificial valves and interfere with the pump itself, or can detach from the device to travel in the bloodstream. This results in clinical effects which depend on where the clot goes. If a clot enters the circulation of the brain the result is often a stroke.
Anticoagulant drugs are required to minimize this risk, and anticoagulation itself carries risks of bleeding. Also, there is the risk of infection developing in the device; mechanical devices are liable to damage the blood, causing rupture of red cells and a risk of kidney damage due to the released haemoglobin from the red cells; and there is a need for regular changes of battery power source.
At present left heart assist devices will allow relatively normal life for many months, reversing many of the adverse effects on the body of long-standing heart failure. Most clinical use has been as a ‘bridge to transplant’, enabling ill patients to survive until a suitable heart becomes available for
transplantation. Occasionally, use of a left heart assist device has been temporary, where the heart has been affected by a condition which is recoverable.
At present, the technology of artificial hearts is advancing rapidly, but the devices currently in use are not as satisfactory as the transplanted human heart.
D. J. Wheatley
See also
heart failure;
prostheses.
