The amount of blood circulating in the left ventricle/body loop must be very close to that in the right ventricle/lung loop. With 100,000 beats a day, even a teaspoon of difference at each beat would add up to 500 litres (110 gallons) of blood in the wrong place. The heart has evolved complex biological mechanisms to make sure this does not happen, but the engineers were having huge battles to try to do the same with feedback systems. For VADs, either the right (or more usually) the left ventricle can be independently supported, taking this problem away.
Left ventricular assist devices, or LVADs, have produced a revolution in care for end-stage heart failure. More than 15,000 LVADs have now been implanted worldwide, and around a third of patients with end-stage heart failure are now supported on LVADs. The intention is usually to bridge the patients to transplant, but in fact the shortage of donor hearts means that patients can often stay on LVAD support for years. Survival rates of over 50% are seen at seven years, and there are reports of patients living up to 13 years on these devices. LVADs have therefore become by default a therapy in themselves. Again, technology has progressed, with newer LVADs performing better.
A breakthrough idea was to stop imitating the heart, with its pulsing action, and move to constant flow of blood. Rotating paddles (impellers) push the blood along in a continuous motion, creating a smooth unbroken stream. This has the curious side effect of creating a patient without a pulse, which can be disconcerting for the unsuspecting physician as well as producing some unwanted side effects as the body adapts to the new flow.
External battery packs are still an inconvenience and a source of infection, but systems are being developed that transfer energy transcutaneously (across the skin) based on induction (like domestic induction stoves). The LVAD units would still need a small, implanted battery in case of a temporary device failure – and it has been known for external battery packs to be snatched from patients by handbag thieves.
The search for a completely implantable total artificial heart continues. Trying to develop external transcutaneous units to fully power the demands of the heart is the biggest barrier. Specifications for a total artificial heart require it to pump eight litres (14 pints) per minute of blood against a blood pressure of 110mmHg (millimetres of mercury). (The biological power storage molecule adenosine triphosphate [ATP] would be needed in quantities greater than half your body weight per day to power your own heart to do that, if ATP were not continually renewed in cells).
Compressors have been miniaturised to be more portable, but it has been a struggle to make them completely implantable. Here it seems that the VAD technology may hold a solution, dispensing with compressors altogether and using instead the impeller devices, with dual right and left VAD working together.
Solutions seem tantalisingly close, but no one is anticipating an easy ride. The many failures over the years have certainly produced in scientists a humility and awe for the natural engineering of the heart.
* This article originally appeared in The MIT Press Reader, and is republished with permission.
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