Earlier this month, the first fully implantable artificial heart was approved by the Food and Drug Administration. It brings hope to patients who are near death from heart failure; yet some major problems remain with it–namely, its large size and relatively short lifespan.
A new concept for an artificial heart could solve some of those issues. But its innovative pulse-free architecture might also raise problems of its own.
Artificial hearts work by pumping deoxygenated blood from the body to the lungs. The device then pumps oxygenated blood through the body. The newly approved device, called AbioCor, made by Massachusetts-based Abiomed, uses an implanted hydraulic pumping system to simulate a natural heart beat. But an alternative design, conceived by O.H. “Bud” Frazier, a prominent heart surgeon and pioneer in the development of cardiac devices at the Texas Heart Institute in Houston, pumps blood through the body continuously, rather than with the periodic beat of the normal heart.
Pumps that work on this principle, known as continuous flow pumps, are already in clinical use as part of “ventricular assist devices,” which are implanted into patients with some remaining heart function to help circulate blood through the body. (Artificial hearts replace the heart entirely.)
“Continuous flow pumps are like little turbo machines,” says Tim Baldwin, program director of the advanced technologies and surgery branch of the National Heart, Lung and Blood Institute in Bethesda MD. “They are more durable and allow you to make smaller devices.”
With Frazier’s continuous flow design for an entirely artificial heart, a severely damaged heart is removed and replaced with two rotor-based pumps that continually cycle blood through the body, completely taking over the function of the heart.
In preliminary experiments conducted over the last two years, Frazier and his team have implanted pairs of commercially available ventricular-assist devices into calves that had their hearts removed. The researchers say the devices were able to pump blood and respond to the animals’ needs based on their activities. “You put this in cattle and they stand up and moo and eat and wonder why everyone is looking at them so weird,” says William Cohn, a collaborator on the research and director of minimally invasive surgical technology at the Texas Heart Institute. “You see a cow wagging his tail and you say, wow, this is the future of the artificial heart.”
The biggest advantage to the rotor-style or axial pumps is that they are small and relatively simple. The AbioCor heart, for example, is so large that it can only be implanted in people with large chest cavities, making it inappropriate for most women. “Axial pumps are about the size of an adult thumb and can pump more blood than a normal heart,” says Frazier.
Continuous flow pumps are also more durable, due to the simplicity of their design–the only moving part is the rotor. “Other pumps work well, but there are lots of moving parts so they are subject to mechanical wear,” says Cohn. The longest the AbioCor heart functioned in clinical trials was 18 months, while continuous flow devices are being designed to operate for 10 or more years.
Frazier also says continuous flow pumps are better able to respond to the body’s changing needs for blood. “If you’re walking, more blood is pumped back to the heart and the heart will automatically pump more,” he says. If pressure on one side of the pump increases, flow through the device automatically increases, allowing the pump to respond like a native heart, Frazier says.
But what about the long-term impact of living pulse-free? That question is a matter of lively debate in the cardiac device community. Akif Undar, a clinician and cardiac researcher at Penn State University, says pulse is important to get blood to all the small capillaries feeding the organs. “I think you would see organ damage in animals given a [non-pulsing] heart,” he says. Others, like Yukihiko Nose of the Baylor College of Medicine, say that animal studies conducted by his group show that continuous flow devices can be as safe as devices that use a pump with a pulse.
Frazier and team aim to answer this question more definitively with long-term animal experiments, pending funding for the project. (The longest experiments the researchers have carried out so far lasted 20 days.) The team is also designing specialized pumps tailored for use as artificial hearts, with rotors that respond more efficiently to changes in flow. “A lot of work needs to be done before this can even be considered for clinical application,” cautions the NHLBI’s Baldwin.
It’s not yet clear who would be the prime candidates for the pulseless heart, should it prove safe and effective in animal studies. Because artificial heart technology is still so risky, the current FDA approval for the AbioCor heart limits the device to patients with heart failure who are not eligible for transplant and would likely die within a month. Cohn hopes that in the future, artificial heart technology will become much safer and easier to use, broadening the potential pool of patients. “It wouldn’t surprise me if at the 2050 Olympics, there were standard and modified [competitor] divisions,” he says.
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