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.