This article discusses recent developments in artificial heart devices. The latest artificial heart devices are intended for patients suffering from the most severe form of heart failure, in which the heart is no longer capable of pumping blood at all. Currently in clinical trials, the AbioCor Implantable Heart, from AbioMed of Danvers, is the first completely self-contained artificial heart available. When the first AbioCor was implanted in 2001 at Jewish Hospital in Louisville, the procedure marked the first artificial heart transplant in close to two decades. Unlike the earlier Jarvik-7, the AbioCor heart is fully implantable. The patient is not tethered to controllers, external pumps, or monitors. The AbioCor is a hydraulically driven pump. A gear inside the pump spins at 10,000 revolutions per minute to create pressure. AbioMed expects to file for US Food and Drug Administration approval for a Humanitarian Device Exemption to allow sale of the AbioCor for use in a clearly defined subset of patients who qualify for heart replacement.
The concept of a mechanical heart is nothing new. The first permanent replacement heart, an air-driven device called the Jarvik-7, was developed by Willem Kolff and Don Olsen. Barney Clark, a dentist from Seattle, received the device in a 1982 implantation surgery performed by William Devries at the University of Utah. Clark lived for 112 days before succumbing to complications caused by the machine.
Four more patients received the Jarvik-7, and also suffered complications ranging from stroke to mechanical failure of the device. Some researchers have said that the Jarvik-7 was a success as a proof-of-concept, but a failure as a medical treatment.
Now, more than 20 years after that first attempt at building a replacement heart, research and technology are finally catching up with intent. The latest artificial heart devices are intended for patients suffering from the most severe form of heart failure, in which the heart is no longer capable of pumping blood at all.
Currently in clinical trials, the AbioCor Implantable Heart, from AbioMed of Danvers, Mass., is the first completely self-contained artificial heart available. When the first AbioCor was implanted in 2001 at Jewish Hospital in Louisville, Ky., the procedure marked the first artificial heart transplant in close to two decades.
Unlike the earlier Jarvik-7, the AbioCor heart is fully implantable. The patient is not tethered to controllers, external pumps, or monitors. The AbioCor is a hydraulically driven pump. A gear inside the pump spins at 10,000 revolutions per minute to create pressure, according to Fred Zarinetchi, vice president for research and development at AbioMed. The AbioCor has left and right ventricles that fully replace the ventricles of the natural heart; the device connects to the patient's natural atria.
Each mechanical ventricle is essentially a polyurethane sac with an inflow and outflow valve. The sacs are worked hydraulically: Hydraulic fluid is injected outside the sacs to compress them, and then drawn away to let them expand.
In a natural heart, the right and left atria contract at the same time to pump blood to the ventricle. The ventricles then contract together to propel blood out of the heart. In the AbioCor, the atria still beat at the same time, but the ventricles alternate pumping. So, when the left ventricle is full, the right ventricle is emptying, and vice versa. This allows the device to send blood alternately to the lungs and then to the body, instead of sending it to both at the same time, as a natural heart does. Despite this difference, the AbioCor does maintain a constant blood volume, Zarinetchi said. The device has an implanted active controller that adjusts the flow of one ventricle versus the other to increase blood flow into the lungs, he said.
The device is "about the size of a large grapefruit" and weighs approximately two pounds, according to Zarinetchi. It is capable of delivering 8 to 10 liters of blood per minute. "This is not sufficient for sustaining a patient through vigorous exercise," Zarinetchi said. "But the main issue is that for these patients, cardiac output has deteriorated so greatly that they're not really aware of a limitation. Bring their cardiac output to 8 liters per minute, and they're reborn."
The AbioCor draws operating power from an external rechargeable battery pack. In addition, it has an internal rechargeable battery that is implanted in the patient's abdomen. This gives the patient 30 to 40 minutes to perform activities, such as showering, while disconnected from the main battery pack.
There are also implantable mechanisms that do not replace diseased hearts, but work with them. Known as left ventricular assist devices, or LV ADs for short, they too are reserved as treatment for the very sick to whom no transplant is available. LV ADs were the subject of an article in the June issue ("The Telltale Heart," page 56).
Unlike the current generation of LV ADs, the AbioCor doesn't run wires out through an opening in the patient's body to hook up to the battery pack. Instead, it uses a Transcutaneous Energy Transfer system that doesn't puncture the skin. The system sends electricity from the battery pack to a radio frequency transmitter located on the abdomen; this device sends the energy to a radio frequency converter implanted inside the body. The device receives the energy and sends it to the internal battery and controller device. The TET system is capable of transmitting 40 to 60 watts of power, according to Zarinetchi.
The primary advantage of the system, in Zarinetchi's opinion, is that, because it doesn't pierce the patient's skin, it provides no pathway for infectious microbes to enter the body. "When you implant a cable that runs outside the body, there is an easy pathway for serious, deep infection," he said. " If sepsis occurs, the heart device and cables all have to be removed in order to clear the infection."
Mapping The Flow
Researchers at the university of virginia in Charlottesville are conducting fluid analysis research in an effort to tackle blood compatibility problems, such as thrombosis, that threaten artificial-heart patients.
Thrombosis, the forming of blood clots, isn't bad for the human body in and of itself, according to Amy Throckmorton, a research associate in the university's Artificial Heart Program. The danger occurs when these clots are dislodged from the surface they've formed on and are expelled into a small blood vessel, where they can form a blockage.
Heart program researchers are using a combination of techniques to analyze the blood flow path and fluid shear stress within an implantable circulatory aid called a left ventricle assist device, in the hopes of minimizing flow stagnation. Researchers are using CFD software, mathematical models, particle image velocimetry, and an experimental flow loop system to identify areas of low shear stress. According to Throckmorton, low shear stress is a likely indicator of fluid stagnation within the device. This stagnation permits the physiological conditions required for the formation of potentially dangerous blood clots.
The hope is that eliminating areas of low shear stress from future heart devices will eliminate the need for the use of special coatings in the devices, which prevent clots from adhering to surfaces where they can grow, and more importantly, lessen the amount of anti-thrombolytic medication that artificial-heart recipients must take on a daily basis -Gayle Ehrenman
As with the LVADs, thrombosis is a major concern for AbioMed recipients. Rather than using a textured surface that catches and holds clots, as the HeartMate LV AD does, for instance, the AbioCor controls the problem by using an exceptionally slick surface that clots can't adhere to. In addition to the device's very smooth surface, its flow shear, controlled geometry, and blood wash go a long way toward keeping clots from adhering to the surface of the artificial heart, Zarinetchi said. He added that patients implanted with the AbioCor take amounts of anti-thrombolytic medications similar to what patients with artificial heart valves take on a daily basis.
AbioMed has implanted the device in nine patients as part of a clinical trial that calls for implanting the device in a total of 15 patients. The company expects to complete this round of clinical trials by the end of this year. Two of the original nine recipients of the device are still alive. Tom Christerson, the longest living recipient of the AbioCor, died in February, after being supported by the device for close to 17 months. AbioMed has determined that Christerson's death was due to the wearing out of an internal membrane of the AbioCor.
The company expects to use the information gained from Christerson's experience to refine the design of the artificial heart, and make it more robust. According to Zarinetchi, the company is striving to make the heart me mechanically sound for up to five years, which he says is the current life expectancy of a transplanted natural heart.
"It has taken 20 years for the technology community to get the major issues with blood compatibility under control," Zarinetchi said. "What we're seeing now represents 40 years of research. Now, we need to create devices that are forgettable for the patients. We need to create devices that allow patients to be discharged from the hospital and enjoy substantial quality of life."
AbioMed expects to file for U.S. Food and Drug Administration approval for a Humanitarian Device Exemption to allow sale of the AbioCor for use in a clearly defined subset of patients who qualify for heart replacement. Under U.S. federal rules, an exemption may be granted for a device that has been shown to be safe, which is applicable to treat a defined patient population of fewer than 4,000 per year, for whom no approved alternative devices exist, and for whom the potential benefits outweigh the risks.
The exemption would move the AbioCor device out of" experimental" status and classify it as a commercially available product unlike anything else that exists currently. It would allow AbioMed to make its artificial heart available to more patients, more quickly than is possible through the traditional FDA approval process. And when you're dealing with patients who need a new heart, every minute counts.