February 3, 2012 (Fort Lauderdale, Florida) — Engineers have been trying to create a completely implantable ventricular assist device (VAD) that has no cords or vents transecting the skin since the earliest days of VAD and artificial-heart development. Now that vision appears more realistic than ever before because of the new generation of wireless power-transfer technologies.
Currently, infections are common in VAD patients because the device is powered by an external battery through a cable that runs through the skin. "Drive-line infection is the number-one problem with these devices, and I think it's a bigger problem than people realize," Dr William Cohn (Texas Heart Institute, Houston) told heartwire following a presentation on the new generation of transcutaneous energy-transfer systems at the Society of Thoracic Surgeons (STS) 2012 Annual Meeting .
"There are people who have had pumps for seven years with no problem, but by and large, if you've had a pump for a year, there's a 70% chance you'll get an infection," and the drive line transecting the skin is the major source of those infections, he said. Cohn also believes that a lot of the embolic events and strokes suffered by VAD patients are likely related to these infections.
Drive-line infection is the number-one problem with these devices, and I think it's a bigger problem than people realize.
Eliminating this source of infection with VADs powered transcutaneously without wires running through an open wound will make the devices far safer. Safer devices would shift the cost/benefit balance in favor of implanting them earlier in the progression of heart failure, at the stage when they could stop or even reverse its progress. Currently, the devices are restricted to very sick patients with no other options because the mortality and hospitalization rates associated with them are still high, especially in destination-therapy patients. "Of course, these patients stay in the hospital a long time--they're almost dead when we get to them," Cohn said.
But with the ongoing development of smaller continuous-flow VADs that can be implanted with a minimally invasive procedure-–or perhaps someday a catheter intervention--VAD implants could someday be as safe and routine as implanting a pacemaker or ICD, he predicts. "Once we get rid of the drive line, the pumps will get better, the pumps will be put in a little earlier, and it may change the whole dynamics of the field," Cohn said.
Dr Francis Pagani (University of Michigan, Ann Arbor) agrees with Cohn that wireless power delivery to LVADs could dramatically change mechanical circulatory-support therapy. "If the wireless system of energy transfer were to live up to its potential, it's a game changer in the field," he told heartwire .
"It would be the single biggest innovation in the field since the development of continuous-flow pumps, which brought LVAD therapy to a different level. Its impact would largely be the improvement in quality of life--greater freedom and mobility, greater patient acceptance, and reduction in infection."
However, Pagani is not convinced that eliminating the drive line will reduce stroke. "Stroke is likely not going to be changed by this technology; that is something inherent in the pump design and patient factors," he said. "Effects on stroke and survival would need further study."
The Magic of Resonant Coupling
The potential advantages of a self-contained mechanical circulatory-support device were foreseen even before ventricular assist devices or artificial hearts became a reality. Cohn noted that in the 1960s and 1970s there were even attempts to create devices that ran on nuclear power, but those efforts were abandoned because the obvious safety issues could not be overcome.
In the late 1990s and early 2000s, Arrow developed the LionHeart totally implantable LVAD but eventually abandoned the project before earning regulatory approval. Cohn explained that the transcutaneous power-transfer system in the LionHeart was very sensitive to changes in the positioning of coils on either side of the skin. When they were not aligned perfectly, they would create heat that could burn the skin.
The LionHeart also used an internal gas compliance chamber to adjust for volume fluctuations with beats of the pump instead of an external vent. Cohn said that these compliance chambers have a tendency to become stiff over time, as scar tissue accumulates around them. Therefore, the external vent was a better option for regulating volume, and since the vent was already transecting the skin, a power and control drive line did not significantly increase the marginal risks of infection, so interest in transcutaneous power delivery waned.
But the newer continuous-flow VADs, such as Thoratec's HeartMate II, do not need a vent or a compliance chamber, and a safe and effective transcutaneous power and control system would allow the VADs to eliminate the drive line and be completely implantable.
WiTricity, a Watertown, MA–based company spun off from MIT, believes it has developed a technology that could make this type of VAD possible. In May of last year, Thoratec and WiTricity announced a deal under which Thoratec will support the development of the Fully Implantable Ventricular Assist System (FILVAS) based on the HeartMate II. The agreement also provides Thoratec with an option to license the FILVAS technology for incorporation into future VADs, including HeartMate III and HeartMate X .
WiTricity's system is based on resonant magnetic coupling, whereby the oscillating magnetic fields exchange energy across space with minimal power loss when the resonant frequencies of two coils are aligned. The power transfer depends on the alignment of the resonant frequency of the fields, not the physical proximity or alignment of the two coils. Therefore, a battery connected to a coil on the outside of the body could deliver consistent energy to the coil connected to the device implanted on the inside of the body without ever overheating, even if the coils' relative position to one another shifts continuously as the patient moves. WiTricity's technology is "a completely different ballgame" than the transcutaneous power-delivery system Arrow tried with LionHeart, Cohn said.
In fact, a resonant frequency system can deliver energy with only 15% power loss between coils set up to 2 m apart, so WiTricity, among other companies, is developing industrial and home applications for wireless power delivery.
While the prospect of eliminating all of those cords under one's desk is very attractive to a lot of people, Cohn insists that there's no more important application of the technology than VADs. "The value proposition of those other applications isn't as great," he said. "Those are nice to have, but this is a must have. Patients die from complications from the [VAD] power line."
A VAD is a big thing in the vascular space with a superfreeway for bacteria to get to it.
Cohn and VAD pioneer Dr OH "Bud" Frazier (Texas Heart Institute) are developing a pulseless totally artificial heart that is essentially two continuous-flow VADs working together. So far, they've successfully implanted it in almost 40 calves and one human patient who was dying of amyloidosis. He survived with it for about a month before eventually succumbing to complications of the amyloidosis.
Cohn believes these types of devices may be the future of cardiac-transplant therapy, "but none of this stuff can take off until we get rid of the drive line.
"There's no other implant that's open to the outside of the body. [A VAD] is a big thing in the vascular space with a superfreeway for bacteria to get to it," Cohn said. "That's horrible and was never part of the [original] plan."
Cohn is a consultant for HeartWare International. Pagani has research grants from HeartWare and the National Heart, Lung, and Blood Institute.
Heartwire from Medscape © 2012 Medscape, LLC
Cite this: 'Tetherless' Power-Transmission Technology Could Radically Change LVAD Therapy - Medscape - Feb 03, 2012.