Innovative Defibrillator Alternative for Regulating Heart Rhythms

October 4, 2012

On July 26 this past summer, two cardiologists at the University of Ottawa Heart Institute implanted a defibrillator. Nothing out of the ordinary, you would think. It is something they do almost daily to provide lifesaving therapy for patients at risk of sudden cardiac death.

But this was out of the ordinary because it was not a standard defibrillator. The device the cardiologists implanted is so new, it has not yet been approved in Canada. It was just the third time one had been implanted in this country, under Health Canada’s special access program. And it marks a major difference in how defibrillators are designed and used.

Conventional defibrillators, known as transvenous defibrillators, are implanted with wires, called leads, that snake through veins into the heart. When the defibrillator identifies any dangerous heartbeat, it delivers a shock through the leads to return the heart to its normal rhythm and allow it to get back to pumping blood steadily throughout the body. These defibrillators have been in use for decades and have prolonged the lives of hundreds of thousands of people.

“It’s a very well-established therapy,” said Dr. Pablo Nery, an electrophysiologist at the Heart Institute who, along with colleague Dr. David Birnie, implanted the new device. “But it’s not always feasible to implant a transvenous device.”

Not all patients are suitable for a conventional defibrillator. In some with congenital heart problems, there is no way to advance the leads into the heart through the veins. Also, the leads may pose a danger due to the risk of blood clots or infection. These patients often have to have a more complex and invasive surgery to attach leads to the outer layer of the heart muscle in order for them to benefit from the use of a defibrillator.

What makes the new device special is that it is entirely subcutaneous. No part of it actually touches the heart. Instead, an electrode is implanted just under the skin near the heart. The defibrillator is connected to the electrode and monitors the heartbeat at all times. If needed, it delivers a shock to the heart muscle to restore its normal rhythm. Developed by Cameron Health, the S-ICD, as it is known (for subcutaneous implantable cardiac defibrillator), is approved for use in Europe and is currently being reviewed in both the United States and Canada.

Conventional ICDs use leads that run from the device through major veins to an anchor point in the heart. These transvenous leads can cause problems in the long term. Despite decades of design improvements, leads can malfunction, break or stop working. Known as lead failure, this results in either inappropriate shocks or lack of proper regulation of the heartbeat. What is  worse is that failed leads often must be removed, which poses serious risks to the patient. Risks that Dr. Birnie, for one, is well-acquainted with. He has participated heavily in research on devices with leads that are prone to breakage.

The goal of the subcutaneous ICD is to reduce or eliminate these problems.  “The subcutaneous ICD provides effective therapy for patients who are either not eligible for or are at high risk with a traditional  ICD. Such patients may now be able to receive protection from a subcutaneous ICD without the risks associated with the standard leads,” explained Dr. Nery.

That made the 18-year-old recipient of the S-ICD at the Heart Institute a perfect candidate. “The S-ICD offers advantages for particular patient subgroups,” said Dr. Nery. “This technology is now an alternative for young patients, in part because lead extraction can be avoided down the road.” Another advantage is more aesthetic in nature but nonetheless important for young people. A conventional transvenous defibrillator sits on the front of the chest, just under the collarbone, and is easy to see. The S-ICD, in comparison, is implanted to the side, under the patient’s arm, and with a much smaller incision than with the transvenous defibrillator. That, said Dr. Nery, is an important consideration for many patients in terms of body image and quality of life.

There are some limitations to the new device. For one, it does not act as a pacemaker, the way a transvenous defibrillator does. So while the S-ICD is effective at dealing with a racing heart—the most common cause of sudden cardiac death—it cannot deal with a slowing heartbeat. That is not necessarily a problem, said Dr. Nery, but it does affect who is selected for each therapy and is one of the key reasons that the S-ICD will not replace, only supplement, the transvenous defibrillator.

As for the 18-year-old who received the S-ICD, he had a congenital problem, a hole in his heart. Drs. Nery and Birnie saw him recently for a follow-up appointment. He is doing very well and is very happy with his recovery.

While the doctors have yet to identify another patient who would benefit from the S-ICD,  they are ready to use it again when the opportunity arises.