The Beauty of Subcutaneous ICDs Is Not Merely Skin Deep

Three decades and 1 million-plus implantations after its introduction, the transvenous implantable cardioverter-defibrillator (T-ICD) is facing stiff competition from a smaller, thinner, more versatile and longer lasting addition to the family tree of implantable devices that shock the heart to prevent sudden cardiac death. The new kid on the branch is the subcutaneous implantable cardioverter-defibrillator (S-ICD). After proving safe, efficacious and not prone to complications in clinical trials, and so earning first FDA approval in 2012, wireless pacemakers today are rising in popularity as the right choice for many patients and preferable to many of the physicians who treat them.

Apart from its diminutive, less intrusive physical profile, one of the S-ICD’s main points of appeal is its placement. While the transvenous implantation requires leads inserted through veins and into hearts, with all the risks such a high level of invasiveness can bring—not to mention the literal beating that leads take inside the heart—the S-ICD uses a lead that does its job under the skin, along the bottom of the ribcage and breastbone. Wireless leaves the heart and vasculature untouched.

More specifically, the S-ICD delivers electric shocks, as needed, by way of a pulse generator implanted in the left lateral chest in combination with a lead-electrode “tunneled” 1 to 2 centimeters to the left of the mid-sternal line. The main rap against it so far is that, unlike the T-ICD, it doesn’t regulate pacing for either fast-heartbeat arrhythmias (tachycardia) or slow ones (bradycardia). But a quick review of the literature shows that, even there, it may come out a winner: Conversations are underway to consider outfitting the S-ICD with Bluetooth radio-wave technology for various pacing purposes.

Meanwhile, newly pooled data on the smaller device point to low mortality, a low or flat complication rate over time and no evidence of problematic implantations, says Martin Burke, DO, director of the Heart Rhythm Center at the University of Chicago. Pointing to an analysis he led of two-year results gleaned from two large prospective studies, publishing the results in the Journal of the American College of Cardiology this past spring, Burke told Cardiovascular Business that the device’s rate of spontaneous event conversion (from arrhythmic to a normal heart rhythm) is “outstanding. It’s comparable to a transvenous system,” which successfully positions it for “non-niche patients at primary prevention or secondary prevention who have no pacing indication. The process of selection now becomes a personal one between physician and patient, with multiple options to choose from.”

Burke and colleagues’ pooled-data analysis (J Am Coll Cardiol 2015;65:1605-15) showed termination of 90.1 percent of ventricular tachyarrhythmia and ventricular fibrillation events after one shock, with some 98.2 percent halted after up to five shocks from the S-ICD.

Numbers like those, coupled with similarly encouraging data on safety and complications—on top of longer battery life and remote follow-up capabilities—are “going to let these devices rule the day for just about anybody who’s at risk for sudden cardiac death and who doesn’t have any pacing indications,” says Burke, who consults for Boston Scientific. (The company’s Emblem S-ICD has CE Mark approval for sales in Europe, and the FDA greenlighted a second-generation version earlier this year.)

“The other thing is, with [the S-ICD’s] longer detection times, we’re seeing a low need for shocking in general,” adds Burke. “We’re seeing the device actually not go off, because the charge time takes longer to occur so that the event spontaneously terminates. We’ve seen this now in greater than 50% of all the spontaneous events.”

Nuanced comparisons 

Among the patients who might not be good candidates for S-ICD implantation are those who need pacing for cardiac resynchronization therapy (CRT). In commentary accompanying Burke et al.’s JACC investigation report, Anne Curtis, MD, a cardiologist and distinguished professor specializing in cardiac electrophysiology at the University of Buffalo, pointed this out by way of noting the S-ICD can be appropriately implanted as a second device for patients who have endocardial pacemakers already in place.

Curtis says she believes S-ICD is breakthrough technology even if it’s only so because of its newness as an innovation. “On the other hand, it’s not going to assume the majority of the ICD market,” she says. “I don’t see that happening simply because there are other reasons for pacing that do need other kinds of available systems.”

She says physicians and patients will need to carefully consider other options in many if not most cases in which an S-ICD is in the running.

“Atrial fibrillation is the most common sustained arrhythmia in the general public,” Curtis says. “When a patient goes into atrial fibrillation and the rhythm goes very fast, sometimes the ICD will deliver a shock in a situation that is not life-threatening. When you look at a single-chamber ICD versus a subcutaneous ICD, they are probably going to be reasonably equivalent in their ability to tell the difference.” However, sometimes a dual-chamber ICD or CRT device would be “a little bit better at telling the difference between these arrhythmias, compared to a single-chamber device or a subcutaneous ICD.”

The call is necessarily nuanced, says Curtis. “You add everything up and, if the patient seems appropriate for the ‘subQ’ ICD, it can be a great way to go for a lot of patients.”

Patients already voting with their feet

Burke concurs with the call for clinical circumspection. “There will be case reports coming down the pike on patients who have ventricular tachycardia below 170 beats per minute, and what do you do with that patient? You’re probably going to go with a transvenous device, because the S-ICD will not treat a tachycardia arrhythmia below 170 bpm,” he says.

Still, allowing for these and other patients with characteristics that automatically rule out the S-ICD, “there is a vast middle ground of patients at risk who have never had an event and are pretty much waiting to be saved by a device,” says Burke.

Their number includes patients for whom the option of a heart-saving device that doesn’t touch the heart, much less reside inside it, is a “no-brainer.” Burke says he has been observing this firsthand while trying to enroll patients in the randomized Praetorian trial comparing the S-ICD with a single-chamber transvenous device. “When presented with both options, the patients don’t even want to be part of Praetorian. They just want to get an S-ICD.”

When there is bias against the S-ICD, it’s generally the physician who holds it, continues Burke. “As a doctor, you attach yourself to things you know,” he says. “The newness of this particular device and the difference in the implant platform [represent] workflow issues. This tends to be a bigger inhibiting factor than anything brought up by the patient.”

“I have faith in electrophysiologists,” says Burke. “Most of them are early adopters in general, but I think they were a little bit careful with this in the beginning—as they should be.” As the documented data on S-ICD start to play out in real-world clinical practice, Burke makes a prediction: “I expect the case will be very compelling.”

Leadless Pacemakers Keeping Pace With S-ICDs

Subcutaneous ICDs aren’t the only smaller profile, less intrusive heart helpers opening new lanes for treatment of arrhythmia patients. A miniature, leadless pacemaker seems to be making its way toward FDA approval faster than expected.

In April, Medtronic’s Micra TPS won CE Mark. In May, lead investigators in the device’s U.S. multicenter trial announced that they had successfully implanted it in 100 percent of the first 140 patients who received it, with mean electrical pacing measurements at all patient visits falling within expected ranges—and with no unforeseen events, dislodgements, infections or serious issues of any kind.

Perhaps most tellingly, enrollment for the next phase of the study completed nearly half a year ahead of schedule. This “proves that patients are enthusiastic because they accept it very well,” said global and European trial Co-chair Philippe Ritter, MD, of University Hospital of Bordeaux.

Ritter’s U.S. Co-chair, Dwight Reynolds, MD, of the University of Oklahoma, put the progress in the perspective of his 35-year cardiology career for Cardiovascular Business. 

“Double- and triple-chamber pacing has been one major development. Defibrillators have been another. And this is the third sentinel development in implantable technologies,” says Reynolds. “If you couple this with the new subcutaneous ICDs that also don’t require intravascular leads—that combination, the ability to avoid indwelling venous catheters—this is really a home run for the implantable electronic device industry.”

Medtronic will probably face competition right out of the gate. Boston Scientific and St. Jude Medical also have devices in the works, and others are expected to jump in as well.

“We all have to continue to be meticulous in our analysis of the data,” says Reynolds. “We all have a responsibility to make sure that everything is fully vetted and fully disclosed, because there are certainly examples in industry of initial exuberance not being warranted in the long run. But I think it would also be fair to say that we are not any less optimistic at this point in our pursuit of these devices than we were at the very beginning, when we had no information about how they were going to work.”