Totally Subcutaneous Defibrillators: Will ‘SubQ’ be ‘Subpar’?

Dear Readers, The implantable cardiac defibrillator (ICD) is one of the most important advances in the field of cardiology over the past several years. With the development of transvenous defibrillator leads, biphasic waveforms, active cans, and device miniaturization, implantation of a defibrillator has become a relatively quick, low risk procedure in the hands of experienced implanters. Despite these advances, modern implantable defibrillator therapy still requires placement of at least one transvenous lead, unless hardware is surgically placed in the epicardial space. Unfortunately, these leads are the weakest link in a defibrillator system. Not only is most of the morbidity associated with device implantation related to lead placement, most of the long-term problems are also related to the lead. According to the national defibrillator database, the risk of a pneumothorax or hemothorax is 0.57%, the risk of perforation is 0.08%, and the risk of lead dislodgement is 1.07%.1 Furthermore, after implantation, transvenous leads can fail due to an insulation breach or a conductor fracture, leading to multiple shocks and hospitalizations. The recent recall of the Medtronic Fidelis defibrillator lead underscores the challenges related to designing and producing a highly reliable transvenous defibrillator lead. Those involved in transvenous lead extractions are especially aware of the morbidity associated with placement of a foreign body in the vascular space, especially in young patients. These issues beg for the development of a defibrillator system that does not require a transvenous lead. There has been progress toward the development of a totally subcutaneous (subQ) defibrillator system. Cameron Health, Inc. has developed a subQ defibrillator that is already commercially available in some countries around the world, and plans to begin a pivotal human trial in the Unites States soon. Such a device could theoretically have a big impact by reducing the morbidity associated with currently available transvenous devices. However, there are skeptics. There are several arguments against a defibrillator that does not have a lead within the heart: the inability of such a device to provide antibradycardia pacing, inability to deliver cardiac resynchronization therapy (CRT), inability to deliver antitachycardia pacing (ATP), and the potential for inappropriate shocks caused by noise detected in the subQ space. However, none of these potential strikes against the concept offer a strong enough case against further efforts to pursue a subQ defibrillator. Let’s first look at the issue of pacing. It is reasonable to predict that patients who require frequent or dual-chamber pacing, or patients who might benefit from CRT will not be optimal candidates for a totally subQ device. So the next question then is: what proportion of ICD recipients has no pacing indication? One way to address that question is to look at the proportion of implanted devices which are single chamber. A recent review of the national ICD registry was published in September 20091 and included a summary of 339,000 device implants. Single-chamber ICDs accounted for only 22% of ICDs. The remaining devices were divided equally between dual-chamber and biventricular devices. This would suggest that a minority of defibrillator recipients would be good candidates for a subQ device. However, it is likely that some dual-chamber devices were implanted for purposes of improving rhythm discrimination rather than for AV sequential pacing. It is also likely that some of the biventricular devices were implanted in patients who had a narrow QRS complex before there was sufficient evidence against the use of CRT in that patient subgroup. What about the argument that a totally subQ device will not be able to deliver ATP? There is a lot of evidence to support the use of ATP to minimize the number of shocks patients receive for ventricular arrhythmias. However, there are many patients who never receive any therapy from a primary prevention ICD, and there are many sequences of ATP that are delivered for a ventricular tachycardia that would have terminated spontaneously. Probably the strongest case against a totally subQ ICD is the problem related to sensing a ventricular rhythm from the subQ space. Because it appears that the heart can be defibrillated without transvenous or epicardial hardware using relatively low energy, the challenge of sensing will be the true test of a totally subQ system. Although it is probable that a sensing algorithm can be developed that successfully uses signals from subQ vectors to detect ventricular arrhythmias, distinguish them from supraventricular rhythms, and rule out myopotentials, the small amplitude of the cardiac signal in the subQ space will make this a challenge. The fact that the Cameron Health device has been implanted in humans outside the United States and is already commercially available in some countries provides some encouragement that this challenge is surmountable. Many may argue that a totally subQ ICD will have few applications and will be prone to spurious shocks from myopotential oversensing. However, there are many potential advantages of a subQ device compared to transvenous systems. In addition to addressing the limitations of transvenous defibrillator leads, a totally subQ system that does not require fluoroscopy to implant could also revolutionize the implant technique and venue. The ability to place a defibrillator outside of an electrophysiology or catheterization lab opens many opportunities for costs savings in the developed world and opens many opportunities for dissemination of life-saving defibrillator therapy in developing world. Disclosure: Dr. Knight has served as a consultant to Cameron Health, Inc. Sincerely, Bradley P. Knight, MD, FACC, FHRS Editor-in-Chief, EP Lab Digest