Are MRI-Safe Pacemakers and Leads on the Horizon? Q & A with J. Rod Gimbel, MD

Tell us about the Medtronic EnRhythm MRI SureScan pacing system. How does it work? The EnRhythm MRI SureScan pacing system consists of the dual-chamber EnRhythm MRI SureScan pacemaker and CapSureFix MRI SureScan pacing leads. It is designed for safe use under specified scanning conditions in MRI machines with a magnetic field strength of 1.5 tesla (T). A tesla is the unit used to describe the strength of a magnetic field. A typical magnet that we place over a pacemaker or ICD is about 10 Gauss. One tesla is 10,000 Gauss. Along with the strong static magnetic field in the MRI, there is also a powerful gradient magnetic field and radiofrequency field being applied. The MRI presents a hardy challenge to the pacing system in terms of electromagnetic interference. Because of all the electromagnetic interference in the MRI suite, traditional pacemakers may misinterpret MRI-generated electrical noise and withhold pacing therapy or deliver unnecessary pacing therapy. Therefore, this investigational system includes a new feature that Medtronic calls SureScan. When programmed on before an MRI scan, this temporarily alters the device s data collection and monitoring functions, while still providing pacing if needed. How does this pacemaker differ from currently available pacemakers? It s important to understand that the EnRhythm MRI SureScan pacing system includes both important modifications to both the pulse generator and leads. The system works together to minimize the risk of MRI exposure. While the EnRhythm MRI SureScan pulse generator has all the advanced features of Medtronic s present EnRhythm pacemaker, including MVP (managed ventricular pacing), the EnRhythm MRI SureScan pacing system includes modified hardware to minimize the level of energy transmitted through the lead/device connection point. The pacemaker also includes a new SureScan feature that can be programmed on before an MRI scan to eliminate the impact of MRI-generated electrical noise, which can alter pacing therapy. To help identify the device, the pacemaker and leads also contain radiopaque marks, viewable via X-ray, to indicate that the system is MR-Conditional (a term identifying a device that may be used during an MRI under specified scanning conditions). The CapSureFix MRI SureScan pacing leads are modified versions of the active-fixation Medtronic CapSureFix ^{Novus leads, and are designed to decrease the risk of overheating during an MRI scan without limiting the ability to pace or sense the heart. What are the risks of having cardiac devices in MRIs? Why is it important to have MRI-safe implantable devices for patients? The risks are many, varied, and complex. Because of electromagnetic interference, MRI scanners may cause traditional pacemakers to pace or inhibit in an unreliable fashion. Additionally, the pacing lead can act as an antennae picking up the energy from the MRI and dissipating the energy as heat at the tissue interfaces. This can lead to deterioration of sensing and capture. Approximately 1.5 million Americans have cardiac pacemakers, and none of these patients have devices that have FDA labeling suggesting they can undergo an MRI procedure. This leaves the clinician in a difficult situation should an MRI be requested. Obviously, MRI is a powerful tool and it s frequently ordered. MRI is also becoming increasingly important in the diagnosis and management of cardiac disease. Estimates vary, but it s felt that approximately 200,000 pacemaker patients are denied MRI annually. Tell us about the trial and study design. How many patients will be involved in the trial? How many sites will be involved? The EnRhythm MRI trial is a prospective, randomized, controlled, unblinded, multi-center study involving up to 470 individuals at sites in the United States, Canada, Europe, Middle East, Africa, and potentially Japan. Eligibility is based on the traditional indications for a pacemaker and a willingness to undergo elective MRI scanning. All patients in the trial will receive an EnRhythm MRI SureScan pacing system (pulse generator and leads), and two-thirds of the participants will receive MRI scans; the trial s first MRI scans will occur approximately two to three months after device implant. The control group is also particularly important in this study. The study will compare the electrical system performance among patients in both groups. In addition, the study will assess any MRI-related complication rates in the month following the MRI scans. When are the start and completion dates of the trial? The trial began in Europe in February 2007; in Canada in June 2007; and in the United States in January 2008. The expected study duration and follow-up time is approximately 30 months, following which the FDA will review all relevant clinical and non-clinical data in order to determine whether or not the EnRhythm MRI system is safe and effective and should be allowed to be marketed. If the technology becomes available in the future, will patients need to inform MRI staff beforehand that they have an implanted pacemaker? Would current non-MRI compatible pacemakers become obsolete? Before undergoing any medical procedure, it is recommended that patients advise their treating doctors that they have an implanted heart device. They should consult with their heart doctor to evaluate any possible associated risk. Scanners differ, and the EnRhythm MRI system is being tested at 1.5 T MRI strengths. Scanners of increasing strength are becoming more common, such as 3.0 T, and while such scanners may look the same, they are not. The current system, if available in the future, will not initially be labeled MRI Conditional for all MRI machines. Non-MRI compatible systems will remain on the market for years to come, and the problem of legacy leads will persist for decades as non-MRI compatible leads will remain in patients. Even if an MRI-compatible pulse generator is attached to a legacy lead at battery ERI, the system itself will not be MRI-compatible because of the lead. Thus, innovative solutions such as EMI attenuators that might be attached to leads during battery changeout need to be developed to solve the problems that legacy systems present. Of course, abandoned leads that are not MRI-compatible are another problem altogether. The engineers of all the device companies have their work cut out for them and will be busy for years to come. Because of the increasing role of MRI in the future, physicians will likely push for devices that are MRI-compatible; it s simply one less thing to worry about in an already hectic, risk-filled day of managing patients. Do you see MRI-safe ICDs becoming available sometime in the future as well? Of course, the need is there. However, like all things, we walk before we run. The entire product portfolio of any company will need to be MRI-safe, ideally under all scanning conditions. Build it and doctors will come. Is there anything else you d like to add? The treatment and prevention of heart failure and sudden death will continue to be the focus of cardiologists for years to come, and implantable rhythm devices will continue to play a central role in the management of these patients. One can envision a day where MRI becomes the one-stop shop for cardiac diagnostics, combining the power of echo, angiography, and traditional radiopharmaceutical scanning all in one. Additionally, entire cath labs are being envisioned around MRI-based interventional and diagnostic tools. The very same patients that will need these tools will also frequently have implantable devices, and so the problem of MRI device incompatibility must be solved. Like so many things with technology, when MRI-compatible devices are finally available, we ll ask ourselves how we ever lived without them. Finally, if any of your readers have patients that they would like to refer to this clinical study, they may contact the sponsor at the following e-mail address: medtroniccrmtrials@medtronic.com. Disclosure: Dr. Gimbel has disclosed that he has a financial relationship with one or more companies that produce or market products or services relevant to the topic of this manuscript.}