Author + information
- Arthur J. Moss, MD∗ ( and )
- Valentina Kutyifa, MD, PhD
- Heart Research Follow-up Program, Cardiology Division, Department of Medicine, University of Rochester Medical Center, Rochester, New York
- ↵∗Reprint requests and correspondence:
Dr. Arthur J. Moss, Cardiology Division, Department of Medicine, University of Rochester Medical Center, 265 Crittenden Boulevard, CU 420653, Rochester, New York 14642-0653.
Implantation of transvenous pacemakers began in the 1960s, and currently several million patients worldwide are living with a life-saving implanted pacemaker. The implanted cardioverter-defibrillator (ICD) was first reported in 3 patients in 1980 (1), with a marked and progressive increase in the number of implanted devices beginning in the mid-1980s. Implantation of cardiac resynchronization therapy (CRT) with and without an ICD began in the mid-1990s, with an increase in application of this effective treatment beginning at the turn of the century. Most of these devices have been implanted in older patients, generally in those >65 years of age, who are likely to have noncardiac as well as cardiac problems. During the 50-year period from 1965 to 2015, new diagnostic techniques were introduced, including magnetic resonance imaging (MRI), which is widely used for diagnosis and evaluation of patients with cardiac and noncardiac disorders. However, the use of MRI diagnostics in patients with implanted devices has remained a controversial issue to the present day.
Early on, it was appreciated that MRI could adversely affect the electrical function of implanted transvenous pacemakers, ICDs, and CRTs, and several deaths were reported during MRI scanning in patients with these implanted devices (2). By way of background (3), the main magnetic coils of an MRI system generate a constant magnetic field, the strength of which is measured in Tesla units—a unit of magnetic flux density. Inside the main magnet are 3 gradient coils that generate on/off magnetic fields. The static and dynamic electrical fields of the MRI can affect the electronic circuits and settings of pacemakers, ICDs, and CRT devices. Most clinical MRI systems operate at 1.5-T, but an increasing number of MRI systems are using 3.0-T output, an amount of magnetic field strength that provides better imaging but is likely to have a greater adverse effect on implanted electrical devices.
The reed switch in a pacemaker or an ICD is unpredictably affected by the electric field generated by the MRI with resultant pacemaker, ICD, or CRT malfunction. Recently, several device manufacturers have developed MRI conditional pacemakers and ICDs that allow for safe MRI scanning with limited exposure in patients with these implanted units (3). At the present time, there is only limited experience with MRI in patients with CRT.
In reviewing the published medical data on this MRI-device topic, almost all of the published papers were experiential studies involving a limited number of patients with no control group and little or no information after MRI scanning in the device-implanted patients. In this issue of the Journal, Gold et al. (4) report the first international, multicenter, prospective, randomized trial evaluating the safety of 1.5-T full-body MRI exposure (chest, cervical, and head regions) in 175 ICD patients, including patients with single- or dual-chamber ICD devices, and with a control group of 88 ICD patients not exposed to MRI. The ICD involved a Medtronic Evera MRI-ICD (Medtronic, Minneapolis, Minnesota) connected to commercially-available Medtronic defibrillator leads. The endpoints included ventricular pacing threshold, decrease in R-wave amplitude, and a limited number of induced and spontaneous ventricular tachycardia/ventricular fibrillation episodes during a 1-month follow-up. The primary findings were the absence of adverse effects with the standardized, comprehensive MRI protocol. Pacing and sensing were not significantly affected by MRI, with normal arrhythmia detection and defibrillation function documented during and following the MRI examinations.
The Medtronic Evera MRI-ICD was specifically designed to reduce interaction between the ICD and the MRI-induced electrical environment, and such electronic devices are categorized as “conditional” (i.e., conditioned to work properly in an MRI environment). As described by Gold et al. (4) in the current paper, the following modifications were included in the Medtronic Conditional Evera MRI-ICD utilized in the trial: “ferromagnetic material was reduced, a hall sensor replaced the mechanical reed switch, filters to prevent gradient and radiofrequency (RF) were added, and battery circuitry protection was added.” Pre-clinical testing substantiated the safety of this system (5). Other manufacturers have developed MRI-compatible pacemakers, but only Biotronik (Berlin, Germany) has developed ICDs that can be used during MRI scanning, as long as it does not involve chest scanning (3).
It is clear that new upgrades of the ICD are becoming safer in patients undergoing MRI studies. It is only a matter of time before all cardiac electronic device manufacturers will have MRI-compatible pacemakers, ICDs, and CRTs available for use with both 1.5- and 3.0 T-MRI, and possibly higher, but that does not exist now. Furthermore, there are large numbers of cardiac electronic devices already implanted that are susceptible to the adverse effects of the generated electromagnetic fields associated with the current generation of MRI techniques.
So, where does that leave us now? Reasonable programming guidance is provided for conventional (nonconditional) implanted pacemaker or defibrillator devices during MRI in the 2013 European Society of Cardiology (ESC) guidelines (6): 1) patients who are not pacemaker dependent should be programmed to VVI/DDI (inhibited mode); and 2) patients who are pacemaker dependent should be programmed to VOO/DOO (asynchronous mode). In addition, it is recommended to deactivate other pacing functions, deactivate monitoring and antitachycardia pacing/shock functions in ICDs, monitor electrocardiography and symptoms during MRI, restore original device programming, and check the device function after the MRI. For patients with MRI-compatible pacemakers or ICDs such as reported by Gold et al. (4) and by other investigators, one should follow the manufacturer’s instructions. Presently, there are no specific validated guidelines for patients with implanted cardiac resynchronization devices, but fixed rate VOO pacing with other parameters turned off in patients who are not pacemaker dependent would seem reasonable. Additional helpful suggestions are provided in the ESC guidelines (6).
The trial by Gold et al. (4) has significantly advanced safe MRI studies using 1.5-T output in patients with the new, conditional ICD. This approach is the beneficial tip of a troublesome iceberg. Millions of patients worldwide have nonconditional devices implanted, and MRI scanning techniques at 1.5-T and above are currently in use, with the full effects on existing cardiac electrical devices yet to be determined. There are many issues still to be resolved, but such is the future of medicine.
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Moss has received research grant support from Boston Scientific to his institution. Dr. Kutyifa has reported that she has no relationships relevant to the contents of this paper to disclose.
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