Author + information
- Jerold S. Shinbane, MD⁎ (, )
- Patrick M. Colletti, MD and
- Frank G. Shellock, PhD
- ↵⁎Division of Cardiovascular Medicine, Keck School of Medicine, University of Southern California, 1510 San Pablo Street #300 North, HCC I, Los Angeles, California 90033
The rapid proliferation of diagnostic and therapeutic modalities using a spectrum of materials and energy sources has led to the potential for interactions that limit or contraindicate the use of magnetic resonance imaging (MRI). This convergence of technologies in patient care due to the frequent use of MRI procedures requires biomedical implant developers to engineer and prospectively study mechanisms to protect against unwanted interactions. The design of a cardiac pacemaker with special attention to the MRI environment represents a paradigm shift in device engineering.
Initial studies of pacemaker/MRI interactions occurred through in vitro experimentation, retrospective case reports, and series of patients with pacemakers accidentally or intentionally placed in the MRI environment (1). Subsequently, prospective studies of carefully monitored patients with pacemakers examined using MRIs under well-defined conditions yielded variable results, as electromagnetic energy conduction can potentially cause tissue damage at the lead tip/endocardial interface, damage to the pulse generator and leads affecting sensing and pacing threshold and impedances, inappropriate pacing acceleration or inhibition, battery depletion, and other issues (1).
Challenges regarding the risks of performing MRI examinations versus the potential benefits to individual patients in situations in which an MRI was essential to patient management (1,2) have led to the first wave of engineering of prospectively designed pacing systems acceptable for use under specific MRI and device conditions (3,4). A “magnetic resonance [MR]-conditional” item is defined as posing no known hazards in a specified MRI environment with specified conditions of use; an “MR-safe” item as posing no known hazards in all MRI environments; and an “MR-unsafe” item as posing hazards in all MRI environments (5).
Currently, there is only 1 published prospective, randomized multicenter study assessing the efficacy and safety of a pacing system engineered to be MR conditional (3). Pulse generator design changes included: 1) reducing ferromagnetic content to minimize magnetic field interactions and to avoid damage or malfunction of components; 2) shielding to minimize the effect of the electromagnetic environments; and 3) changing the reed switch to a Hall sensor, which allows for predictable behavior in a magnetic field. The lead wire winding pattern was designed to minimize potential heating of the pacing system and cardiac tissue due to conduction of electrical impulses from time-varying magnetic fields and radiofrequency energy from the MR scanner to the pacing lead. Dedicated programming modes include asynchronous (5.0 V/1.0 ms) pacing and nonstimulation choices. Labeling to identify the device and components as MR conditional include radio-opaque markings on the pulse generator, leads with a unique radio-opaque indicator, and identification cards with an MR-conditional icon and information. Study conditions and results are listed in Table 1. Importantly, no significant complications were observed in this study. Further post-marketing data will help define potential issues with application to larger groups of patient in other clinical settings.
Three medical device companies (Biotronik, Berlin, Germany; Medtronic, Inc., Minneapolis, Minnesota; and St. Jude Medical, St. Paul, Minnesota) have MR-conditional cardiac pacemakers available in Europe and other countries. In the United States, the U.S. Food and Drug Administration approved the Revo MRI SureScan Pacing System (Medtronic, Inc.). Because the leads and pulse generator technology were specifically designed for use in 1 integrated system, these systems are not indicated for generator changes using pre-existing leads or situations in which abandoned leads are still present in the patient. Currently, in the United States, the Medtronic, Inc. system has scan-positioning limitations (landmark isocenter of radiofrequency coil superior to C1 or inferior to T12), whereas in Europe, MR-conditional pacemaker isocenter conditions include: Medtronic (no scan exclusion zone), St. Jude Medical (contraindication to use of transmit radiofrequency coil directly over the pacing system) and Biotronik (scan exclusion zone with the maximum allowed positioning mark for the isocenter starting from the foot at the hip level and the maximum allowed positioning mark for the isocenter from the top of the skull at the level of the eyes). Because there are no specific guidelines regarding scanning beyond the specified limitations of an individual MR-conditional device, reliance on the current literature and the guidance of institutional review bodies regarding the risks and benefits in these situations are important considerations (1,2).
The spectrum of electronically activated biomedical devices with potential MRI interactions has significantly increased and includes implantable cardioverter-defibrillators, resynchronization devices, loop memory recorders, implantable physiological measurement devices, and neuromodulation devices. Similar design, engineering, and prospective testing will be required for these devices relative to use in patients undergoing MRI procedures.
The current MR-conditional pacemakers provide an important step in device engineering, allowing patients to have greater access to what is considered to be one of the most important noninvasive diagnostic imaging procedures: the MRI. Future questions relate to how to safely and cost-effectively engineer, evaluate, and implement new MR-conditional systems (Table 2). A continued multidisciplinary approach directed toward understanding the interface of medical devices, diagnostic imaging studies, therapeutic procedures, and environmental energy exposures is required to prevent or minimize adverse interactions related to the convergence of these technologies.
Please note: Dr. Shellock has received research grant support from Medtronic, Inc.; Boston Scientific Corporation; Biotronik; and St. Jude Medical. Dr. Shinbane and Dr. Colletti have reported that they have no relationships relevant to the contents of this paper to disclose.
- American College of Cardiology Foundation