The EnRhythm MRI Trial: Evaluating the Use of Pacemakers in MRI

MRI examinations are contraindicated due to pacemaker ferromagnetic components and electromagnetic interference (EMI), which could result in oversensing or heating in pacemaker leads due to the electromagnetic radiofrequency (RF) field. In addition, other potential problems concerning the interaction between MR imaging and cardiac pacemakers include induction of ventricular fibrillation, motion, dislocation, torquing of the pacemaker and/or pacemaker leads, changes to the pacemaker program, sensing and pacing failure, and damage to the pacemaker components, etc. As many as 10 patients with permanent pacemakers are reported to have died after they underwent MR imaging. MRI-induced heating is also a possible hazard in patients with biologic implants made of metallic materials.³ Maximum RF-induced heating occurs at the electrode-tissue boundary; that is, the area of endocardium and myocardium close to the tip of the electrode has a potential risk of thermal injury, which results in the deterioration of pacing thresholds or atrial/ventricular perforation. Heating worsens when objects are configured in a loop or coil, as conducting loops are known to provide a high current density in low impedance. In addition, to the described loop currents, the presence of extended wires that act as dipole antennas will pick up the electrical component of the RF field. Energy absorption is indicated by the specific absorption rate (SAR). The SAR value varies according to the magnetic field and parameters used in a given MR imaging pulse sequence. In vitro studies have shown temperatures increase in some leads up to 23 ºC at only moderate RF exposures of 1.3 W/kg. This increase in temperature occurs in a range that is used for temperature-controlled RF catheter ablation of the accessory pathways, and is probably capable of inducing tissue injury at the lead tip. Background In the past few years, a number of in vitro and in vivo investigations have shown safety of pacemaker and MR imaging at field strengths of 0.35-1.5T.^3-9 In these studies, patients with a history of permanent pacemaker implant were monitored before, during and after undergoing MR imaging. Cardiac telemetry as well as pacemaker threshold changes were evaluated; overall, no patient deaths occurred and only minor threshold changes were observed. However, these studies had several limitations, such as small sample size, no patient randomization, and no utilization of pacing systems specifically designed for use with MRI compatible technologies. Thus, the potential for thermal injury is present if a large population is exposed to an MRI environment. It is important to recognize that failing to identify an adverse event is not equivalent to demonstrating safety. We can gather from these studies that only under very special circumstances (i.e., where MRI is the only diagnostic modality and no alternative acceptable imaging modality is available) can MRI studies be conducted in patients with pacemakers. However, MRI examinations must be performed using proper monitoring (e.g., programming to an asynchronous mode and allowing limited RF exposure to avoid thermal injuries) in patients with implanted pacemakers. New Technology and Clinical Trial Medtronic has designed the EnRhythm ^® MRI SureScan pacing system, a cardiac pacing system designed specifically to be utilized in MR imaging environments. MRI compatibility was based on preclinical testing. MRI radiofrequency characterization was tested via simulation and in vitro testing, through three-dimensional (3D) human model development, and via preclinical studies to establish correlations between changes in pacing capture threshold (PCT) and RF power deposition. A prospective randomized clinical trial EnRhythm ^® MRI utilizing this system is now underway. This clinical trial has already started enrolling patients in Europe and Canada, and may soon start enrollment in the United States. The EnRhythm ^® MRI SureScan pacing system programs the device to a magnet mode (asynchronous pacing mode) to prevent erroneous operation due to improper activation or noise coupled into the lead system, and hardware changes to the lead interface to minimize induced currents on the lead electrodes. The leads were modified to change their characteristic impedance and attenuate the coupling of radiofrequency energy onto the leads in order to minimize lead tip heating. In addition, this pacing system incorporates all diagnostic and therapeutic features of the EnRhythm ^® pacemaker, including Managed Ventricular Pacing (MVP ®), which enables the device to be programmed to deliver pacing pulses to the right ventricle only when necessary. The EnRhythm ^® MRI clinical trial plans to enroll 364 subjects among 35 sites in Europe, the U.S., and Canada. Subjects will be randomized to receive clinically relevant head and lumbar MRI scan sequences 9-12 weeks post-implant or no MRI testing at all. Conclusion In summary, although over 60 million MRI scans are performed annually, MRI examinations are contraindicated in most medical centers for pacemaker patients. Yet up to 75% of pacemaker patients will need an MRI at some time. A number of investigations in the past few years have shown safety of pacemaker and MR imaging at field strengths of 0.35-1.5T; in these studies, no patient deaths occurred and only minor threshold changes were observed. However, these studies had several limitations. Therefore, the potential for thermal injury is still present if a large population is exposed to an MRI environment. Ultimately, we can only conclude from these previous studies that there are limited circumstances in which MRI studies can be conducted in patients with pacemakers. Medtronic s EnRhythm ^® MRI clinical trial will evaluate if this new pacemaker system is designed for safe use with MRI scanners. Whether patients with a permanently implanted pacemaker will be able to undergo routine MRI testing will depend on the results of this very important clinical trial.