Commentary from the Section Editor

DOI: 10.19102/icrm.2012.030205

Editor-in-Chief

Magnetic Resonance Imaging (MRI) and Cardiac Pacemaker: From Foes to Friends

Using MRI on patients with implantable cardiac devices has been contraindicated due to interactions that may increase the risk of serious adverse events. There are several theoretical risks that can occur in a patient with a pacemaker undergoing MRI examination: electronic circuit malfunction, programming reset, lead movement, and electrode tip heating effects. There have been case reports of deaths attributed to use of MRI in pacemaker patients worldwide. However, none of these reported cases occurred when there was a physician present directly supervising the scan. Nevertheless, not until recently, patients with cardiac devices were labeled as MRI-contraindicated.

Each year, an estimated 200,000 patients receive a pacemaker in the US. The number approaches 5 millions worldwide. As patients age, they are more likely to need an MRI. It is estimated that approximately 50–75% chance of a patient with a pacemaker or implanted cardiac device will need an MR scan at some point in their life. Therefore, the availability of MR-conditional cardiac devices enables diagnostic capabilities provided by MRI to be available to previously denied patients.

In this issue of Innovations of Cardiac Rhythm Management, Sabin and Clair reviewed recent studies on patients with pacemakers undergoing MRI examinations, as well as pointed out some of the MRI advantageous features of the new Medtronic EnRhythm MRI SureScan (Revo MRI) system. In addition, they reported their own experience in implanting the Revo MRI system at the Vanderbilt Heart and Vascular Institute.

Sabin and Clair cited three landmark studies by Naele et al, Nazarian et al, and Wilkoff et al in their article. In all three studies, 1.5T MRI scanners were used. None of the MRI study was performed in the thoracic field. The procedure was carried out in a controlled environment with the presence of a physician, an experienced MRI personnel, and a device company representative. The leads were implanted for at least 6 weeks or more in Nazarian et al, 3 months or more in Naele et al, and 9–12 weeks in Wilkoff et al. In all three studies, there was no untoward event reported.

The FDA approval of the Revo MRI system was based primarily on the results of the prospective study by Wilkoff et al. Heart Rhythm 2011;8:65-73. In this multi-center, worldwide clinical trial, 464 patients were randomized to not undergo or undergo an 1.5T MRI scan between 9 and 12 weeks post implantation. In 258 patients who underwent MRI scans, there was no MRI related complications during or after the scan. The FDA approval was announced on February 8 2011, however, limits the system's use to specific patients, MRI imaging of certain body parts, and under certain scanning parameters.

What makes the Revo system MRI friendly? First, the lead has been designed to be a poor conductor of radiofrequency energy. As mentioned in Sabin and Clair et al, the active fixation leads were designed with an inner coil that uses only two filars instead of four. The internal circuitry of the pacing system has been improved to lessen the chance of cardiac stimulation and disruption of the internal power supply. The generator was manufactured with less ferromagnetic material and can be specifically set to an MR-safe mode for the purposes of the scan and then reset afterwards. Both the generator and the lead have radio-opaque marker to make them easily identifiable on X-ray.

Despite all the attractions it gathers, the Revo MRI system still has its limitations. First, it does not allow for rate drop response. Therefore, the system will not be appropriate for every patient. Moreover, preexisting non-MRI conditional models have various different features and the physician will decide which will provide the most benefit for an individual. Secondly, it can only be implanted as a dual chamber system. Thirdly, it incurs higher cost at the present time. Thus, we need to limit its implantation to patients who are most likely to be candidates for MRI scanning in the near term. Last but not least, the handling of the CapsureFix MRI 5086 lead may prove problematic since the lead is relatively thicker and stiffer as experienced by the implanters at the Vanderbilt Heart and Vascular Institute. From their experience, of the 16 complete Revo systems implanted since February 2011, there were 3 atrial lead dislodgements.

To maintain the integrity of the system, the implanting physician must not mix MRI-conditional components with standard ones. The MRI-safe label is only valid as an intact pacing system. In addition, the FDA requires that patients must not have previously implanted, active or abandoned medical devices, leads, lead extenders or adaptors, and broken or intermittent leads. The system must be implanted in the left or right pectoral region for at least 6 weeks prior to any MRI scan.

Patients with an MRI-conditional pacemaker can undergo imaging only with 1.5T MRI machines. In the US, 1.5T machines are currently used in only about 70% of all MRI scans.

It is also of paramount importance to emphasize the multidisciplinary approach required in the use of these pacing systems. It is clearly essential for an electrophysiology technician or a personnel trained in MRI-conditional devices to be present to program and reprogram the device before and after the scan. An experienced physician is also required to supervise the procedure.

The arrival of an MRI-conditional pacing system also triggers multiple intriguing questions to ponder. If the system has an insulation failure, would it still be considered MRI-conditional? Will the system still be MRI-conditional if the battery approaches its end of life? When changing out an end-of-life standard pacemaker generator in a patient with possible MRI-scanning need in the future, what should be done with the preexisting standard leads? Under what conditions is it reasonable to extract standard leads in order to implant a complete MRI-conditional system? The ongoing MagnaSafe Registry is a prospective study examining the safety of patients with MRI-non-conditional pacemakers and implantable cardiac defibrillators (ICD) who undergo MRI scans. Preliminary data presented at the AHA 2011 showed an acceptable safety profile with a low incidence of battery voltage, lead impedance, and pacing threshold change after MRI. The final results of this study and the current availability of the MRI-conditional pacemaker system will influence future guidelines to address the new practice.

Currently, device industry is also expanding its MRI-conditional pacemaker system to implantable defibrillator and biventricular systems. In Europe, Biotronik has already received an approval for an MRI-conditional ICD system. If continuing with the current pace, we are certainly looking at a near future with many breakthrough technological innovations in the field of implantable cardiac devices.

Ulrika Birgersdotter-Green, MD, FACC, FHRS
ubgreen@mail.ucsd.edu
Professor of Medicine
Director of Pacemaker and ICD Clinic
University of California - San Diego
San Diego, CA

Huy M. Phan, MD, PhD
m3phan@ucsd.edu
Clinical Electrophysiology Fellow
University of California - San Diego
San Diego, CA