A Review of Magnetic Resonance Imaging in Patients with Implantable Cardiac Devices

It is estimated that each year there are more than 900,000 implantable cardiac devices prescribed worldwide. There have also been an increasing number of MRI systems worldwide, with about 35 million magnetic imaging procedures performed yearly.^3,4 Because of the expanding clinical indications for both implantable devices and MRI, there are an ever-increasing number of patients who require both of these medical modalities. Due to the widespread application and powerful diagnostic capability of MRI, there are instances where denying a patient MRI evaluation may influence the quality of health care received. One study found that 17% of patients with pacemakers were denied MRI in the previous year. This may affect over 1 million patients worldwide.⁵ Studies to determine the safety of MRI in patients with devices are limited. Despite these concerns, several hundred non-pacemaker-dependent device patients have undergone MRI without complications while monitored under a number of different protocols established for safety. Various strategies have been used to minimize the risk of performing MRI procedures in device patients. Several investigators have manipulated various aspects of the device programming features, such as programming the device to subthreshold or placing the device in asynchronous mode. Others have limited imaging to non-pacer-dependent patients. Furthermore, MRI protocols themselves have been modified to attenuate radiofrequency (RF) power or to limit studies to those where the pulse generator is positioned outside the bore of the MRI system. Finally, the most conservative approach to date has been to explant the pulse generator prior to MRI imaging.^3,6Concerns Addressed A number of legitimate concerns regarding MRI studies in patients with cardiac devices have been noted. Most of these concerns have not come to pass. The fear of devices and/or leads being explanted from patients due to the MRI magnetic force has not occurred. The feared malignant arrhythmias have not been seen in controlled settings. Animal research demonstrated lead/device heating, which may lead to the potential complication of significant changes in device/lead parameters such as thresholds, impedance and pacing voltage. However, histologic evaluation of the animal myocardial tissue revealed no damage. The study also determined that in chronic leads, the temperature increase was lower than in acutely placed leads and that this temperature was below the critical temperature needed for cell damage.² In a second study, there was a significant change in thresholds after an MRI in 10% of device patients. However, these changes were not clinically significant.⁶ Most studies show that if the device leads are not in the MRI scanning field, the rise in temperature seems to be within acceptable limits (< 2 degrees centigrade).⁷ Martin et al established that no clinically relevant changes occur in lead pacing parameters with regard to the anatomical location of the MRI study.⁶ These data taken together seem to support the view that lead heating does not cause clinically significant changes to pacing parameters. Key Studies Depicting Safety in Device Patients Four key studies have been instrumental in providing evidence that patients with implantable devices may undergo MRI studies with a relatively safe benefit to risk ratio. Each study explores one of the four major groups that device patients fall into (non-pacer-dependent, pacer-dependent, ICD, and loop recorder patients). The first study was performed by Martin et al in non-pacemaker-dependent patients. The group studied 54 non-pacer-dependent patients who underwent a total of 62 MRI studies at 1.5 Tesla (T). This study was very liberal as to patient selection and pacemaker settings, allowing for the study of a broad population of patients. No limitations were placed on the type or duration of the MRI procedure, pacemaker, lead models, or proximity of the imaged anatomy relative to the pacemaker. Pacemakers were not programmed to the asynchronous mode. Only pacemaker-dependent patients were excluded, but it was later found that one pacemaker-dependent patient was inadvertently studied with no complications. There were no clinically relevant complications or malfunctions. It has been postulated that limiting the distance from the device to the anatomical region being imaged may be beneficial to MRI imaging safety. This study demonstrated that anatomical region/distance from the imaging area to the device was not associated with clinically relevant changes in pacer parameters such as threshold changes.⁶ The second study by Gimbel et al looked at 10 pacemaker-dependent patients undergoing 11 MRI scans of the head and neck. All patients were scanned with a head coil, limiting RF power deposition. This approach avoided RF exposure to the device and leads. This should theoretically limit heating within the leads and the lead-tissue interface. There was no clinical evidence of device malfunction or alterations in programming.¹ The third and fourth studies, again by Gimbel, explored MRI imaging of patients with ICDs and loop recorders. Both studies found it reasonable to image these patients if the proper precautions were in place. No clinically relevant malfunctions or alterations occurred.^8,9Our Experience We are currently providing MRI imaging at our practice for patients with devices. Our group provides this service to the surrounding community on a case-by-case basis. We have provided MRI at our institution to eight patients with devices, including 1 pacemaker-dependent patient, without complications or significant changes to device settings. Our patients and/or their physicians usually hear of this service by word of mouth and call us to ask if we can perform an MRI. We then contact the physician who is requesting the study, and make sure that the study results will directly impact patient care. We have developed a protocol that allows us to provide an MRI study to patients with a device with an acceptable (low-level) risk. The protocol consists of: 1. Informed consent from the patient. 2. Good working relationship with the MRI staff. It is helpful if the same staff is used for each study if possible. 3. An electrophysiologist, pacemaker company representative, and EP nurse are present during the procedure. 4. The pacemaker is interrogated and placed in OOO mode. ICD is turned off. 5. Transcutaneous pacer and resuscitation materials should be available in the MRI suite. 6. Cardiac and pulse oximetry is monitored, and verbal communication is obtained from the patient. 7. After the study, the pacer is interrogated. Device Company and FDA Recommendations In the April 2005 issue of PACE, Medtronic,¹⁰ Guidant,¹¹ and St. Jude Medical¹² had guest editorials regarding cardiac devices and MRI. Medtronic and Guidant's editorials were the most conservative of the three. Their view was that until FDA approval, they would continue to provide safety information to physicians who decide to provide a device patient with an MRI study. However, they would not endorse the MRI scanning of their devices until they had been specifically engineered to undergo MRI imaging. St. Jude Medical's statement was less conservative, stating the company is confidant that the likelihood of damaging or reprogramming one of our current devices is remote. They added When there is an appropriate clinical justification for an MRI study, the theoretical risks can be managed by appropriate programming and monitoring on the part of the clinical team caring for the patient. Future of MRI and Cardiac Devices The major providers of cardiac devices are all working on devices engineered for MRI compatibility, and FDA permission to begin the first patient tests are pending. The new technology involves the use of MRI-compatible non-ferrous metals and added filters to prevent the pacemaker and leads from picking up MRI signals, which may confuse the device and cause it to react in an erratic manner. In February 2007, Medtronic, Inc. announced the start of a clinical study to evaluate the safety and efficacy of the first pacemaker system engineered for safe use in MRI imaging. The system consists of the dual-chamber EnRhythm^® MRI SureScan pacemaker and the CapSureFix^® MRI SureScan pacing leads. Approximately 350 patients will participate in the prospective, randomized controlled, unblinded, multicenter study. Guidant and St. Jude Medical are also soon likely to follow, as they have been developing similar systems. Discussion A number of legitimate concerns regarding MRI studies in patients with cardiac devices have been noted. Most of these concerns are theoretical in nature and/or secondary to anecdotal incidents of device malfunction. To date, 10 deaths have been attributed to the use of MRIs in device patients.⁶ Most of these deaths were reported to the FDA; however, these reports were not published in peer-reviewed medical literature. These fatalities were poorly characterized, with no ECG data available. In most of these cases, the patients were imaged by MRI with the medical staff unaware that the patient had a cardiac device. Therefore, the necessary precautions needed to safely perform the test were not followed. Conversely, no permanent sequela has been reported when patients have been carefully monitored and the device underwent reprogramming before the scans. Of note, more than 230 patients have been safely imaged using MRI systems operating from 0.350 to 1.5 T. If an MRI will significantly impact their care, device patients should not be excluded from this testing modality. Conclusion Our group at Providence Hospital and Medical Centers has established a protocol in which MRI at 1.5 T can be done at a low level of risk. Close cooperation with the MRI staff is critical, though. Strict adherence to an established protocol is also key for patient safety. As we become more enthusiastic in imaging patients with devices, it will always be important to stay vigilant and heed the comment from Gimbel et al,⁹ who warned that failing to identify an adverse event is not equivalent to demonstrating safety.