Study Overview
Objective. To assess the risks associated with magnetic resonance imaging (MRI) in patients with a pacemaker or implantable cardioverter-defibrillator (ICD) that is “non–MRI-conditional.”
Design. Prospective cohort study using the multicenter MagnaSafe Registry.
Setting and participants. Patients were included in the registry if they were 18 years of age or older and had a non–MRI-conditional pacemaker or ICD generator, from any manufacturer, that was implanted after 2001, with leads from any manufacturer, and if the patient’s physician determined that nonthoracic MRI at 1.5 tesla was clinically indicated. Exclusion criteria included an abandoned or inactive lead that could not be interrogated, an MRI-conditional pacemaker, a device implanted in a nonthoracic location, or a device with a battery that was near the end of its battery life. In addition, pacing-dependent patients with an ICD were also excluded.
Main outcome measures. The primary outcomes of the study were death, generator or lead failure requiring immediate replacement, loss of capture (for pacing-dependent patients with pacemakers), new-onset arrhythmia, and partial or full generator electrical reset. The secondary outcomes were changes in device settings including: a battery voltage decrease of 0.04V or more, a pacing lead threshold increase of 0.5V or more, a P-wave amplitude decrease of 50% or more, an R-wave amplitude decrease of 25% or more and of 50% or more, a pacing lead impedance change of 50 ohms or more, and a high-voltage (shock) lead impedance change of 3 ohms or more.
Main results. Between April 2009 and April 2014, clinically indicated nonthoracic MRI was performed in a total of 1000 pacemaker cases (818 patients) and 500 ICD cases (428 patients) across 19 centers in the United States. The majority (75%) of the MRI examinations were performed on the brain or the spine. The mean time patients spent within the magnetic field was 44 minutes. Four patients reported symptoms of generator-site discomfort; one patient with an ICD was removed from the scanner when a sensation of heating was described at the site of the generator implanted and did not complete the examination.
Regarding primary outcomes, no deaths, lead failures, losses of capture, or ventricular arrhythmias occurred during MRI. One ICD device was left in the active mode for anti-tachycardia therapy (a protocol violation) and the generator could not be interrogated after MRI and required immediate replacement. Four patients had atrial fibrillation and 2 patients had atrial flutter during or immediately after the MRI. All 6 patients returned to sinus rhythm within 49 hours after MRI. No ventricular arrhythmias were noted. There were also 6 cases of partial generator electrical reset with no clinical significance.
Regarding secondary outcomes, a decrease of 50% or more in P-wave amplitude was detected in 0.9% of pacemaker leads and in 0.3% of ICD leads; a decrease of 25% or more in R-wave amplitude was detected in 3.9% of pacemaker leads and in 1.5% of ICD leads, and a decrease of 50% or more in R-wave amplitude was detected in no pacemaker leads and in 0.2% of ICD leads. An increase in pacing lead threshold of 0.5 V or more was detected in 0.7% of pacemaker leads and in 0.8% of ICD leads. A pacing lead impedance change of 50 ohms or more was noted in 3.3% of pacemakers and in 4.2% of ICDs.
Conclusion. Device or lead failure did not occur in any patient with a non–MRI-conditional pacemaker or ICD who underwent clinically indicated nonthoracic MRI at 1.5 tesla when patients were appropriately screened and had the cardiac device reprogrammed in accordance with the protocol. Substantial changes in device settings were infrequent and did not result in clinical adverse events.
Commentary
It is estimated that 2 million people in the United States and an additional 6 million worldwide have an implanted non–MRI-conditional cardiac pacemaker or ICD [1]. At least half of patients with such devices are predicted to have a clinical indication for MRI during their lifetime after device implantation [2]. The use of MRI poses concerns due to the potential for magnetic field–induced cardiac lead heating, which could result in myocardial thermal injury and detrimental changes in pacing properties [3,4].
In this study, Russo and colleagues assessed the risks for patients with a non-MRI-conditional pacemaker or ICD receiving an MRI scan using a pre-scanning protocol. If the patient was asymptomatic and had an intrinsic heart rate of at least 40 beats per minute, the device was programmed to a no-pacing mode (ODO or OVO). Symptomatic patients or those with an intrinsic heart rate of less than 40 beats per minute were determined to be pacing-dependent, and the device was reprogrammed to an asynchronous pacing mode (DOO or VOO). All bradycardia and tachycardia therapies were inactivated before the MRI. Based on this standardized protocol, no major adverse outcomes occurred. All pacemaker or ICD device were reprogrammed in accordance with the pre-specified protocol except one case where the ICD device was left in the active mode for anti-tachycardia therapy (a protocol violation) and the generator could not be interrogated after MRI and required immediate replacement. In addition to patient safety, the authors also measure the functionality of the devices pre-MRI and post-MRI. One of these measurements were battery voltage changes, a small decrease was noted for both pacemakers and ICDs as expected. The radiofrequency energy generated during MRI scanning creates a temporary decrease in battery voltage, which had resolved in all pacemaker cases although some ICD voltage decreases of 0.04 V or more had not resolved by the end of the 6 month post-MRI follow-up.
Several limitations exist. The study registry included devices and leads from different manufacturers, but did not report outcomes by manufacturer. While overall it appears to be safe to conduct an MRI study for patients who have non–MRI-conditional devices, this study did not provide enough information for patients younger than 18 years of age, patients who required repeat MRI studies, MRI examinations of the thorax, or higher MRI field strengths—the newer 3 tesla high-resolution MRI machines.
Applications for Clinical Practice
This multicenter prospective cohort study provides strong evidence that patients with a non–MRI-conditional pacemaker or defibrillator can receive nonthoracic MRI studies at 1.5 tesla when a straight pre-scanning device interrogation is performed per the standardized protocol.
—Ka Ming Gordon Ngai, MD, MPH