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MRI scanning may be safe even for patients with legacy pacemakers or implantable cardioverter-defibrillators (ICDs) not known to be MRI-ready, as long as a safety protocol is followed, a prospective study suggested.

The most important event to watch out for was immediately after scanning, according to Saman Nazarian, MD, PhD, of University of Pennsylvania Perelman School of Medicine, and colleagues. This happened in nine out of 2,103 scans (0.4%), where eight of those resets were transient.

The one case where a reset couldn't be fixed by device reprogramming was when a patient had a legacy pacemaker with less than 1 month of battery life left; the device had to be replaced, the authors reported online in the New England Journal of Medicine.

"If power-on reset occurs, the device reverts to an inhibited pacing mode. Therefore, in pacing-dependent patients, the device may transiently cease pacing because of electromagnetic interference, and electrocardiographic monitoring and pulse oximetry are warranted so that the scanning can be stopped if inhibition of pacing occurs," they said.

A prespecified safety protocol dictated that pacing be switched to asynchronous mode for pacing-dependent patients; pacing placed in demand mode for others; and tachyarrhythmia functions disabled.

When it came to changes in device parameters, 1% of patients had P-wave amplitude fall by at least 50% after going into the MRI.

No adverse events observed in the 63% of patients that had a follow-up at 1 year. By then, device interrogations showed that the most common changes (by ≥50%) were:

• Decreases in P-wave amplitude (4%)
• Increases in P-wave amplitude (4%)
• Increases in atrial capture threshold (4%)
• Increases in right ventricular capture threshold (4%)
• Increases in left ventricular capture threshold (3%)

"No change in device parameters that occurred either immediately after the MRI or at long-term follow-up in any patient was large enough to result in lead or system revision or device reprogramming," Nazarian's group emphasized.

Their study population was a group of 1,509 patients who underwent 1.5-Tesla MRI scans deemed clinically necessary despite having a pacemaker (58%) or ICD (42%) not meeting FDA criteria for being MRI-conditional. Clinicians performed device interrogation at baseline and immediately after the MRI.

"In our smaller study that was reported previously, we noted an association between thoracic imaging and changes in long-term right ventricular sensing and capture threshold. However, the current larger study, in which the follow-up period was longer, does not suggest any association between the region of imaging and detrimental changes in device parameters," according to Nazarian's group.

"The primary detrimental associations were a larger reduction in right atrial and right ventricular lead sensing immediately after the MRI with ICD systems than with pacemakers, as well as a larger reduction in long-term right ventricular lead sensing with longer lead length than with shorter lead length."

Besides being a single-center study, the analysis included patients with a wide variety of cardiac devices, so the number of each pacemaker or ICD included was small. Furthermore, a 20% rate of patients lost to follow-up was a major study limitation.

Even so, they maintained, the present findings complement those of the older, similar MagnaSafe Registry.

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