Medical Device Daily National Editor
BOSTON — MRIs are safe ... almost all of the time.
But it's well-known that the strong magnetic fields of these machines can produce some rather unexpected and amazing effects, such as pulling or throwing loose metal instruments about the MRI room, occasionally causing injuries.
And there's one other thing: they can heat up implanted metal devices.
Rather significantly, the magnetic fields can heat up pacemakers, potentially turning their leads into hot tips which then ablate portions of the heart, death being the possible, though occasional, result.
"Every once in awhile there's going to be a disaster," said Bruce Wilkoff, MD, director of cardiac pacing and tachyarrhythmia devices, Department of Cardiovascular Medicine and professor of medicine at Cleveland Clinic Lerner College of Medicine of Case Western Reserve University (Cleveland), presenting the results of a pivotal trial of a new MRI-safe pacemaker at this year's scientific sessions of the Heart Rhythm Society (HRS; Washington).
"Cooking the myocardium" is how Wilkoff described what can happen when the leads of a standard pacemaker heat up during MRI, and he noted the increasing population of older people with pacemakers who also require assessment via this modality.
Other disastrous or near-disastrous effects of MRI for those with pacemakers may be that the devices are compromised so that they are unable to deliver therapy or they deliver unnecessary pacing. Thus, these patients are generally advised not to undergo what may be a rather necessary assessment.
The pacemaker studied in the trial is the EnRhthm MRI SureScan system from Medtronic (Minneapolis), CE-marked but not yet approved by the FDA, noted Wilkoff, a Medtronic consultant helping to develop the system.
Wilkoff said that the increase in the use of pacemakers – "over the years, many millions implanted," he noted – and a continually growing number of these implantations in those over the age of 65 who may need MRI assessment. Thus, he said, "the indication for MRI doubles, the overlap of the two populations [with pacemaker/needing MRI] is enormous."
The pilot trial tracked 464 patients implanted at 41 centers in North America (U.S. and Canada), Europe and the Middle East, with enrollees randomized to either MRI or no MRI. Following implantation with pacemakers, the MRI group received various types of head and lumbar scans with 1.5 Tesla MRI machines.
Of this group 444 patients were later followed, the follow-ups finding "no MRI-related complications or MRI-attributed sustained ventricular arrhythmias, asystole episodes, or pacemaker malfunctions."
There were some slight decreases in pacemaker sensing amplitudes in some devices. But, according to the study abstract: "There was no evidence of clinical (bradycardia or tachycardia), subclinical l(pacemaker performance) or technical (pacemaker or lead damage) adverse events observed in patients receiving an MRI."
Asked what constituted the changes in the pacemaker providing its safety in MRI, Wilkoff told Medical Device Daily that there was no one thing but rather "multiple small changes" in its "circuit, materials, geometry." The lead is still of metal material but with associated changes in the generator and the lead-generator architecture, he said.
In response to questions from MDD, company representatives said that the enRhythm pacemaker is the only one yet designed for use with MRI and that it has been well received in the countries where it has been introduced.
The device utilizes "modified hardware to minimize the level of energy transmitted through the lead/device connection point." The device also includes a feature called SureScan that, "when programmed on before an MRI scan, temporarily alters the device's data collection and monitoring functions while still delivering asynchronous pacing if needed. The device also contains a radiopaque mark, viewable via X-ray, to indicate that the device is MR Conditional.
Filing for a PMA with the FDA, is being made "via a series of modules being submitted to FDA." The company predicts going before an FDA advisory panel, but the Medtronic representatives said it is not yet offering an estimate of regulatory approval or U.S. commercialization. They also said it is "premature" to state pricing.
Rather than specifying a marketing strategy for the system, they underlined the potential need for the device by that group identified by Wilkoff, those over 65 with a pacemaker and likely to require an MRI diagnostic procedure at some time, and that this type of pacemaker then would likely be used under the guidance of a physician.
In post-presentation questioning, Wilkoff said that the use of MRI for patients implanted with standard pacemakers can't be ruled out categorically but should be considered, based on a risk-benefit analysis.
Many ICD-implanted patients may need such assessments, he said, given ongoing chronic diseases, and that the risk of an MRI scan in these cases is less than that offered by avoiding a needed diagnostic. In such cases, he said, the condition of the patient and assessment of the pacemaker's operation, should receive close and continuous monitoring.