MRIs safe for ICDs and pacemakers, if proper protocol followed

Magnetic resonance imaging (MRI) can be performed safely in patients with certain pacemakers and implantable cardioverter-defibrillators (ICD) as long as expert staff are on hand to monitor and intervene if needed.

Up to 75% of patients with pacemakers and ICDs develop an indication to undergo MRIs. To define a protocol for scanning such patients, researchers at Johns Hopkins University in Baltimore, and Rambam Medical Center, in Haifa, Israel, enrolled patients from primary and subspecialty physicians between February 2003 and April 2010.

As most ICDs lack asynchronous pacing capacity, pacemaker dependent patients with ICDs were excluded. Patients with newly implanted ICD or pacemaker leads (less than 6 weeks) and those with abandoned or epicardial leads also were excluded, resulting in 438 patients with devices (54% with pacemakers and 46% with ICDs) who underwent 555 MRI studies. Results appeared in the Oct. 4 issue of Annals of Internal Medicine.

MRIs were done at a magnetic strength of 1.5 T. Pacing mode was changed to asynchronous for pacemaker-dependent patients and to inhibited for others. Tachyarrhythmia functions were disabled. Blood pressure, electrocardiography, oximetry and symptoms were monitored by a nurse with experience in cardiac life support and who had immediate backup from an electrophysiologist.

Researchers looked at whether exposure to electromagnetic interference might cause implantable devices to revert to a back-up programming mode known as “power-on-reset.” In this mode, pacing is set to an inhibited mode and tachycardia therapies are enabled. There were three power-on-reset events. One occurred during cardiac MRI in a patient with a single-chamber ICD that had been implanted in 1999. The ICD did not attempt to deliver tachyarrhythmia therapy, but the patient experienced a pulling sensation in his chest and the MRI examination was discontinued.

The other two patients had pacemakers implanted in 1997 and 2003 and were undergoing brain and cervical spine examinations, respectively. Both patients had occasional pacing inhibition associated with programming reversion to the inhibited pacing mode, but they were not pacemaker-dependent and completed their MRI examinations. None of the three patients had device dysfunction during long-term follow-up.

When the device was located in the MRI field of view, image distortion, signal voids or bright areas, and poor fat suppression occurred. Selecting imaging planes perpendicular to the plane of the device generator, shortening the echo time, and using spin echo and fast spin echo sequences reduced the effect. Artifacts occurred during thoracic exams, and because thoracic MRIs have a greater effect on the devices, they should be reserved for patients with an absolute clinical need, the study authors said.

Researchers noted that they did not test all available cardiac devices, that follow-up was unavailable in 43 patients (10%), and that some data were missing. Defibrillation threshold testing and randomization to a control group were not performed.

An editorialist commented that MRIs are not risk free, but the risks are quantitatively small. “[T]he risks of MRI in the presence of CRMDs [cardiac rhythm management devices], although potentially serious, have probably been overestimated and can be managed effectively in many cases. In our opinion, the presence of a CRMD should no longer be considered an absolute contraindication to MRI. Rather, the risks and benefits of MRI in a patient with a CRMD should be assessed on an individualized basis, as with any important medical decision.”

To manage risks, practitioners should conduct MRIs only at 1.5 T, implants should be in place for at least six weeks to reduce movement of leads, and no epicardial or nonfunctioning leads should be present. “Most important, the equipment and experienced personnel necessary to manage any eventuality must be immediately available,” the editorial concluded.