Study bolsters viability for MR of implanted devices

MR imaging of pacemakers and implantable cardioverter devices is not for every imaging service. But the procedure is feasible, despite American College of Radiology recommendations to the contrary, according to a study from Oklahoma.

Dr. Edward T. Martin, director of cardiovascular MRI at the Oklahoma Heart Institute, came to the Society for Cardiovascular Magnetic Resonance annual meeting armed with data showing that-with the proper precautions-pacemakers and ICDs need not be contraindications for cardiac MR.

The demand for MRI by affected patients is growing, Martin said. He estimated that more than 400,000 patients with pacemakers could benefit from MRI annually. As many as 200,000 patients with ICDs were denied MRI last year because of contraindications, according to Dr. J. Rod Gimbel, a cardiologist from Knoxville associated with ICD maker Medtronics.

While admitting that it can be dangerous, Martin argued that the history of MR imaging of pacemakers and ICDs does not justify blanket exclusion. About 250 successful MRI procedures have been performed on pacemaker patients, and 17 deaths worldwide occurred soon after the procedure, Martin said. None of those deaths arose during monitored scanning, however.

A prospective study involving 61 MR exams and 54 pacemaker patients who were scanned without incident in 2004 at Oklahoma Heart identified criteria to optimize safety [J Am Coll Cardiology 2004;43(7):1315-1324]. Pacemaker-dependent patients were excluded. The minute ventilation feature was disabled, and full interrogation was performed pre- and postscanning. Asynchronous mode was not used, and subthreshold pacing was not performed because the risk of developing a lethal arrhythmia was considered extremely low. Continuous MR-compatible ECG monitoring was performed and intermittent voice contact maintained. Resuscitation equipment was available, and an electrophysiologist was present for all studies.

Martin offered the following guidelines for safely scanning patients with pacemakers based on this group's experience with 157 MR procedures performed on such patients and a thorough literature review:

- Document that a clinically necessary MR study is warranted in a patient with a pacemaker.

- Obtain informed consent.

- Have emergency equipment and advanced cardiac life support (ACLS)-trained personnel readily available.

- Scan only non-pacemaker-dependent patients.

- Interrogate the pulse generator immediately before and after MRI and reprogram if necessary.

- Disable the minute ventilation feature.

- Maintain voice contact throughout the procedure and continuously monitor heart rhythm and rate. Pulse oximetry monitoring is not necessary but can be used if an additional level of comfort is needed.

- Make sure a physician adept in the ways of pacemaker programming is present during scanning.

- Limit scanning to modern pacemakers (after 2000).

Subthreshold output programming is reasonable but may not be necessary if these guidelines are followed.

ICDs are trickier than pacemakers, but several trials suggest that safe MRI of these devices is possible, Martin said. Gimbel's experience with eight MR procedures performed on seven ICD patients in 2005 found no change in pacing, sensing, impedance, charge time, or battery status. One ICD exhibited a "power on" reset similar to rebooting a computer.

The Oklahoma Heart Institute has performed MRI on 17 patients with ICDs. Two devices caused problems. One could not be interrogated immediately after scanning, and the other developed a power on reset during imaging, then failed defibrillation threshold testing, suggesting that this was not a benign event. ICDs are also more susceptible than pacemakers to malfunctions, leading the Oklahoma group to add more restrictive selection criteria for ICD patients to the guidelines above:

- Disable therapy and detection for tachycardia/bradycardia modes with pre-MRI programming.

- Perform postscan device reprogramming and defibrillation threshold testing.

This special section was compiled by James Brice, senior editor of Diagnostic Imaging.