MRI may be safe for patients with some types of pacemakers, ICDs

Susan Jeffrey

August 04, 2004

Baltimore, MD - New data from in vitro and animal studies suggests that magnetic resonance imaging (MRI) may be safely used in patients with one of a number of pacemaker systems and selected ICD systems manufactured after 2000[1]. Researchers report that at 1.5 T, no significant problems were seen with most pacemakers, although MRI did interfere with older models of defibrillators.

The new report appears in the August 3, 2004 issue of Circulation.

More confirmation is required from further studies before firm recommendations can be made, the researchers, with senior author Dr Henry R Halperin (Johns Hopkins Medical Institutions, Baltimore, MD), write.

However, at their own institution, they are already offering MRI to patients with newer device models and critical indications for MRI such as suspected cancer, Halperin told heartwire . "If a patient has a device that we've tested and has a critical need for an MRI scan, we'll do it," he said. They take certain precautions, though, including turning the device off during the scan and having a cardiologist present to monitor the patient. "We've scanned 23 patients already, and there have been no problems."

MRI contraindicated

Currently, the presence of implantable devices is considered a contraindication to MRI. Concerns have centered on the potential of MRI to inhibit the function of the device, to induce rapid pacing, mechanical pull or rotation of the device, device failure or reprogramming, or heating of leads. Newer devices, however, are smaller, contain less magnetic material, and have improved protection against environmental electronic interference, Halperin and colleagues note.

To determine whether modern systems might be used during MRI, the researchers carried out a series of in vitro and in vivo experiments with a range of current devices and lead systems.

Testing was first carried out using "phantoms," boxes filled with liquid that mimics the conduction properties of the body, Halperin said. They assessed whether there was any heating in the leads or pull on the devices during scanning and whether any malfunction of the device became apparent.

Pacemakers and ICDs manufactured after 2000 had few interactions "of any consequence" during scanning, he said. "One pacemaker reset itself, which wouldn't have any safety implications for the patient."

Older ICDs, though, were damaged by MRI, and some were destroyed. "They didn't blow up or anything, but they lost communication with the programmer that's used to interrogate and program them, so they would then be pretty much nonfunctional."

No thermal damage

Subsequently, late-model ICDs from all three of the largest device manufacturersMedtronic, St Jude Medical, and Guidantwere implanted in dogs for four weeks. The animals underwent three- to four-hour MRI scans, at higher energy levels than those used in regular scans. No device dysfunction occurred, they report, and images were of high quality, with distortion dependent on the scan sequence and plane. Pacing threshold and intracardiac ECG amplitude were unchanged over eight weeks, except in one animal that had transient capture failure lasting less than 12 hours after MRI.

After eight weeks, the dogs were autopsied. Of note, Halperin said, there was no evidence of thermal damage at the tip of the lead in the animals. Limited necrosis present was not different from three control animals that had the devices implanted but did not undergo MRI.

Although they now scan patients with newer devices, the better alternative would be for device manufacturers to prospectively design devices to be compatible with MRI, Halperin added. "For it to become 'prime time,' the device manufacturers should certify their devices as MRI-safe, and they're working on that," he said. "They're starting to feel the heat, if you will."


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