(Reuters Health) - A small study adds to mounting evidence that MRI scans may be safe for people with pacemakers or implanted defibrillators.

Manufacturers currently warn against putting the devices into MRI scanners, whose strong magnetic field might in principle cause the metal wires from the devices to heat up and burn the heart tissue.

Apart from injuring the heart, that could also upset the electric properties of the delicate devices.

But more and more research suggests those worries, which are entirely theoretical, could turn out to be unfounded, said Dr. Christopher Kramer, a spokesperson for the American Heart Association (AHA), who was not involved in the new work.

"With this study, and the several studies prior, there are really no clinically relevant changes that occur in 99.9% of the devices that get scanned," he told Reuters Health.

The AHA does not endorse MRI, short for magnetic resonance imaging, in pacemaker patients and Medicare does not pay for the scans unless the patients are part of a registry used to investigate the safety of the procedure.

In the new study, Dr. Robert Russo and colleagues at Scripps Clinic in La Jolla, California, reviewed medical records for 109 patients with pacemakers or implantable cardioverter-defibrillators (ICDs). Each had undergone one or more medically necessary MRIs such as scans to look for brain tumors.

The devices were turned off during the scans or, if the patients didn't have a viable heartbeat without them, set to a constant rhythm that wouldn't be upset by the scanner's magnetic field.

There were no deaths, no device failures and no heart rhythm disturbances in any of the patients, according to the study, which was published in the American Journal of Cardiology.

And while there were slight changes in the electrical measurements before and after the scan, they weren't deemed big enough to have any impact on the patients.

What's more, the researchers found changes of the same magnitude when they looked at 50 patients with heart devices who hadn't been scanned, Russo told Reuters Health, indicating that the variation is normal.

He said the findings are not definitive yet and need to be confirmed in a bigger study, such as the MagnaSafe Registry, which Russo is currently running.

Both Russo and Kramer said more and more centers have started doing MRIs on patients with heart devices if there are no other good alternatives.

"If it is clinically necessary the clinician needs to weigh the risk and benefits on a case-by-case basis," Russo said.

Although U.S. health regulators have already approved one MRI-compatible pacemaker, thousands of patients still live with older models. And between 50 percent and 70 percent of those might eventually need an MRI, Russo said.

SOURCE: bit.ly/U8RH0N American Journal of Cardiology, online August 27, 2012.

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