It might be possible for people with an older pacemaker to safely undergo magnetic resonance imaging after all.

A new study suggests that avoiding an MRI of the chest and reprogramming the pacemaker before and after the scan can prevent malfunction of cardiac pacemakers, making it easier for doctors to diagnose problems that can be difficult to detect without an MRI.

"This is very significant in terms of clinical practice because for years and years it's always been that scanning pacemaker patients with MRI was contraindicated unless they had a newer device specifically designed to be safe during the test”, said Robert Watson, chairman of neuroradiology at the Mayo Clinic in Rochester, Minnesota.

People with the implantable devices are often excluded from MRIs out of concern that the magnetic fields emitted by the machines can overheat the lead wires implanted in the heart and interfere with the pacemaker's sensitive electronics.

The Russo team used a registry, developed in 2009, logged cases of adults with a programmable defibrillator or pacemaker implanted after 2001 who got an MRI.
If the patient had an adequate heart rate of at least 40 beats per minute, the devices were turned off.

If the rate was lower, the devices were set to blindly make the heart beat at a steady rate, eliminating the usual feedback the devices get to readjust their electrical pulses. After the MRI, the settings were restored.

At 19 US centres, 818 patients with pacemakers and 500 with implantable defibrillators were evaluated, according to the report in the New England Journal of Medicine, February 22.

None of the MRI recipients died and only one needed to have their implanted device replaced due to a generator failure apparently caused by a failure to properly reprogram the implantable defibrillator before the MRI.

The team logged some voltage changes, but "all the changes we saw were relatively small and not clinically significant," said Dr Russo.

In addition, the number of MRIs that patients received did not seem to affect the risk.

"One patient had a total of 11 MRIs of the brain performed because of a history of primary cancer of the brain," he said.

That case showed why MRIs are important for pacemaker patients because, after chemotherapy, radiation and surgical treatment, "CT scanning suggested there was no recurrence. But MRI showed there was residual tumour and more treatment was necessary," he said.

Patients need to tell medical personnel if they have an implantable device, Watson stressed. But with MRI available to them, and the proper protocols to avoid or deal with any unexpected device malfunctions, "you can make life-changing diagnoses".

Whether chest MRIs are just as safe remains an open question. Dr Russo said he suspects that they are.

Dr Watson said the Mayo Clinic, using a multidisciplinary team, has "done a fair number of cardiac MRIs and have not experienced an adverse patient outcome."

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