The impact of portable devices on pacemakers and defibrillators has been the subject of fiery disputes among health specialists for some time now. In the most recent conference of the American Heart Association, researchers found that while iPods, Bluetooth headsets, iPhones, electric blankets, hand-held airport metal detectors or pills swallowed to perform video endoscopy do not trigger adverse reactions in pacemakers and implantable cardioverter defibrillators, MP3 player headphones do.

The headphones usually contain neodymium, a magnetic substance that can influence the way pacemakers and defibrillators send signals to the heart. According to William h. Maisel, senior author of the study and director of the Medical Device Safety Institute of Beth Israel medical Center in Boston, Mass., implanted cardiac devices may stop looking for abnormal heart rhythms in the presence of magnets.

The researchers tried to establish whether magnets outside the clinical setting could be potentially dangerous for patients using this technology. In order to do that, they tested eight different models of MP3 player headphones by placing them on the patients’ chests, over where the devices were implanted, and checked for interference.

In 14 patients, accounting for 23 percent of those tested, the researchers found evidence of interference: in 15 percent of pacemaker patients and 30 percent of the defibrillator patients had a magnet response. The signals returned to normal after removing the headphones, researchers said.

With the help of a gauss meter to measure the units of the magnetic charge produced, they found that a field strength of 10 gauss at the site of the pacemaker has the potential to interact with the device. That is particularity worrying, considering that MP3 player headphones have field strengths of 200 gauss or more.

“For patients with pacemakers, exposure to the headphones can force the device to deliver signals to the heart, causing it to beat without regard to the patients’ underlying heart rhythm,” Maisel explained. “Exposure of a defibrillator to the headphones can temporarily deactivate the defibrillator.”

However, there’s also good news: even at the headphones’ high field strength, researchers found no interaction if the headphones were held at least 3 cm (1.2 inches) from the skin surface. This was reported in all types of headphones, no matter the brand, and patients should be safe as long as they avoid keeping them too close to the implanted devices.

Researchers began looking into the interference matter earlier this year as well, after one cardiac patient reported dizziness while using an iPod. However, after examining the magnetic fields produced by four different iPod models, they found no interference with the pacemakers.

Pacemaker or defibrillator patients don’t usually need to make major changes in their lifestyle; however, they are advised to avoid activities that involve magnetic fields, including the magnetic resonance imaging procedure.