If you have an implanted cardiac device, you might want to double-check where you drop your MP3 player's earphones as they could interfere with the proper performance of your heart device, a new study presented during a conference in New Orleans revealed.

According to Harvard researchers, the strong little magnets within the headphones of an MP3 player could throw off pacemakers and implantable cardioverter defibrillators (ICDs) when placed within an inch of the devices. Pacemakers are designed to boost slow heart rhythms, and when exposed to magnets, they may deliver signals that tell the heart to beat faster, whether it needs to or not.

A team of scientists tested eight models of earphones, including clip-on and earbud types, on 60 patients with defibrillators and pacemakers by varying the distance of the earphones from the devicecs, study leader Dr. William Maisel, director of the Medical Device Safety Institute at Beth Israel Medical Centre in Boston said.

"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," he said. "Exposure of a defibrillator to the headphones can temporarily deactivate the defibrillator."

The study results showed that while MP3 players themselves caused no electromagnetic interference, different brands of headphones generated magnetic fields, which caused some interference when they were placed within an inch of their chest in 23% of the patients. A magnetic field with the strength of 10 gauss can cause pacemaker trouble, and most portable headphones have strength 20 times that, Maisel said. However, the strength of the field decreased when earphones were moved just a small distance from the implanted devices.

"Magnetic field strength falls off very rapidly with distance," Maisel explained. "If you double the distance from the pacemaker or defibrillator from one inch to two inches, the magnetic field strength falls off by a factor of eight. Even moving a magnet away from a device just a small amount can have significant impact."

Previous unrelated studies revealed that MP3 players including iPods, Bluetooth devices and electric blankets didn't affect pacemakers.