Heart patients wearing pacemakers and implanted defibrillators should avoid putting iPods or other media players in their breast pockets as the magnets in the headphones can deactivate the devices, according to a Medical Device Safety report.

The study, led by Dr William Maisel of the Medical Device Safety Institute at Boston's Beth Israel Medical Center, found that earphones could interfere with pacemakers when placed within 1.2in of the devices.

The Medical Device Safety Institute reported its findings to the American Hearth Association last week, and said strong magnets inside the headphones caused problems for one in four patients, and particularly those with a defibrillator. The researchers said the magnets could prevent electrical impulses being sent from the device to the heart, possibly leading to palpitations or arrhythmia.

"The main message here is: it's fine for patients to use their headphones normally, meaning they can listen to music and keep the headphones in their ears," said Dr Maisel "But what they should not do is put the headphones near their device."

Eight models of headphones were tested with 60 patents with defibrillators and pacemakers. Researchers stressed that MP3 players themselves posed no threat to pacemakers and defibrillators.