The American Heart Association has warned that MP3 players may adversely affect some heart-monitoring electronics.

New research suggests that the neodymium magnets used in the headsets of many players can cause harmful interference to implanted pacemakers and cardioverter defibrillators.

The study found that the magnets in the headphones caused the interference when placed within 3cm of the implanted device. The magnets do not pose any risk beyond this range.

"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 patient's underlying heart rhythm," said Dr William H Maisel, senior author of the study and director of the Medical Device Safety Institute at the Beth Israel Deaconess Medical Center.

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

This is not the first time that portable media players have been cited as harming implanted medical devices.

Earlier this year, the US Food and Drug Administration conducted a study into possible negative health effects of the iPod following a presentation from a high-school science student.

The players were eventually found not to pose any health risk, but hardware manufacturers are having to pay more attention to the possible side effects of their devices in light of an increasingly litigious customer base.