Magnetic interference from iPod headphones could pose a risk to patients with surgically implanted heart monitoring devices, according to a study involving 60 pacemaker and defibrillator patients.

Interference from MP3 player headphones could prove fatal by temporarily deactivating a device, and the research team said patients should keep headphones more than 3cm away from their pacemaker or defibrillator.

"For family members or friends of patients with implantable defibrillators, they should avoid wearing headphones and resting their head right on top of someone's device," said William Maisel, director of the medical device safety institute at Beth Israel Medical Centre in Boston, Massachusetts. Patients themselves should avoid keeping headphones in a chest pocket or draping them over the chest, he added.

Maisel and his team tested eight different models of headphones, including clip-on and ear-bud varieties. These were all connected to an iPod, although any brand of MP3 player would have had the same effect. Most headphone types contain small amounts of the magnetic metal neodymium.

The team placed the headphones on the skin above the implanted device of 60 defibrillator and pacemaker patients and detected an impact on the device's operation in 14 patients.

"When a magnet is placed on a defibrillator, it temporarily deactivates the device," said Maisel. "In other words the defibrillator stops looking for dangerous fast heart rhythms, and if a patient had a life-threatening heart rhythm problem while the magnet was over the device, it would not treat it."

Pacemakers, on the other hand, are designed to treat slow heart rhythms. They are meant to send signals to the heart when the heart rate slows, but magnetic interference can cause a pacemaker to start pacing without regard to the patient's underlying heart rate. "This can cause palpitations (a feeling of heart irregularity) or can rarely induce an abnormal heart rhythm - although we did not observe any of these in our study," said Maisel.

The team did not find any magnetic interactions when they placed headphones 3cm or more from the device.

Maisel, who presented his work yesterday at the American Heart Association's meeting in New Orleans, said he did not know of any cases in which headphones had caused death or injury.

A Food and Drug Administration report concluded earlier this year that interactions between MP3 players themselves and implanted cardiac devices were unlikely.

A separate study presented at the conference found that wireless devices such as Bluetooth are also unlikely to cause a problem.