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Most people with pacemakers or implanted defibrillators enjoy their iPod or other MP3 player just as much as anyone else, but a new study suggests they should be cautious about where they store the headphones.

The headphones contain magnets that could potentially cause interference if placed directly on the chest above the heart device, according to a report presented this week at the American Heart Association meeting in New Orleans.

"For defibrillator patients, it is a much bigger concern because the magnet can temporarily deactivate it," says the study's senior author, William H. Maisel, MD, director of the Medical Device Safety Institutethis link opens in a new tab at Beth Israel Medical Center in Boston. 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.

About 250,000 people in the United States each year are given pacemakers. An additional 125,000 receive implantable cardioverter defibrillators (ICDs), which can shock the heart back into a normal rhythm.

In the new study, the researchers attached eight types of headphones to iPods and tested them on 60 patients with ICDs. The earphones were either earbuds or clip-ons, not the larger noise-canceling varieties favored by business travelers and DJs.

The headphones were placed on the chests of patients, directly over the ICDs. Electromagnetic interference occurred in 14 patients, or 23%. There weren't any problems if the headphones were 3 centimeters or more above the skin's surface.

Dr. Maisel said patients with pacemakers and ICDs are told that magnets can interfere with their implant's function, but they may not be aware that headphones contain the magnetic substance neodymium, which helps with sound reproduction.

The magnetic strength of the headphones varies by brand and model, but the study found that 3 centimeters, or about 1.2 inches, was a safe distance for all.

When in use, headphones are obviously a safe distance from the implanted device. However, Dr. Maisel says pacemaker and defibrillator patients shouldn't tuck them in their front pockets, drape them over their shoulders, or allow a loved one to rest his head on their stomach while mellowing out to an iPod.