NEW YORK (Reuters Health) - Magnets made from three elements - neodymium, iron and boron (NdFeB), sometimes referred to as "rare earth" magnets -- have the potential to interfere with cardiac pacemakers and implantable cardioverter-defibrillators (ICDs), investigators from Switzerland warn in the current issue of the medical journal Heart Rhythm.

"What makes the new NdFeB magnets so special is their high magnetic field strength at very small sizes," Dr. Thomas Wolber of the Rhythmology Service, Cardiovascular Center, University Hospital Zurich told Reuters Health.

These magnets have recently gained popularity and are incorporated in toys, jewelry, name tags and are also available as larger magnets for household and office use. "Due to their very small size, their strength may be underestimated by patients," Dr. Wolber said.

"Magnetic fields," he said, "can change the mode of pacing in pacemakers and can deactivate arrhythmia detection and treatment in ICDs. While interference with a pacemaker does not put the patient at substantial risk, deactivation of an ICD could prevent the device from delivering lifesaving therapies."

Wolber and his colleagues tested the effects of two round NdFeB magnets, of 8 and 10 millimeters diameter, and one necklace made of 45 round magnets on 41 patients with pacemakers and 29 with ICDs.

These small magnets caused interference in all patients at distances up to 3 centimeters, Wolber and colleagues report. The cardiac devices resumed normal function immediately after the magnets were removed.

"Small magnets (up to 8 grams) such as those tested in the study are safe as long as they are not used in close proximity (within 3 cm) to a cardiac device, as may be the case with jewelry (necklaces, earrings, brooches) or name tags," Wolber said.

"Spreading information about potential interference is essential because device inactivation for several hours, as may occur for example by a name-tag attached near the device all day, might be a real problem," he added.

In a commentary, Dr. Huagui Li of the Minnesota Heart Clinic in Edina says this study is "timely and important to attract the attention of both the public and the medical profession about the potentially serious health consequences of magnets used in decoration products. For an ICD patient, the magnet interference can be fatal."

Li and Wolber recommend that manufacturers who use these magnets in products should be required to include warnings in their product information.