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ZURICH, Switzerland, Dec. 4 -- Rare-earth magnets -- now being packaged into necklaces and bracelets -- may have a deadly attraction for people with pacemakers and implanted defibrillators, warn researchers here.

Although they have a range of a little more than three centimeters, that was enough for neodymium-iron-boron magnets to cause interference in the heart devices in a cohort of consecutive patients, according to Thomas Wolber, M.D., of University Hospital Zurich.

It has long been known that magnetic fields can affect such devices, especially those that use magnetism for some of their functions, but the usual magnets in the home or office aren't normally strong enough to cause problems, Dr. Wolber and colleagues reported in the December issue of Heart Rhythm.

But jewelry wasn't considered a problem until recently when manufacturers began using the powerful rare-earth magnets to make such things as necklaces, bracelets, and nametags, which are typically worn near the skin, they said.

"Physicians should caution patients about the risks associated with these magnets," Dr. Wolber said. "We also recommend that the packaging include information on the potential risks that may be associated with these types of magnets."

Rare-earth magnets are used in multi-part jewelry, which can be assembled into various configurations and held together by magnetism. For example, one manufacturer offers a ring with three semi-circular parts, which can be expanded by placing extra segments between the sections.

Dr. Wolber and colleagues enrolled 41 ambulatory patients with a pacemaker and 29 patients with an ICD who were being seen for a routine outpatient service. The researchers tested two spherical magnets, a necklace made of 45 spherical magnets of different sizes, and a magnetic name-tag, each weighing less than eight grams.

They were first placed on the skin near the implant, the researchers said, and moved until signs of magnetic interference were seen. Then they were lifted in one-centimeter increments until interference ceased.

All of the devices showed signs of magnetic interference when the magnets were placed on the skin, documented by continuous ECG-monitoring for pacemakers and by emission of audible tones or annotation of magnet detection during marker-channel recording for ICDs.

The study found that pacemakers were more sensitive , affected to a mean distance of 2.1 centimeters, compared with 1.6 centimeters for the ICDs and the difference was statistically significant at P=0.007.

The maximum distance at which interference was seen was three centimeters.

Dr. Wolber and colleagues suggested that patients may be in danger if such magnets are brought close to their implants. Many pacemakers, for instance, use magnetic switches and interference could cause asynchronous pacing. At least one brand of ICD uses a magnetic switch to control some functions and could be unintentionally deactivated by magnetic jewelry, they said.

"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," said Huagui Li, M.D., of the Minnesota Heart Clinic in Edina, Minn.

In an accompanying commentary, he cited a recent case, in which a patient called him in alarm after hearing a beeping sound coming from her ICD.

Investigation showed that a brooch she was wearing was activating the device's magnetism warning system, Dr. Li said, and removing the magnetic jewelry solved the problem.

But, he said, a patient with poor hearing might not have heard the warning and been left with a device that was not functioning properly, with potentially fatal consequences.

"For an ICD patient, the magnet interference can be fatal," Dr. Li said.

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