Portable media players such as iPods are unlikely to interfere with heart pacemakers, a U.S. Food and Drug Administration researcher reported on Thursday.

His tests of a variety of iPods showed they did not produce enough of an electromagnetic field to interfere with the devices.

FDA researcher Howard Bassen and colleagues set up a complex experiment using a saline-filled bag to simulate the human body and a coil sensor designed to pick up electromagnetic emissions.

They measured the magnetic fields produced by four different iPod models: a fourth-generation iPod and an iPod with video, and an iPod nano and an iPod shuffle. They also measured the voltages delivered to the inside of the pacemaker by the magnetic fields from the iPods.

All their measurements indicated the iPods could not affect cardiac pacemakers, they reported in the journal BioMedical Engineering OnLine.

We measured magnetic field emissions with a 3-coil sensor

placed within 1 cm (half an inch) of the surface of the player. Highly localized fields were observed (only existing in a one square cm area), they wrote.

Based on the observations of our in-vitro study we conclude that no interference effects can occur in pacemakers exposed to the iPods we tested, they concluded.

Two reports had suggested otherwise. Last year cardiologists operated an iPod during a patient's examination, and reported in the journal Heart Rhythm that they had seen interference with the pacemaker.

And last May, a study presented by a 17-year-old high school student to the Heart Rhythm Society annual meeting in Denver caused a stir when he said he detected electrical interference half of the time when the iPod was held 2 inches from the patient's chest for 5 to 10 seconds.

The FDA researchers did not examine any portable music devices other than iPods, which are made by Apple Inc..

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