The study, published in Heart Rhythm, challenges an earlier report that said iPods can cause implantable cardiac pacemakers to malfunction. The study found patients showed no change in any of 255 separate tests and no patients had symptoms.

"This provides reassuring evidence that should allay the fears of people using iPods and other digital music players," the study's senior investigator Dr. Charles Berul of Children's Hospital Boston said in a statement.

The tests were run on 51 patients with pacemakers or implantable cardioverter-defibrillators ranging in age from 6 to 60 that came in for appointments from September to December 2007. One of four digital music players -- either the Apple Nano, the Apple Video, the SanDisk Sansa or the Microsoft Zune was placed directly on the chest as patients were tested lying down.

Berul and colleagues said they are reassured by their own findings but acknowledge their testing was only short-term.

"We can't conclude that it's completely safe to have an iPod right on top of the device for hours at a time,"Berul said. "That's why we suggest the precaution of keeping it at least 6 inches away."