A small, new study revealed that placing the ubiquitous ear buds or other headphones too close to the chest could interfere with the proper functioning of the devices used to keep hearts beating at their proper rhythm.

Patients should not place the headphones, which contain magnets, in shirt pockets or drape them over their chest, lest they risk havoc with their heart-rhythm devices, researchers said on Sunday.

Pacemakers treat slow heart rhythms, while implantable cardioverter defibrillators (ICDs) jolt dangerously racing hearts back into a normal rhythm.

'For patients with pacemakers, exposure to the headphones can force the device to deliver signals to the heart causing it to beat without regard to the patients' underlying heart rhythm,' said Dr William Maisel of Beth Israel Medical Centre in Boston and the study's lead investigator.

Exposure of ICDs to magnets in headphones may deactivate them, causing the device to stop looking for abnormal heart rhythms, Dr Maisel explained.

Results of the 60-patient study were presented at the American Heart Association scientific meeting in New Orleans.

Researchers tested eight different models of MP3 player headphone with iPods. They found a detectable interference with the heart devices in 14 patients - 30 per cent of ICD patients and 15 per cent of the pacemaker patients.

In most cases, removal of the headphones restored normal device function and there were no problems when the headphones were at least 3 centimetres from the skin's surface, Dr Maisel said.

Earlier this year US health regulators reported that interactions between MP3 players and implanted heart devices are unlikely to occur. That may be the case - as long as you keep those ear buds where they belong. -- REUTERS