Headphones from personal music players can interfere with pacemakers or implanted defibrillators, a medical trial has concluded.

The American Heart Association (AHA) found that magnets used inside some headphones designs, including clip-on models and earphones, can potentially interrupt a pacemaker’s beat or deactivate a defibrillator, even if the cans aren’t being used.

The ASA tested eight different headphone designs, each connected to iPods, on 60 defibrillator and pacemaker patients. Researchers found a “detectable interference” in 14 patients, while 15 per cent of the pacemaker patients and 30 per cent of the defibrillator patients experience a response to the cans

The field strength of each pair varied considerably and although some cans had field strengths of around 200 Gauss, some only measured strengths of around 10 Gauss.

“Exposure to the headphones can force [pacemakers] to deliver signals to the heart, causing it to beat without regard to the patient's underlying heart rhythm,” concluded William Maisel, senior author of the study and a director of the Medical Device Safety Institute at Beth Israel Deaconess Medical Center.

He added that defibrillators can be temporarily deactivated by headphones, but stressed that removing the cans does – in most cases – restore the medical hardware to its normal function.

It’s worth noting that the trial placed the headphones onto patients’ chests, directly above their medical tech. Although this isn’t really representative of where you’d normally keep your cans, we’re sure you’ve fallen asleep with headphones on your chest before.

The research concluded that the best way to keep yourself safe – if you use a pacemaker or defibrillator, that is – is to make sure your headphones are at least 3cm away from the device at all times.