The study, conducted by William Maisel of Beth Israel Medical Centre in Boston, tested eight different brands of headphones, including ear buds, all hooked up to an iPod. The team placed the headphones above the patients defibrillators and pacemakers and found that they impacted the device in 14 of the cases.

"When a magnet is placed on a defibrillator, it temporarily deactivates the device," Maisel told The Guardian. "In other words the defibrillator stops looking for dangerous fast heart rhythms, and if a patient had a life-threatening heart rhythm problem while the magnet was over the device, it would not treat it."

The results were less pronounced in pacemaker patients. The magnets' interference can disrupt the device's effect. "This can cause palpitations (a feeling of heart irregularity) or can rarely induce an abnormal heart rhythm - although we did not observe any of these in our study."

An FDA study earlier in the year, meanwhile, found that interaction between headphones and pacemakers was not a major concern.