Calls made on cell phones do not affect hospital medical devices, U.S. researchers said Friday, but store anti-theft alarms might make implanted heart devices misfire.

Tests at the Mayo Clinic in Rochester, Minnesota, showed normal use of cell phones, also called mobile phones, caused no noticeable interference with patient care equipment. But a portable CD player caused an abnormal ECG (electrocardiographic) reading when a patient used it near one of the leads of the device.

And at least two reports suggest that anti-theft devices set up near the doors of retail stores can cause implantable rhythm devices such as pacemakers and defibrillators to malfunction.

Most hospitals forbid the use of cell phones.