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, they said.

But a portable CD player caused an abnormal electrocardiographic (ECG) reading when a patient used it near one of the leads of the device, according to one of several reports in the March issue of Mayo Clinic Proceedings.

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.

Dr. David Hayes and colleagues said their tests suggest the ban is unmerited. They tested cell phones using two different technologies from different carriers, switching them on near 192 different medical devices.

During 300 tests run over five months, they reported no trouble with the equipment.

But not all technology mixes with medical devices.

Dr. J. Rod Gimbel of East Tennessee Heart Consultants and Dr. James Cox of the University of Tennessee Medical Center in Knoxville described two cases in which anti-theft devices apparently caused implanted heart devices to malfunction.

One of the patients had a pacemaker and she collapsed after pausing in a store doorway. Another had an implantable cardiac defibrillator that shocked him after he stood near an anti-theft unit.

The devices are called electronic article surveillance or EAS systems and use an electromagnetic field.

"More than 1 million EAS systems are installed worldwide," Gimbel and Cox wrote.

Store employees need to know of the danger, they cautioned.

"Simply moving the person away from the anti-theft device may save their life," Gimbel said in a statement.

"Many times with public safety issues we wait until something bad occurs before we act. Here's an opportunity where we can make our knowledge public and head off future problems."