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People fitted with pacemakers should be aware that carrying a digital cellular phone could cause the heart-regulating devices to slow down, shut off or even speed up the heart rate, a researcher says.

The first large-scale study on the subject, released Thursday, found the digital cellular phones caused interference with heart pacemakers in more than half of the 975 patients tested.Dr. David Hayes, a co-author of the study, said interference occurred most often when a phone's antenna rested directly over the pacemaker, and rarely when a phone was held at the ear.

"I would tell patients that analog phones are certainly the safest," said Hayes, director of pacemaker services at the Mayo Clinic in Rochester, Minn. "Whatever kind of phone you're using, it's best to avoid carrying it around turned on in your breast pocket."

The patients were tested with five different types of digital cellular phones held at the ear and at different positions over the pacemaker. Overall, some interference, as defined by changes in an electrocardiogram, was seen in 53.6 percent of the patients.

Analog phones, the most common type of wireless phone available in the United States, caused interference in just 3.1 percent of the tests.

The results, which were similar to those of earlier investigations, were presented at the annual meeting of the North American Society of Pacing and Electrophysiology.

Problems created by the cellular phones included "temporarily inhibiting or turning off the pacemaker and causing the pacemaker to pace the heart at an inappropriately fast rate," Hayes said.

Tim Ayers, a spokesman for the Cellular Telecommunications Industry Association, said the study contained no surprises. He said new methods to shield medical devices and cell phones are helping to lessen the problems.

Many hospitals already restrict the use of cellular phones or recommend against their use around pacemakers and other electronic medical equipment.