Cellular telephones and pacemakers.

People who rely on a pacemaker to regulate their heartbeat should heed a new report. It suggests that some cellular telephones can interfere with the device, which emits an electric signal to keep the heartbeat on track.

David L. Hayes of the Mayo Clinic in Rochester, Minn., and his colleagues studied 980 people with pacemakers. The team had these people use a variety of cellular phone models in more than 5,500 tests. Results of the study appear in the May 22 New England Journal of Medicine (NEJM).

The findings apply only to cellular phones, not cordless household phones.

The researchers found that in 20 percent of the tests, the cellular phone altered the pacemaker's function, resulting in abnormalities in heart rhythm. Only rarely did people experience noticeable effects such as dizziness or lightheadedness, the researchers said. These effects nearly always occurred when a volunteer held the phone directly over the pacemaker. When patients held the phone to their ear, they did not experience any such effects, Hayes says.

Patients who use cellular phones should keep them out of their breast pockets, where the phones may be close to or directly over a pacemaker, comments Donald M. Witters Jr. of the Food and Drug Administration in Rockville, Md. To receive calls, a cellular phone must periodically transmit a signal to its home base. That signal can interfere with a pacemaker's regulation of the heartbeat, he says.

Hayes contends that "there's no significant public health risk" to pacemaker patients who use cellular phones properly. Nonetheless, FDA has asked manufacturers to begin designing pacemakers that reject any interference from cellular phones or other wireless technology.

The NEJM study was supported by grant money from Wireless Technology Research, a Washington, D.C., group funded primarily by the cellular phone industry.