BOSTON: It may not be a killer, but for people with pacemakers, a digital cell phone is anything but a friend, the Mayo Clinic reports. When such a phone is positioned close to the heart, it can cause symptoms of heart trouble, such as palpitations, light-headedness or dizziness. Researchers found such symptoms 7 percent of the time in pacemaker users, according to the study reported in the New England Journal of Medicine. The research analyzed the effects of five different types of digital cell phones on a focus group of 980 pacemaker users. Noticeable interference in the pacing mechanism was detected 20 percent of the time, with potentially serious disruptions occurring in 6.6 percent of the cases. Despite their inferior communications performance, older phones, which employ an analog signal, caused relatively little interference with the pacemakers. Experts caution that further research is needed to determine how significant the cell phone risk may be.