Wireless Technologies and the National Information Infrastructure.
Chapter 12 - Electromagnetic Interference and Wireless Devices:
Interference With Medical Devices

Medical devices can be affected by interference from radio devices, including cellular telephones, and this has recently become a public issue. (see footnote 5) Pacemakers, apnea monitors, blood-gas pumps, hearing aids, wheelchairs, and electronic imaging devices have reportedly been interrupted or interfered with in the presence of cellular telephones or other radio devices. (see footnote 6) In some cases, deaths have occurred, though none have been attributed to cellular telephones. In spring 1995, pacemaker wearers were warned not to use new digital cellular phones because of interference problems. (see footnote 7)

Specific problems have surfaced with new digital mobile telephones and hearing aids. Time division digital transmissions can produce loud audio tones in some hearing aid models and other analog audio devices from up to 100 feet away. The tones can reach 130 dB--the sound of an airplane taking off as heard by a person standing on the runway. (see footnote 8) The interference lasts as long as the hearing aid is close to the digital phone, but returns to normal when the phone is turned off or moves out of range.

Shielding can reduce the amount of interference hearing aids encounter, but there are limits to what shielding can be done. There are three types of hearing aids, those worn in the ear, outside the ear, and in a pocket and attached by wire. Hearing aids worn in the ear, by far the most popular, are least amenable to shielding, because they are already very small; hearing aids worn in the pocket are most susceptible to EMI, but can be easily shielded.

There are about six million hearing-aid users in the United States today, and the number is projected to increase as the baby-boomer generation ages. It is not known what types of hearing aids (in-ear, on-ear, or pocket), or how many (one or two ears) are used, nor is it known how many hearing- impaired people use cellular telephones. The projected cost of retrofitting hearing aids to eliminate interference is unknown; this may not be feasible given their small size and life span.

The potential for EMI has long been studied and understood by radio engineers and medical technologists, and a substantial body of technical work and engineering expertise exists. Like other forms of electromagnetic interference, shielding devices against electromagnetic radiation and controlling the output levels of emitting devices are the two main ways compatibility is attained. Another is the proper installation and spacing of medical equipment to minimize the potential for interaction.

Standards have been set for both transmitting devices and for shielding of computing and medical devices, based on both lab testing and field experience. Voluntary standards were promulgated in 1979 by the Food and Drug Administration (FDA) specifying that medical equipment should be protected against interference up to seven volts per meter between 450 and 1000 MHz. (see footnote 9) A more recent standard issued by the International Electrotechnical Commission, one of the main standards' bodies in this area, relaxes suggested permitted exposure to three volts per meter in the frequency range from 26 to 1000 MHz. (see footnote 10) The Association for the Advancement of Medical Instrumentation, a voluntary standards body in the United States, has convened a committee to address EMI problems. (see footnote 11) Table 12-1 gives the FDA's 1994 draft suggestions on the minimum distance that should be maintained between transmitters of various power outputs and medical devices with various amounts of shielding.

However, with the growing number of both radio and medical devices and their shrinking size, more interference is likely to occur. Because transmission equipment can rarely be altered to reduce interference, regulators think the best solution is for device manufacturers to pay close attention to shielding, working in consultation with the designers and manufacturers of emitting devices. (see footnote 12) Other measures by themselves may not be sufficient. For example, proposals have been made to restrict the use of wireless devices in hospitals and clinics, but the ubiquity and small size of such devices makes policing difficult. Moreover, health care is becoming more decentralized with sensitive medical equipment increasingly housed in homes and outpatient clinics. Mobile care-givers, in turn, are becoming more reliant on wireless communications to interact with doctors and technicians at hospitals in other locations. This evolution in care-giving requires that medical equipment and wireless communications exist side-by- side. Users of medical or radio devices are generally unaware of field strengths, frequencies, the position, or in some cases even the presence of electromagnetic radiation. Warnings, when they do exist, rarely tell users what to do beyond "avoid electromagnetic interference."

Clearly, incorporating shielding into medical devices early in the development process is essential. Other measures may provide some help in minimizing interference problems: promulgating strong standards, limiting radio devices in well-identified areas, and providing good consumer education of the dimensions of EMI.

Regulatory and Legislative Initiatives

In October 1994, the Subcommittee on Information, Justice, Transportation, and Agriculture of the House Government Operations Committee held hearings on medical device interference from wireless and cellular devices. (see footnote 13) The Federal Communications Commission (FCC) and FDA have primary oversight responsibilities for this area, and have consulted frequently on design and standards issues. However, legislative interest in this issue appears to have precipitated action in the industry to address EMI problems. For example, the Cellular Telecommunications Industry Association (CTIA) and the Health Industry Manufacturers Association have jointly funded a Center for the Study of Wireless Electromagnetic Compatibility at the University of Oklahoma to study medical device interference. This center convened a Forum on Electromagnetic Compatibility in September 1994, which discussed these issues.

The Hearing Aid Compatibility Act of 1988 required that all telephones be made compatible with hearing aids by 1991. (see footnote 14) However, in a concession to the cellular telephone industry, the act excluded mobile phones. The act did permit the FCC to revisit the issue at a later date, with the presumption that new technologies would be made compatible with hearing aids. The FCC has determined that PCS equipment will be exempt from compliance with the act, noting that U.S. operators who choose GSM will use a different frequency than their European cellular counterparts, that few hearing-aid users will be affected, and that cost-effective solutions to mitigate interference are available. (see footnote 15) There is some concern in the hearing-aid users' community that PCS operators will choose GSM as their standard. The FCC has convened an advisory committee to examine this issue.