Protecting Devices from Radio-Frequency Transmitters

Medical Device & Diagnostic Industry Magazine
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An MD&DI April 1998 Column

EMI FIELD NOTES

New EMC guidelines will help engineers test and manage electromagnetic interference from nearby transmitters.

The recommendations contained in two new guidance documents will place new pressures on health-care facilities to use the outlined methods when testing existing and new medical devices' interactions with other electronic devices. Device manufacturers must ensure that all their devices can withstand the basic electromagnetic compatibility (EMC) checks recommended in the new documents, even if they are not designing equipment to meet international EMC standards. The documents, which describe the test methods and the theory behind the need to perform the tests, are: ANSI C63.18, "Recommended Practice for an On-Site, Ad Hoc Test Method for Estimating Radiated Electromagnetic Immunity of Medical Devices to Specific Radio-Frequency Transmitters," and AAMI technical information report (TIR) 18, "Guidance on Electromagnetic Compatibility of Medical Devices for Clinical/Biomedical Engineers—Part 1: Radiated Radio-Frequency Electromagnetic Energy."

Figure 1. The medical device should be 10 ft from the radio-frequency transmitter. (Source: IEEE Standards Draft of ANSI C63.18.) Illustration by James Schlesinger

ASSESSING IMMUNITY

The American National Standards Institute (ANSI) accredits a number of engineering committees that develop standards, guides, and recommended practices. The accredited engineering committee for EMC is C63. It oversees six subcommittees, including SC-8—Medical Device EMC Test Methods, whose working group 1 developed C63.18 over the past three years.

The recommended practice is meant to guide clinical and biomedical engineers in assessing the immunity of medical devices to the electromagnetic fields radiated through the air by portable radio-frequency (RF) transmitters. The document mainly covers existing inventories of medical devices that were not necessarily designed, developed, tested, or manufactured to any kind of EMC engineering standard. By using commonly available RF transmitters with power outputs of 8 W or less, clinical and biomedical engineers can evaluate the sensitivity of their medical equipment to the wireless-transmitter environment.

The document is also intended to help device users obtain objective data on RF immunity and improve the consistency of EMC measurements among health-care facilities. It recommends that when selecting which medical devices to test, the health-care organization consider the criticality of the medical device, the impact on the patient if the device fails, the device's history of unusual failures, and the proximity of the medical device to RF transmitters such as walkie-talkies and cellular phones.

The facility should designate a suitable test area away from any critical-care areas. A bare room of at least 400 sq ft in the basement of the health-care facility would make an excellent setting for immunity testing.

When the actual testing is carried out, the medical device should be placed as shown in Figure 1. In general, the transmitters should be held 10 ft from the device to prevent damaging it. During the test, the RF transmitters should be operated as they are normally, except that the antenna should be held first parallel to the floor and then perpendicular to the floor. In consideration for other health-care personnel, an announcement should be made in advance that testing will be performed in this frequency range. The phrase "This is a test" should be repeated during transmission.

The recommended guidance document stresses that such testing should be performed cautiously. All health-care personnel should be aware of the test, its location in the facility, and its potential effect on electrical medical devices in the vicinity.

Some health-care organizations may choose to perform immunity testing at the sites where the medical device will be used in the facility. They should be aware, however, that results will vary in this instance due to differing wall absorptions, wall reflections, and the influence of neighboring objects.

Test results can be used to determine a priority list of sensitive equipment and the minimum separation distances necessary to prevent RF transmitters from interfering with the normal operation of medical devices. The recommended practice suggests that the test results be documented for future reference and so management can set appropriate facility guidelines.

When new communications systems are installed in a facility, the RF characteristics of the transmitters should be researched and compared to the facility's existing inventory of RF sources. If the new sources differ significantly in frequency or modulation characteristics, they should be used as transmitters in the next periodic test of electrical medical devices.

MANAGING THE EMC ENVIRONMENT

AAMI's EMC committee produced the TIR on EMC. Its purpose is to help clinical engineers and biomedical personnel assess the EMC environment of their facilities and to support reasonable actions to minimize undesirable electromagnetic interference interactions in health-care surroundings.

The TIR references ANSI C63.18, but it goes beyond that document to provide technical information on EMC to make health-care personnel "experts" in EMC engineering. For example, the TIR provides abbreviations and definitions relevant to EMC to assist health-care professionals with interpreting EMC standards and specifications.

Further, about 15% of the TIR's content is devoted to explaining the basic physics of electromagnetic energy and how it may interfere with the normal operation of electronic devices. The document explains the coupling mechanisms of radiation, conduction, and induction, while illustrating the "upset" phenomena of junction rectification, bit corruption, and cochannel interference. The textbook portion concludes with a technical description of frequency and wavelength of radio waves and how they relate to the power and energy of electromagnetic waves.

The guidance specifies about 15 recommendations (in corresponding chapters) that health-care organizations should consider, including selection of a focal point for EMC, management of the EMC issue, and purchase of appropriately designed medical devices. One chapter is devoted to how to assess RF environments in health-care facilities, providing guidelines for identifying intentional and unintentional RF sources and for measuring the facility's ambient signal levels.

For health-care organizations planning new facilities, the TIR gives specific recommendations on site selection, followed by hints on how to design a building to minimize EMC problems. In addition, it lists specific construction guidelines designed to lower the cost of meeting EMC shielding, filtering, and grounding restrictions.

The TIR concludes by listing selected case studies of typical EMC problems in health-care organizations. Most of the case studies focus on radiated radio wave problems and their effect on devices such as ventilators, dialysis equipment, and other electrical medical devices.

CONCLUSION

The two new guidance documents should ensure that clinical and biomedical engineers better understand the impact of RF sources on the operation of electrical medical devices and provide reasonable guidelines for the use of wireless communications devices in health-care facilities. Management of the electromagnetic environment should include maximizing the compatibility of the electromagnetic sources and receptors within the control of the health-care organization by performing electromagnetic interference testing.

Dan Hoolihan is vice president of TÜV Product Service (New Brighton, MN).

The C63.18 Recommended Practice may be ordered from ANSI, 11 W. 42nd St., New York, NY 10036; 212/642-4900; fax 212/302-1286.

TIR 18 may be ordered from AAMI, 3330 Washington Blvd., Ste. 400, Arlington, VA 22201-4598; 703/525-4890, fax 703/276-0793.

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