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About Electromagnetic Interference in Hospitals

28 December, 2011 | imaging equipment, medical equipment parts, Radio Frequency Interference (RFI)

Radio Frequency Interference (RFI) and Medical Equipment

With the assimilation of wireless communication technology into hospital infrastructures, hospitals are becoming concerned about the impact of radio frequency (RF) electromagnetic interference (EMI) between wireless technologies and medical equipment. Such interference may cause undesirable changes to medical equipment, possibly resulting in misdiagnosis, mistreatment, and/or patient injuryThese wireless communication devices include: wireless Local Area Networks (LAN), Bluetooth, wireless telecommunications, paging, two-way radios, telemetry devices, cell phones, wireless Personal Digital Assistants (PDA), and PC tablets/laptops. While most medical devices are manufactured now with a recommended 3V/m (10V/m for life support devices) immunity level against interference from RF emissions (IEC 60601-1-2), older equipment may have inadequate shielding and, therefore, be more susceptible to interference.

Preventing Interference

Wireless devices are becoming mainstream in today’s society. The pervasiveness of the mentioned wireless devices in the medical field is unavoidable and here to stay, with more being introduced all the time. So, what is the most logical solution for hospitals? One option is to do nothing and deal with the RFI interference if and as it happens. Another option is to test every electronic-based medical device in the hospital to gauge and measure the potential of interference issues. Unfortunately, it is virtually impossible to test every combination of transmitting wireless device and electronic-based medical device.

There have been many discussions, meetings, publications, and suggestions regarding the use of portable communications equipment in the vicinity of medical equipment over the past 20 or so years. Most of these dealt with the early problems reported in infusion pumps, electric wheelchairs, pacemakers, and anesthesia machines; which due to inadequate shielding, temporarily failed while exposed to EMI.

Also, early on in the life of cellular phones, analog systems operated with quite extensive RF outputs. There were reports of interference with electric wheelchair circuits, cardiac pacemakers, and anesthesia equipment. Of course, telemetry equipment was particularly vulnerable. It was then that the fear of cellular phones in hospital began. Consequently, many hospitals totally or partially banned cell phone use in the building.

The Hazards of Electromagnetic Interference

Electromagnetic interference with electronic circuits can be dangerous in many ways. As a result, for many years, military, aircraft, and automotive electronics systems have been required to meet strict RFI requirements for immunity to up to 200 V/m, because these systems could encounter such levels during normal operations. The technology has already been developed to “harden” most medical devices against fields that are much more intense than the 3 V/m level specified in present RFI standards for medical devices. Most hardening techniques are not costly if they are incorporated into the initial design of the electronics system. Standard RF immunization techniques include the use of shielding, grounding and filtering. Shielding includes enclosing the device in metal boxes or in plastic boxes coated with metallic paint.

Grounding of electronics circuitry and cable shields is an inexpensive but necessary step toward ensuring RFI immunity. RF filtering of signal-carrying conductors, especially in sensitive patient monitoring equipment, should be performed carefully. The potential for the success of these techniques has been demonstrated in implanted cardiac pacemakers, which commonly achieve immunity of up to 200 V/m, even though these devices monitor weak electrophysiological voltages.

RF Filters and Shielding

The use of RF filters preceding the input circuitry of an implanted medical device is uncomplicated and basic. However, patient-connected medical devices, which are powered by an AC line, must accommodate the safety requirements for electrical leakage currents, as well as RFI immunity requirements. Therefore, patient connection leads on devices that obtain power from AC lines must utilize special techniques to simultaneously meet both types of safety requirements. Techniques for isolating patients, which incorporate optical or transformer coupling, may be required. In addition, designers can add interference recognition and fail-safe circuitry to their medical devices. For example, many cardiac pacemakers are protected from erratic operation by being programmed to revert to a fixed rate when RFI is detected.

Mobile RF and wireless communications systems can be optimized for compatibility with medical electronics. The modulation frequencies of RF transmitters should be outside the physiological passband of most or all medical devices. Digital modulation schemes that use time division multiple access (TDMI) and the associated amplitude modulation pulses, should be carefully designed to avoid RFI. Frequency modulation, or non-pulsed, spread spectrum modulation techniques, such as certain forms of code division multiple access (CDMA) can be used.

Managers of facilities where sensitive medical devices are used should control RFI by careful planning and system design. For example, the radiated power of many modern handheld and portable cellular phones is under the control of the base station. When close to a base station, handheld and portable phones may operate at power levels far lower than the maximum power of 600 miliWatt (for handheld phones) or 3 Watt (for portable phones). Thus, when a base station is located near a health care facility or when low power base stations (microcells) are used within the facility, cellular phones will normally operate at low power. However, the base station itself must be properly sited to avoid causing RFI. If deemed necessary, RF sources can be restricted from the more sensitive areas of a hospital, such as intensive care units.

Administrators of healthcare facilities can impose restrictions on the use of mobile RF transceivers. The concept of a specific minimum separation distance for each type of mobile transceiver has recently been proposed. For example, handheld cellular phones that radiate 600 mW would have to be kept at least one meter from a medical device that is immune to 3 V/m. A 3 Watt handheld transceiver would have to be kept 2.4 meters from the same device. In practice, an additional safety factor should be required to account for enhancement of signals by field reflections.

FDA Recommendations on Dealing with RFI Issues

FDA recommendations which were published in 1994 can still be considered as the proper guidelines on how to deal with RFI problems:

• Be aware that EMI can cause steady, momentary, or intermittent disruption of the performance of medical devices.
• Follow the recommendations of the device manufacturer for avoiding EMI.
• Purchase equipment that conforms to EMC standards.
• Consider preventing known sources of interference (e.g. cellular phones, hand-held transceivers) from coming too close to patient monitors and other sensitive electronic medical devices.
• When an EMI problem is suspected, contact the device manufacturer for assistance. Local clinical engineers may also be able to assist in identifying and correcting the problem.
• Report device problems to FDA’s MedWatch Program and note if the problem is believed to be linked to interference from a recognizable source of EM energy in the vicinity.