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Medical Device Interference

“Medical devices” are electronic devices either implanted in the human body or worn on the body with some type of delivery system, such as a pump that supplies medicine. Pacemakers were the first implanted medical devices. When pacemakers first came into use, there were concerns that they would malfunction in the presence of significant electromagnetic fields. At the same time, the first microwave ovens were appearing in homes. And these early microwave ovens tended to leak energy around the edges of the door when food got trapped in the RF gasket that was supposed to prevent RF leaks. Thus, it was common to see signs warning people with pacemakers that a microwave oven was in use.

 

Today, microwave ovens rarely leak. And the device manufacturers have made pacemakers more immune to interference from electromagnetic energy. But the problem with medical device interference has grown far more complicated for two reasons.

  • The numbers and types of medical implants have grown dramatically. Microminiaturization has revolutionized the medical device industry, resulting in smaller devices requiring less power but capable of performing more functions.

  • There has been tremendous growth in the sources of RF energy. Twenty years ago, people did not carry cell phones, pagers, or laptop computers with wireless modems installed.

Over the years, many incidents of suspected electromagnetic interference (EMI) with medical devices have been documented. Defibrillators are one type of medical implant that has had problems due to electromagnetic interference that has been well documented in medical journals. There is heightened concern for the safe and effective use of devices in an environment that has become crowded with potential sources of EMI. Because of its concern for public health and safety, the Center for Devices and Radiological Health (CDRH), which is part of the Food and Drug Administration (FDA), has been in the vanguard of examining medical device EMI and providing solutions. Extensive laboratory testing by CDRH and others has revealed that many devices can be susceptible to problems caused by EMI. The CDRH began investigating incidents with the advent of cardiac pacemakers in the late 1960s. According to the CDRH:

“The key to addressing EMI is the recognition that it involves not only the device itself but also the environment in which it is used, and anything that may come into that environment. More than anything else, the concern with EMI must be viewed as a systems problem requiring a systems approach. In this case, the solution requires the involvement of the device industry, the EM source industry (e.g., power industry, telecommunications industry), and the clinical user and patient. The public must also play a part in the overall approach to recognizing and dealing with EMI. Electromagnetic compatibility, or EMC, is essentially the opposite of EMI. EMC means that the device is compatible with (i.e., no interference caused by) its EM environment, and it does not emit levels of EM energy that cause EMI in other devices in the vicinity. The wide variation of medical devices and use environments makes them vulnerable to different forms of EM energy which can cause EMI: conducted, radiated, and electrostatic discharge (ESD). Further, EMI problems with medical devices can be very complex, not only from the technical standpoint but also from the view of public health issues and solutions.”

The strength of an electromagnetic field at a given distance from the source is proportionate to the radiated power from the source and inversely proportionate to the distance. In many cases, the field strength falls off inversely with the square of the distance from the source. The CDRH gives some good examples to illustrate the point. Note that an electric field strength of 3 V/m (Volts per meter) is often referenced because it is the most common standard that is cited for medical device immunity.

  • The relatively low-power cellular telephone can create a field strength of 3 V/m at one meter.

  • A hand-held CB transceiver creates the same field strength at 5 meters (16.4 feet).

  • A high-power TV transmitter creates this same field strength at a distance of 1,000 meters (1 km).

It is important to note that device susceptibility can be very frequency dependent. This makes it much more difficult and expensive for which to test. For example, a particular device might have good immunity at all but one or two narrow frequency ranges. Unless tests are made in small increments of frequency, the problem might not be detected. Conversely, the problem might never occur in the real world because there is nothing operating at the problem frequencies.

The EM environment that surrounds the devices can vary widely, from the rural setting to the urban setting, to the commercial setting, and of course, the hospital setting. The International Electrotechnical Commission (IEC) has classified the EM environment into eight areas and defined the typical EM environment in each area. Within each area, there are conditions for the location and power of local EM energy sources (e.g., transmitters), which if exceeded would result in higher EM field strengths. Table 1 indicates the general classifications and the upper ranges of radiated EM field strength specified for each environment.

While the major focus of the CDRH and the IEC has been in hospital settings where there is a great deal of sensitive medical devices (not just implanted medical devices), there is also a concern for people who work in or visit RFR “sites.” Consider that even if a site is fully compliant with any of the major standards, these standards are designed to protect people from the biological effects of RF radiation—not from EMI with their medical implants. For example, most of the major standards have limits in the human resonant region of 1.0 mW/cm² for Occupational exposure and one-fifth of that level (0.2 mW/cm²) for General Population exposure. An equivalent power density of 0.2 mW/cm² is equal to an electric field strength of 27.4 V/m. Looking at it the other way, a 3 V/m field is equivalent to an equivalent power density of 0.0024 mW/cm². This is about 1 percent of the public limits and 0.2 percent of the occupational limits!


Table1. IEC ElectroMagnetic Environments
Classification Signal Strength
Residention: Rural up to 3 V/m
Residential: Urban up to 10 V/m
Commercial up to 10 V/m
Light Industrial up to 3 V/m
Heavy Industrial up to 30 V/m
Traffic up to 30 V/m
Dedicated Communications Center up to 1 V/m
Hospital up to 3 V/m


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