Biomedical Engineering Reference
In-Depth Information
19
Electromagnetic Interference
JOSEPH H. McISAAC, III
19.1 Overview of EMI
The modern world is increasingly dependant upon
electronics. It is difficult today to find a useful
device that is not dependent on the transistor
or integrated circuit. From the computer to the
cellular telephone to the automobile, the common
theme is the extensive use of low-powered digital
electronics. Hospitals make even greater use of
these devices than average. Even the ubiquitous
paper chart is giving way to the electronic medical
record. Along with this convenience comes the
risk of electromagnetic interference (EMI). EMI
is not new, European agencies established stan-
dards as early as 1936. In the United States,
EMI requirements were instituted via MIL-STD-
461 for government work and in 1979 by the
Federal Communications Commission under Part
15 regulations [1,2]. Presently, medical devices
are covered under the Food and Drug Adminis-
tration (US) [3] and International Electrotechnical
Commission (Europe) [4,5] (see Figure 19.1).
The EMI can be natural (static electricity, light-
ning) or man-made. It can be intentional, such
as the effect from a high altitude detonation of
a nuclear weapon (HEMP) or unintentional, as a
byproduct of a digital circuit (see Figure 19.2). It
can be radiated or conducted [1].
All electronic devices function according to
Maxwell's Laws [1]. The movement of elec-
tric charges generates an electromagnetic field
which propagates through space as a wave. The
wave contains both an electric field vector and a
magnetic field vector which are coupled together
(see Figure 19.3). Either
an electronic device through capacitive (E-field)
or inductive (B-field) coupling; the device func-
tioning as an antenna.
Narrow-band (<1% center frequency) RF from
0.2 to 5 GHz tends to be very high energy and
produce permanent failures. Wideband RF consists
of short, often repetitive pulses over much broader
spectrum. Energy is less concentrated but more
likely to find system resonance and thus disrupt
function. Frequencies below 10MHz propagate
Association for the Advancement of Medical Instru-
mentation, http://www.aami.org
•
American Medical Association Council on Scien-
tific Affairs, “Report 4: Use of wireless radio-
frequency devices in hospitals,” http://www.
ama-assn.org/ama/pub/article/2036-2918.html
•
ANSI Accredited Committee C63, http://c63.
ieee.org/
•
ECRI, http://www.ecri.org
•
FDA/CDRH EMC web page, http://www.fda.gov/
cdrh/emc/index.html
•
FDA/CDRH recommendations for wireless medical
telemetry, http://www.fda.gov/cdrh/emc/wmt2.html
•
FDA/CDRH Safety Alerts, Public Health Advisories,
and Notices, http://www.fda.gov/cdrh/safety.html
•
FDA MedWatch safety information and adverse
event
reporting
program,
http://www.fda.gov/
medwatch/
•
University of Oklahoma Center for the Study
of Wireless EMC, http://www.ou.edu/engineering/
emc/
•
UK
Medical
Devices
Agency,
“Mobile
Communications - Summary”
Figure 19.1
FDA Recommended Sites for Resources and
field can couple to
Standards.
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