Biomedical Engineering Reference
In-Depth Information
Operation of medical devices and their susceptibility
to EMI
RF propagation modalities and interaction theory
Spectrum-analysis systems and technique (preferably
with signature analysis capabilities) and calibrated
antennas
Established methodology of investigating suspected
EMI problems, which includes testing protocols and
standards
Both standard test procedures adapted for the clinical
environment and personnel trained in RF behavior in-
crease the odds of proactively controlling EMI in the
clinical environment, thus providing a safer and more
effective patient care environment. The methods em-
ployed in the following procedures are variations of the
OATS technique (ECRI, 1992; ECRI, 1988), a standard
for open site testing (Southwick, 1992; Bennett, 1993;
ANSI, 1991) and ANSI C63.4-1991 (ANSI, 1991) and
ANSI C63.18-1997.
The selection of the spectrum analyzer and the op-
tions installed in it were influenced by several factors. A
spectrum analyzer of the communications system test
type was deemed desirable because there is no better
way to characterize devices that emit RFd intentional or
incidentaldand the environment in which those devices
operate. Broadband devices indicate relative RF activity
but do not indicate the operating frequencies or modu-
lation types. Both characteristics, independently and
together, affect the susceptibility of clinical devices.
A digitally based communications analyzer was needed
to archive the results of the EMI tests; both fingerprints
and footprints. The flexibility of performance require-
ments of the device was important, and as a cost-saving
benefit, Texas Children's Hospital also uses it to maintain
the hospital radio communications systems.
Since no two modified OATS environments are iden-
tical, no two results obtained under the same testing
parameters will be identical. Many factors affect the
detailed test results, including complex absorption and
reflection variables that are totally site-dependant.
environment. The hospital administrators decided that
a demonstration of EMI using actual medical equipment
would meet the program's objectives of demonstrating
the variability of effects of a common source radiator on
identical clinical devices.
Two of the same model of hemodialysis machines
from the same manufacturer were used in the demon-
stration. Under the supervision of the clinical engineer,
the machines were prepared in such a way that acces-
sories, calibrations, wiring positions within the devices,
and location within the demonstration area were as
identical as possible.
An intentional radiating source was placed in transmit
mode at a distance of 1 meter from the clinical devices.
The source was a walkie-talkie commonly encountered in
the environment (151.625 MHz frequency band, mea-
sured power output of 4 watts at the transmitter, antenna
efficiency of approximately 40%, frequency modulation
at 5-KHz deviation). The results illustrated the issue of
variable susceptibility. One device failed repeatedly in
a noncatastrophic condition, while the other device, lo-
cated nearby, was unaffected.
Demonstrations such as this, performed on demand,
tend to support the many anecdotal EMI-related reports
that our department receives daily from other institu-
tions. Regrettably, many EMI incidents go unreported
due to a lack of specific programs to address them.
The varied nature of EMI-related equipment
malfunctions and the associated risks mandate a pro-
active program of EMI identification and methodology
for risk reduction. An effective program relies on the
cooperation of all parties potentially affected by EMI;
medical staff, plant engineering, information services,
biomedical engineering, and device manufacturers. Due
to the highly diversified knowledge and experience re-
quired to coordinate detection and mitigation of EMI,
the clinical engineering department and its personnel
experienced in RF must take responsibility.
An operational protocol must be developed to address
EMI issues. (See
Figure 5.6-2
.) This provides a defined
structure to process requests for EMI investigations and
a structure for processing and reporting the results of the
investigations.
Footprinting
At Texas Children's Hospital, the EMI testing program
has evolved from years of experience and analysis. This
program is not static. Sources of EMI and susceptibility
characteristics of devices are in constant change. As
new threats arise, the plan is periodically reviewed and
modified to contend with them. Further modifications to
the plan and procedures are based on a continuous review
of wireless industry trends. They represent a proactive
response to perceived future threats. At present the
program consists of the procedural plan outlined above
and three series of tests: (1) area characterization,
Programs and procedures
Testing for electromagnetic compatibility (EMC) in the
clinical environment introduces a host of complex con-
ditions not normally encountered in laboratory situa-
tions. In the clinical environment, various RF sources of
EMI may be present anywhere. Isolating and analyzing
the impact from the sources of interference involves
a multidisciplined approach based on training in and
knowledge of the following:
