Biomedical Engineering Reference
In-Depth Information
relatively simple electronic circuits. Here comes a story: Circa 1995, there were various
reports about implantable de
brillator patients being shocked when passing through elec-
tronic article surveillance (EAS) systems, such as those used in stores against shoplifting.
There were also reports about pacemaker patients who fainted in the proximity of these
systems, presumably because their pacemakers were inappropriately inhibited by EMI
from the EAS.
EAS systems consist of electromagnetic
fi
fi
field emitters which illuminate and interrogate
a uniquely identi
fi
able “tag” which is a
xed to an object. When the tagged object is in the
field produced by the EAS equipment, the EAS sensors detect its presence and activate a
response. Back in our Intermedics
3
days, we conducted much of our research together with
Dr. Thomas Fåhræus in Lund, Sweden. He became very interested in the potential problem
of pacemaker interference by EAS systems. After all, EAS systems are widely used to
track and monitor merchandise, for inventory control and theft prevention, and in some
cases, to track living specimens. Because of their widespread use in retail stores and com-
mercial establishments, wearers of implantable medical devices have a high likelihood of
entering the
fi
fields produced by such systems.
Thomas terminated a ventricular bipolar pacemaker lead with a LED to be able to tell
when the pacemaker paced normally, and built a simple setup simulating an implant
con
fi
guration (Figure 6.29). He programmed the pacemaker to the VVI mode and a low
pacing frequency (e.g., 40 ppm). He then toured the shops in Lund looking for signs of
EAS interference on his pacemaker implant simulation. If the LED stopped blinking, that
meant that the EAS system was inhibiting the pacemaker. If the LED started to blink rap-
idly (e.g., 70 ppm), on the other hand, that meant that the pacemaker had entered its noise
reversion mode.
Thomas called us in alarm when he found that most of the EAS systems in town inter-
fered with his pacemaker setup. However, that observation was not consistent with what
we were seeing with real implants. Patients were not falling dead like
fi
flies when walking
through EAS systems. In fact, most of the cell phone and EAS interference reports were
highly anecdotal, to the point where they reeked of urban myth. It wasn't di
fl
cult to
explain the di
erence between the test setup and a real implant. The LED was acting as
the detector diode in a crystal radio. The interfering signals were really generated by the
LED's nonlinearity, an element that is negligible in the real implant situation.
Thomas was very ingenious in replacing his pacing detector by a more realistic kind. He
ff
fi
figured out that the tongue can detect the pacing pulses while making the electrode-tissue
interface similar to that of the real implant. This time he made us all accompany him with
di
erent pacemaker models on his interference hunting trip. It was not even April 1, and
there we were, in the best stores of downtown Lund, with wires stuck in our mouths, mov-
ing bizarrely next to the shoplifting gates. Good that everyone in town knew that Fåhræus
was a respectable physician. End of the story: Our not-too-scienti
ff
c study found no
evidence of pacemaker interference by EAS systems. No one really wanted to check
whether or not a de
fi
fi
brillator would react di
ff
erently.
ects of electromagnetic environments on
implantable medical devices, it is essential that the measurements be performed while oper-
ating the test devices in an environment that simulates the absorption and shielding charac-
teristics of the human body. A common physical test environment consists of a 0.027
M
saline solution in a rectangular tank that is essentially transparent to the incident radiated
fi
This is why when determining the e
ff
field. The 0.027
M
NaCl concentration has a resistivity of approximately 375
Ω
cm, which
is used to simulate the electrical characteristics of the human body tissue and
fl
fluid.
3
Intermedics was the world's third-largest implantable cardiac pacemaker/de
fi
brillator company. It was acquired
by Guidant Corporation in 1998.
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