Biomedical Engineering Reference
In-Depth Information
1 4
Electromagnetic Compatibility
and Technical Requirements
In various environments in which patients might find
themselves (home, means of transport, a medical center, a
workplace), electromagnetic fields may occur. The external
fields may affect electronic devices that pacing technology
primarily makes use of, and this phenomenon is called elec-
tromagnetic interference (EMI). Directives on medical
devices require active implantable medical devices to be
designed and produced so that EMI-related risks are
eliminated or minimized.
immediate effect. At these frequencies, the human body is
almost transparent for the magnetic component of the field;
magnetic field of internal tissue is thus level with external
magnetic field. At low frequencies (below 100 kHz), the elec-
tric component is much weaker than the external field because
tissue is conductible, and the human body tissue attenuates
electromagnetic fi elds. Starting from frequencies of around
5 MHz, body tissue attenuates electromagnetic fields. At fre-
quencies over 10 MHz, the tissue is heated at lower levels.
Contrary to nerve stimulation, heating is an accumulative
process. For frequencies ranging from 100 kHz to 10 MHz,
both effects (stimulation and heating) may occur.
In people with implants, the field effects on body tissue
may intensify. A metal implant may heat up and, as a conse-
quence, heat the surrounding tissue. The field may also
increase the current density in the body tissue around the
device. To eliminate clinically significant effects, the density
of the induced current must be reduced.
Heart tissues are connected to pacing devices by leads. As
far as electric and magnetic components are concerned, the
inductive methods and voltage values in the leads depend on
the configuration (unipolar or bipolar) of the applied pacing
and dimensions. The bipolar configuration is far less sensi-
tive to interference than the unipolar configuration. The field
evaluation criterion is thus based on the EMI-sensitivity of
pacemakers with unipolar leads. Bipolar lead systems
(defibrillators and the majority of pacemakers) are less sensi-
tive to interference. An external electromagnetic field may
induce voltage and currents in the lead, which will be sensed
by the device. In stronger fields, these signals will be evalu-
ated as an intrinsic cardiac activity and will result in inhibi-
tion of pacing or detection of tachycardia. Moreover, pacing
devices make use of inductive coupling or a radio connection
to communicate with a programmer. If the frequencies of
external electromagnetic fields are close to those used for
communication with the programmer, the communication
may be interfered with or disrupted. In such cases, the pro-
grammer must be placed further from electrical devices and
cables must not be crossed.
14.1
The Electromagnetic Field Effects
on Pacing Technology
Electromagnetic fields are characterized mainly by a domi-
nant frequency and intensity. Their intensity decreases with
increasing distance from a radiation source. Such a decrease
in intensity depends on whether a person is in a near or far
radiation field of the source. In most cases, people find them-
selves in the far-field; however, on certain occasions, it may
be the opposite. The distance from the source where the near-
field passes into the far-field depends on the relationship
between the size of the field source and the radiation wave-
length. In the near-field, the intensity is, as a rule, inversely
proportional to the third power of the distance; in the far-
field, the intensity is inversely proportional to the first power
of the distance. In fact, the situation is more complex -for
example, the source geometry, whether the electric or mag-
netic component of the electromagnetic field prevails also
matter. In general, then, the electromagnetic susceptibility of
pacing technology depends on maximum field amplitudes
rather than effective values.
Even in people without implanted active implantable
medical devices (AIMDs), the external electromagnetic field
induces currents in body tissue. Depending on the frequency,
the external field may stimulate nerves, damage cell mem-
branes, or heat tissues. At frequencies below 100 kHz, nerves
may be paced at lower levels of the field. The effects on
nerve stimulation decrease with the frequency and have an
