Biomedical Engineering Reference
In-Depth Information
Connectors of implantable devices, which connect the
devices and leads, must be identified by their type. Connector
retention force shall be
voltages are induced in proportion to the field frequency. The
induction loop area is considerably larger in unipolar lead
conductors than in bipolar conductors.
At higher frequencies, leads function as dipole antennas.
In bipolar leads, voltages are induced between lead proximal
and distal electrodes. EMI susceptibility tests are carried out
by means of a voltage clamp. The frequency range of the test
voltages is wider than the frequency range of physiological
signals, and as the voltage level grows, an implantable device
may start detecting interference at a certain level. This may
affect the therapeutic behavior of the implantable system
(e.g., pacing inhibition, tachycardia detection). More test
signals are applied, taking into consideration various inter-
ferences a person may be exposed to at public places.
Per the ANSI/AAMI PC69 standard, a procedure is applied
to testing radiation susceptibility with use of mobile phones.
During this test, a device together with all leads is immersed
into saline solution, simulating body tissue, and is exposed to
a near-field of an electric dipole. According to ANSI/AAMI
PC69, two field levels are applied for testing. The lower level
tests the system's function upon exposure to a mobile phone
with 2 W output at a distance of 15 cm. Passing the test is
mandatory. The higher level tests the exposure in the distance
of 2 cm, which simulates a mobile phone placed above an
implanted device. Passing the test is not mandatory yet. Per
ANSI/AAMI PC69, the test signal is modulated. This test
also guarantees susceptibility in the far-field of powerful
transmitters, such as mobile phone base stations.
As far as protection from magnetic fields is concerned, the
device must be susceptible to exposure to weak magnetic
fields up to the value of magnetic induction 1 mT to prevent
activating the magnetic switch. Nevertheless, strong magnetic
fields (more than 50 mT) may interfere with the operation of
an implantable medical device. The magnetic switch may be
activated and telemetry may be interfered with or a therapy
may be suppressed. The manufacturer must also assess the
risk arising from permanent activation of the magnetic switch.
The tests usually are conducted up to the effective value of the
magnetic fi eld intensity of 150 A/m [ 17, 72 ] .
³
7.5 N [ 17, 72 ] .
14.2.3 EMI Susceptibility Requirements
As mentioned earlier, external electromagnetic fields may
induce currents flowing from leads to the heart, which may
result in fibrillation or local overheating. Moreover, the fields
may induce voltage in the lead conductor, potentially damag-
ing the implantable device or preventing it from properly
sensing the intracardial signal. Thus, implantable systems
ought to have a certain susceptibility to electromagnetic
interference.
Technical standards [ 17, 72 ] de fi ne test procedures and
reference levels to protect implanted systems from:
Damage or fibrillation caused by currents induced directly
•
in implanted leads or conducted by interference currents
from the device
Permanent failure caused by voltages induced in implanted
•
leads
Unacceptable changes or operation modes caused by
•
voltages induced in implanted leads
Temporary changes in the device's therapeutic behavior
•
caused by voltages induced in implanted leads
Temporary changes in the device's therapeutic behavior
•
caused by weak static magnetic fields (1 mT) affecting
any magnetic-sensitive components of an implantable
device
Permanent failure caused by stronger static magnetic
•
fields (50 mT) affecting any magnetic-sensitive compo-
nents of an implantable device
Persisting malfunction of the device caused by time-vary-
•
ing magnetic fields applied to an implantable device
EMI susceptibility tests are conducted in several fre-
quency bands, ranging from 16.6 Hz to 3 GHz. Special atten-
tion is paid to certain frequencies used in power engineering.
Authors of technical standards have agreed on a bipolar sen-
sitivity of 0.3 mV as suitable for EMI testing with frequen-
cies of more than 1 kHz. In patients with ICDs, sensing
sensitivity set below 0.3 mV increases the risk of undesirable
sensing of remote signals.
Permissible human exposure to an electromagnetic field
is limited by a number of both national and international
directives and recommendations. Previously, standards only
applied to a limited range of frequencies (up to 30 MHz).
Today, the standard also includes higher frequencies to cover
contemporary telecommunication devices. Implanted leads
function as antennas in the electromagnetic field. Voltages
induced in a lead depend on the position and characteristics
of the lead as well as frequency, polarization, and the orien-
tation of the electromagnetic field. At low frequencies up to
several megahertz, each lead forms a conducting loop, where
14.3
Ionizing Radiation Effects
The operation of pacing devices may also be affected by ion-
izing radiation. In addition to possible EMI by ionizing radi-
ation sources (e.g., radiology devices), ionizing radiation
may effect the operation of device electronics and the bat-
tery. Recent devices making use of complementary metal
oxide semiconductor technology are, however, much less
sensitive to the ionizing radiation. Based on the results of
studies prepared by the American Association of Physicists
in Medicine, recommendations were issued in 1994; these
are usually included in the manufacturer's documentation
but are not generally known.
