Biomedical Engineering Reference
In-Depth Information
A low frequency respiratory component; (fundamen-
tal
<
1Hz).
A higher frequency cardiac component;
(fundamental
ΒΌ
1-3 Hz).
A calculated mean impedance; (0Hz).
As shown in
Fig. 4.1-13
, the higher frequency cardiac
component is superimposed on the low frequency re-
spiratory component. The higher frequency cardiac
component represents the impedance change during each
cardiac cycle that occurs immediately following the QRS
deflection on the electrocardiogram. The low frequency
respiratory component represents the impedance change
during respiration, due to the expansion of the lungs and
thorax. Moreover, each component has a different fun-
damental frequency, typically 1.0-3.0 Hz for cardiac ac-
tivity and 0.1-1.0 Hz for respiratory activity (Hettrick
and Zielinski, 2006). This frequency differentiation
allows extraction of each signal by specific filtering tech-
niques. Mean impedance, is represented by a ''DC shift''
in impedance, changing according to the amount of static
conductive fluid in the electrode vector lead field config-
uration as a function of time (hours/days).
More specifically, change in impedance waveform
morphologies may be an indicator of change in blood
volume, interstitial volume or tissue integrity. Deviations
in the impedance waveform morphologies such as in the
positive or negative slope, time duration between
minimum and maximum magnitudes, delta between
the minimum and maximum magnitudes, changes in the
minimum and maximum first derivative, changes in the
area under a specific waveform or other deviations in
the waveformmorphology of complex impedance may be
indicative of a vector specific change in chamber or vessel
blood volume, such as in heart disease, tissue degradation,
such as in myocardial ischemia, or interstitial fluid accu-
mulation, such as in peripheral or pulmonary edema, all
secondary to cardiac, vascular or renal disease.
are currently employed. Improved circuit designs planned
for future generation devices will be ideally suited for
implantable impedance applications where high resolu-
tion, real-time complex impedance waveform data is re-
quired. Second, impedance may be able to provide useful
diagnostic information about multiple physiological pa-
rameters including heart rate, CO, respiratory rate,
minute ventilation, thoracic fluid accumulation, myocar-
dial contractility and ischemia detection. Third, imped-
ance is a well established sensing means that it may
provide relevant clinical diagnostic information used in-
dependently or in conjunction with other sensors such as
pressure transducers or accelerometers.
4.1.11.1 Physiological impedance
components
Impedance signals are acquired from selected implant-
able electrode vector configurations, defined in this
context as the electric field generated by the injection
current field electrodes and the voltage measured by the
sense field electrodes. Signals acquired from each elec-
trode vector configuration can be either bipolar where
the injection current electrodes and sense field elec-
trodes are the same, or tetrapolar, where the injection
current electrodes and sense field electrodes are isolated
from each other. Electrodes in implantable pacing de-
vices consist of unipolar or bipolar electrodes positioned
on the distal end of conventional pacing leads. Pacing and
ICD leads can be implanted in the right atrium, right
ventricle, superior vena cava and left ventricular cardiac
vein.
Data acquired from the various possible electrode
vectors typically contain three major physiological com-
ponents that may provide useful information for di-
agnostics or implantable device control:
High frequency
cardiac component
Inspiration
Mean impedance
Low frequency
respiratory component
Expiration
Time
Figure 4.1-13 Simulated impedance waveform consisting of a higher frequency cardiac component superimposed on a low frequency
respiratory component: the dotted line is the calculated mean impedance (measured during two respiratory cycles. An implantable
impedance sensor may be able to leverage all three signal components in order to provide useful diagnostic or device control information.
