Biomedical Engineering Reference
In-Depth Information
contraction). In healthy humans, the stroke volume of the heart has been found to remain
relatively constant over a wide range of exertion.
Now, cardiac output equals stroke volume multiplied by heart rate. Increases in cardiac
output required to meet physiologic needs are provided primarily by increased heart rate, but
some of the cardiac output during exertion is also provided by the heart, increasing its stroke
volume. The stroke volume cannot increase, however, by a factor more than about 2 to 2
1
2
.
Beyond a certain rate, the heart does not
ciently, and increased heart rate beyond
this point can actually result in decreases of stroke volume and ultimately, cardiac output.
The point at which further increase in heart rate does not result in an increase in stroke vol-
ume is typically shifted markedly toward low rates for patients su
fi
fill up su
ering from diseases of the
heart muscle. The idea then is to use an impedance sensor to estimate stroke volume, and
adjust the pacemaker's base rate and other parameters to optimize cardiac output.
ff
IMPEDANCE TECHNIQUE
Impedance plethysmography
(also known as
impedance rheography
) is one of the oldest
applications of impedance measurement on living tissues. It is based on the fact that the
impedance of body segments re
filling state of the blood vessels contained. This
principle has been used in such diverse applications as monitoring cardiac hemodynamics
(impedance cardiography), monitoring lung function and perfusion (rheopneumography),
and monitoring cerebral blood
fl
ects the
fi
fl
flow (rheoencephalography). Since the conductivity of the
body depends on the
fluid content in various intracellular and extracellular compartments,
body composition estimates can also be made using impedance measurements by assum-
ing that bodily
fl
fluids subdivide the body mass into fat mass and lean body mass. In addi-
tion, impedance measurements are not limited to estimating the volume of bodily cavities.
Bioimpedance techniques have also been used at the level of individual cells and small
groups of cells to discriminate pathological states from changes in their equivalent resis-
tive and capacitive parameters. For example, such changes have been detected in cancer-
ous cells.
In the case of intracardiac impedance measurements, the impedance between two elec-
trodes in the ventricular blood pool decreases as the ventricle is
fl
filled (since there are more
conduction pathways for electrical currents), reaching a minimum at end diastole. At end
systole, when the ventricle has expelled as much blood as possible, impedance measure-
ments reach their highest values. Figure 8.14 depicts the most common methods for meas-
uring the impedance of tissues. Here a constant-current source injects an ac current of
constant amplitude into the tissue through two current-injection electrodes. This current
causes a potential di
fi
erence to be developed between any two points between the current-
injection electrodes. This potential di
ff
erence is related to the resistivity of the tissue
between the voltage-sensing electrodes. The
equivalent resistance
is de
ff
fi
ned as the ratio of
the voltage di
ff
erence between the two voltage electrodes and the current
fl
flowing through
the tissue.
Two-terminal measurements introduce some errors because the potential di
erence
sensed between the two electrodes includes nonlinear voltages generated by the current
fl
ff
flowing through the polarization impedance at the electrode-tissue interface. The four-
electrode con
fi
guration yields a more precise measurements since the highly nonlinear
e
ects of electrode-tissue contact impedance are reduced, as the sites of current injection
and voltage measurement are physically separated. With a constant-current source, the
injected boundary current becomes essentially independent of the contact impedance.
Using voltage ampli
ff
fi
ers with su
ciently high input impedance ensures that voltage meas-
urements are virtually una
ff
ected by the electrode-tissue contact impedance. For example,
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