12 Biomedical Engineering Fundamentals
585
the cross-sectional dimensions. In impedance cardiography method, an electric
current is introduced and the resulting voltage is measured across the axial direction
of the thorax while most of electric current follows the path of least resistance
and seeks the path of blood-filled aorta. The thoracic impedance is represented
by measured voltage. During the cardiac cycle, the volumetric changes of the
aorta induced thoracic impedance changes allowing for the determination of the
cardiac stroke volume in absolute units. This method is also known as electric
field plethysmography. In another pressure pulse contour method for blood flow
registration, a generalized transfer function is used to derive the aortic pressure
from the radial pressure and then the aortic flow obtained with applying an aortic
impedance model. This model indicates the ratio of aortic pressure and flow. During
a cardiac cycle, the stroke volume is the integral of the flow waveform. This method
requires a previous calibration to achieve absolute blood flow values [34].
Arterial Radius
The mean value of the arterial radius and its pulsatile waveform are interesting
topic in physiological phenomena. There are various invasive and noninvasive
methods for the monitoring of arterial radius like blood pressure and blood flow.
The methods used to calculate arterial radius are somewhat similar which are used
for blood pressure measurement. However, the measurement of arterial radius is
more sensitive than that of blood pressure measurement due to the reason that the
radius changes up to 10%, while blood pressure may change up to 50% [34].
The invasive methods for the measurement of arterial radius are based on
resistance/inductive strain gauges, photoelectric devices, and transit-time ultrasonic
approach [34].
In the first method, the resistance/inductive strain gauges are fixed directly to
the outer artery wall or even inserted into the artery in order to measure radius
by the catheter. The second method is based on photoelectric devices in which a
pulsating artery casts a shadow on a photocell. The transit-time ultrasonic approach
is similarly used like those for blood flow registration. In this method, the two
ultrasound transceivers are placed opposite to each other on the outer sides of the
arterial wall. The time taken between the impulse emission and its reception on the
opposite side is proportional to the arterial radius [34].
The most popular noninvasive methods for the registration of arterial radius are
based on ultrasonic beams, optical plethysmography, and mechanical plethysmography [34].
In ultrasonic beam method, the reflections of the ultrasound waves are used
and the time taken between the impulse emission from the ultrasound probe on
the skin and reflected impulse reception from both arterial walls is calculated,
which delivered the arterial radius. The method based on optical plethysmography
is an indirect method to assess the local pulsatile volume of the transilluminated
artery. In this method, the arterial radius increases with each blood pulse and the
transilluminated region encloses with an increased ratio of blood which strongly
585
the cross-sectional dimensions. In impedance cardiography method, an electric
current is introduced and the resulting voltage is measured across the axial direction
of the thorax while most of electric current follows the path of least resistance
and seeks the path of blood-filled aorta. The thoracic impedance is represented
by measured voltage. During the cardiac cycle, the volumetric changes of the
aorta induced thoracic impedance changes allowing for the determination of the
cardiac stroke volume in absolute units. This method is also known as electric
field plethysmography. In another pressure pulse contour method for blood flow
registration, a generalized transfer function is used to derive the aortic pressure
from the radial pressure and then the aortic flow obtained with applying an aortic
impedance model. This model indicates the ratio of aortic pressure and flow. During
a cardiac cycle, the stroke volume is the integral of the flow waveform. This method
requires a previous calibration to achieve absolute blood flow values [34].
Arterial Radius
The mean value of the arterial radius and its pulsatile waveform are interesting
topic in physiological phenomena. There are various invasive and noninvasive
methods for the monitoring of arterial radius like blood pressure and blood flow.
The methods used to calculate arterial radius are somewhat similar which are used
for blood pressure measurement. However, the measurement of arterial radius is
more sensitive than that of blood pressure measurement due to the reason that the
radius changes up to 10%, while blood pressure may change up to 50% [34].
The invasive methods for the measurement of arterial radius are based on
resistance/inductive strain gauges, photoelectric devices, and transit-time ultrasonic
approach [34].
In the first method, the resistance/inductive strain gauges are fixed directly to
the outer artery wall or even inserted into the artery in order to measure radius
by the catheter. The second method is based on photoelectric devices in which a
pulsating artery casts a shadow on a photocell. The transit-time ultrasonic approach
is similarly used like those for blood flow registration. In this method, the two
ultrasound transceivers are placed opposite to each other on the outer sides of the
arterial wall. The time taken between the impulse emission and its reception on the
opposite side is proportional to the arterial radius [34].
The most popular noninvasive methods for the registration of arterial radius are
based on ultrasonic beams, optical plethysmography, and mechanical plethysmography [34].
In ultrasonic beam method, the reflections of the ultrasound waves are used
and the time taken between the impulse emission from the ultrasound probe on
the skin and reflected impulse reception from both arterial walls is calculated,
which delivered the arterial radius. The method based on optical plethysmography
is an indirect method to assess the local pulsatile volume of the transilluminated
artery. In this method, the arterial radius increases with each blood pulse and the
transilluminated region encloses with an increased ratio of blood which strongly