12 Biomedical Engineering Fundamentals
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external pressure sensor. Although these methods are precise and direct, they are
not popular due to their invasiveness and related complications for routine use.
On the other hand, the noninvasive methods are popular for the determination of
blood pressure characteristics. These methods include the auscultatory method,
oscillometric method, volume clamp method, and tonometric method [34].
In auscultatory method, the Korotkoff sounds are detected by a stethoscope to
determine systolic and diastolic values. In this method, an inflatable cuff encircles an
extremity (upper arm) and the cuff pressure is increased until a complete cessation
of downstream blood circulation is observed. The first release of the cuff pressure
after cessation resulted in the Korotkoff sound which indicates the time instant when
the upper (systolic) part of the blood pressure pulse wave passes under the cuff and
the cuff pressure is equal to systolic value. On the other hand, the transition from
muffling to silence indicates the time instant when the lower (diastolic) part of the
pulse wave passes and at this time instant the cuff pressure is equal to diastolic
value [34].
The second method is the successor of the ancient mercury sphygmomanometer.
It is based on the principle that the pulsatile blood flow generates radial oscillations
of the arterial vessel wall. These radial oscillations are transmitted to the cuff
encircling an extremity and then to a pressure sensor kept inside it. During
the deflation, the intra-arterial blood pressure exceeds the cuff pressure and the
oscillations of the vessel walls are strengthened due to turbulent flow of blood and
progressing arterial decompression. The cuff pressure during the initial increase in
oscillation amplitude is proportional to the systolic value and the diastolic value is
proportional to cuff pressure value at the time of subsequent rapid decrease in the
oscillations. In this method, the maximal amplitude of the oscillations for the vessel
walls and cuff pressure is obtained when the cuff pressure passes the mean arterial
pressure [34].
The volume clamp method is another noninvasive method in which a miniaturized cuff fixes on a finger. This cuff is equipped with as optical transmission
sensor. This method is based on the principle that the radius (volume) of the
finger artery tends to increase at the time of the blood pressure (volume) pulse
and this increased radius is detected by transmitted light intensity. Afterward, the
cuff pressure (volume) is increased just enough to keep the radius and transmural
pressure constant. The resulting cuff pressure is proportional to blood pressure
waveform because the cuff pressure follows the intra-arterial pressure up to a
constant factor at constant transmural pressure. A pneumatic feedback system is also
used in this method for cuff pressure (volume) control so that a maximum pulsatile
change of the vessel radius is achieved when transmural pressure approaches zero.
The cuff pressure pulsations roughly equal to intra-arterial pressure at the time
of zero transmural pressure. The main advantage of this method is that it does
not require previous calibration with patients to attain absolute blood pressure
values [34].
The last one method is the tonometric method which is a successor of the ancient
sphygmograph. In this method, a rounded probe over a superficial (radial or carotid)
artery which has a backside support of bone is pressed, allowing the artery to be
583
external pressure sensor. Although these methods are precise and direct, they are
not popular due to their invasiveness and related complications for routine use.
On the other hand, the noninvasive methods are popular for the determination of
blood pressure characteristics. These methods include the auscultatory method,
oscillometric method, volume clamp method, and tonometric method [34].
In auscultatory method, the Korotkoff sounds are detected by a stethoscope to
determine systolic and diastolic values. In this method, an inflatable cuff encircles an
extremity (upper arm) and the cuff pressure is increased until a complete cessation
of downstream blood circulation is observed. The first release of the cuff pressure
after cessation resulted in the Korotkoff sound which indicates the time instant when
the upper (systolic) part of the blood pressure pulse wave passes under the cuff and
the cuff pressure is equal to systolic value. On the other hand, the transition from
muffling to silence indicates the time instant when the lower (diastolic) part of the
pulse wave passes and at this time instant the cuff pressure is equal to diastolic
value [34].
The second method is the successor of the ancient mercury sphygmomanometer.
It is based on the principle that the pulsatile blood flow generates radial oscillations
of the arterial vessel wall. These radial oscillations are transmitted to the cuff
encircling an extremity and then to a pressure sensor kept inside it. During
the deflation, the intra-arterial blood pressure exceeds the cuff pressure and the
oscillations of the vessel walls are strengthened due to turbulent flow of blood and
progressing arterial decompression. The cuff pressure during the initial increase in
oscillation amplitude is proportional to the systolic value and the diastolic value is
proportional to cuff pressure value at the time of subsequent rapid decrease in the
oscillations. In this method, the maximal amplitude of the oscillations for the vessel
walls and cuff pressure is obtained when the cuff pressure passes the mean arterial
pressure [34].
The volume clamp method is another noninvasive method in which a miniaturized cuff fixes on a finger. This cuff is equipped with as optical transmission
sensor. This method is based on the principle that the radius (volume) of the
finger artery tends to increase at the time of the blood pressure (volume) pulse
and this increased radius is detected by transmitted light intensity. Afterward, the
cuff pressure (volume) is increased just enough to keep the radius and transmural
pressure constant. The resulting cuff pressure is proportional to blood pressure
waveform because the cuff pressure follows the intra-arterial pressure up to a
constant factor at constant transmural pressure. A pneumatic feedback system is also
used in this method for cuff pressure (volume) control so that a maximum pulsatile
change of the vessel radius is achieved when transmural pressure approaches zero.
The cuff pressure pulsations roughly equal to intra-arterial pressure at the time
of zero transmural pressure. The main advantage of this method is that it does
not require previous calibration with patients to attain absolute blood pressure
values [34].
The last one method is the tonometric method which is a successor of the ancient
sphygmograph. In this method, a rounded probe over a superficial (radial or carotid)
artery which has a backside support of bone is pressed, allowing the artery to be