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Theory
of CardioVision Operation
1. CardioVision measures blood pressure, records pulse, pulse pressure
[systolic minus diastolic blood pressure] and provides additional
cardiovascular information.
Blood
pressure is a function of cardiovascular dynamics, which are
determined by the strength of the heartbeat and the overall condition
of the arterial system.
2. CardioVision utilizes the Oscillometric method of blood pressure
measurement as opposed to the traditional method of Auscultation using
a stethoscope.
This
method indirectly measures blood pressure using the relationship
between the cuff pressure and the amplitude of the pulse waves caused
when the brachial artery in the upper arm [i.e. which the pressurized
cuff is wrapped around] expands each time the heart pumps.
The cuff
pressure is first increased to a level above the systolic pressure
[i.e. the pressure that is reached at heartbeat] and then gradually
reduced. At it's maximum, the cuff pressure constricts the brachial
artery and prevents pulsation.
When the
cuff pressure descends to a level near the systolic pressure, the
pulsation of the brachial artery appears again. This pulsation is
transmitted to the cuff as a minute change in the volume of the arm,
which in turn changes the volume of the cuff. Since the cuff is filled
with air, the cuff volume change can be measured as a change in inner
pressure. When the inner pressure of the cuff falls below the systolic
pressure, minute pressure changes appear. As air within the cuff is
released, and the pressure falls, the amplitude of the pressure
variations increases. The artery itself does not completely open until
the cuff pressure falls to the diastolic level.
As the
blood pressure rises only in the systole, when the cuff pressure is
between the systolic and the diastolic pressure, the artery will open
only if the blood pressure is higher. When cuff pressure becomes equal
to the MEAN blood pressure, the amplitude of the pulse pressure
reaches its highest level. Because of the arterial pressure/volume
properties, when the cuff pressure equals the mean blood pressure, the
elastic modulus of the brachial artery is at a minimum [i.e. the
expandability of the artery is at its greatest]. Therefore, when the
cuff pressure decreases to the mean blood pressure, the artery
increases in elasticity and the arterial volume change caused by the
pulse pressure of the blood vessel increases. As a result, the
pulsatile volume change in the cuff is also increased.
When the
cuff pressure falls below the mean blood pressure, the expansive
pressure of the blood within the artery causes the arterial elasticity
to diminish. This in turn causes the amplitude of the pulse wave to
decrease.
In sum,
the Oscillometric method determines blood pressure according to the
arterial volume change patterns, which are caused by the steadily
decreasing cuff pressure.
3. CardioVision employs data obtained from Oscillometric pulse
measurement [obtained over the course of the entire cuff pressure drop
from systolic to diastolic] to provide 5 identified, distinct,
graphical patterns (and combinations thereof), which show close
correlation to known cardiovascular conditions.
Pattern
A: an artery free of impedance or occlusion will produce a
sharp-peaked mountain.
Pattern
C: an occluded artery will have low elasticity producing a flat-topped
graphical pattern.
Pattern
D: when a patient has arrhythmia, the blood volume per each heartbeat
is not constant. A large pulse pressure is obtained after a long
diastolic period due to large stroke volume, whereas the pulse
pressure obtained after a short diastolic period is small. Due this
instability in the changes in arterial volume, a very choppy graphical
pattern is demonstrated.
Arterial
Stiffness Index [ASI]
ASI is an inverse measure of the
fall-off in arterial distensibility as the cuff pressure is lowered
below mean arterial pressure. Stiff arteries have the least fall-off
in distensibility whereas elastic arteries have the most fall-off.
The
Arterial Stiffness Index is calculated as the pressure width (mmHg X
10) of the Oscillometric curve at 80 of the mean blood pressure.
Elastic
arteries have narrow Oscillometric peaks. Stiff arteries have rounded
peaks and high Arterial Stiffness Indexes.
Both the
ASI and pulse pressure are estimates of cardiovascular risk.
TABLE
1: ARTERIAL STIFFNESS INDEX
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