Model
Digital Document
Publisher
Florida Atlantic University
Description
This study evaluates the use of an electrical impedance
plethysmograph as a noninvasive technique for early detection of
atherosclerosis. The instrument is inexpensive, easily portable and
causes no health risks. Thus, the system is ideally suited for mass
screemng and epidemiological studies, if proven to be effective.
We have conducted experiments usmg a three-channel
impedance plethysmograph once every 8 - 10 weeks on a colony of
20 male cynomolgus monkeys (macaca fascicularis). Five monkeys
were on a control diet (monkey chow) and fifteen on a high
cholesterol diet (1 mg cholesterol/Kcal with 40% of the calories derived from fat). The diet period for the monkeys ranged from 16-28 months (25 months typically).
We wrapped a pressure cuff with one pair of electrodes around
the upper left leg of the monkey. Two other sets of electrodes were
wrapped, one distal to the pressure cuff on the lower left leg and the
other as reference on the upper arm. We measured impedance
pulses at these three different sites simultaneously using a three
channel impedance plethysmograph. The signals were recorded
when the pressure in the pressure cuff was changed from 200 mm
Hg to 20 mm Hg in steps of 10 mm Hg. Arterial volume change was
evaluated from this. Experiments were repeated with the cuffed
segment on the right leg, and then on the left arm.
The arterial volume change vs cuff pressure (V- Pc)
characteristics were used to follow the progression of the disease.
The V- Pc characteristic, initially with a well defined peak, changed
to a flatter characteristic with increased period on the cholesterol
diet. Monkeys on the control diet showed no flattening of the curve
with time. In order to understand theoretically the effect of disease on
the compliance - transmural pressure (C-Pt) characteristic (and hence
V - Pc characteristic), we developed an arterial model to study the
pressure - radius relationship of an artery under different disease
states. We have also developed an expression for the equivalent
incremental modulus of elasticity based on the incremental modulus
of elasticity of the individual arterial wall layers. The resulting expressions were used to study the effect of increase in stenosis and
calcification on the V - Pc and C-Pt characteristics.
The simulation results obtained using the arterial model match
our experimentally observed data of decrease m peak compliance
with disease. The peak compliance was seen to decrease m
amplitude and shift left (towards decreasing transmural pressure) as
the artery got thicker with atherosclerotic disease. The V - Pc
characteristic, initially with a well defined peak, got flatter with
disease.
Our simulation results lead us to believe that the noninvasive
technique 1s sensitive enough to follow progressiOn of the
atherosclerotic disease. Morphometric and histochemical data were
collected subsequent to the sacrifice of the monkeys. Evaluation of these data and correlations with our compliance data will lead us to a more definitive statement on the method's sensitivity.
This however, is beyond the scope of this dissertation.
plethysmograph as a noninvasive technique for early detection of
atherosclerosis. The instrument is inexpensive, easily portable and
causes no health risks. Thus, the system is ideally suited for mass
screemng and epidemiological studies, if proven to be effective.
We have conducted experiments usmg a three-channel
impedance plethysmograph once every 8 - 10 weeks on a colony of
20 male cynomolgus monkeys (macaca fascicularis). Five monkeys
were on a control diet (monkey chow) and fifteen on a high
cholesterol diet (1 mg cholesterol/Kcal with 40% of the calories derived from fat). The diet period for the monkeys ranged from 16-28 months (25 months typically).
We wrapped a pressure cuff with one pair of electrodes around
the upper left leg of the monkey. Two other sets of electrodes were
wrapped, one distal to the pressure cuff on the lower left leg and the
other as reference on the upper arm. We measured impedance
pulses at these three different sites simultaneously using a three
channel impedance plethysmograph. The signals were recorded
when the pressure in the pressure cuff was changed from 200 mm
Hg to 20 mm Hg in steps of 10 mm Hg. Arterial volume change was
evaluated from this. Experiments were repeated with the cuffed
segment on the right leg, and then on the left arm.
The arterial volume change vs cuff pressure (V- Pc)
characteristics were used to follow the progression of the disease.
The V- Pc characteristic, initially with a well defined peak, changed
to a flatter characteristic with increased period on the cholesterol
diet. Monkeys on the control diet showed no flattening of the curve
with time. In order to understand theoretically the effect of disease on
the compliance - transmural pressure (C-Pt) characteristic (and hence
V - Pc characteristic), we developed an arterial model to study the
pressure - radius relationship of an artery under different disease
states. We have also developed an expression for the equivalent
incremental modulus of elasticity based on the incremental modulus
of elasticity of the individual arterial wall layers. The resulting expressions were used to study the effect of increase in stenosis and
calcification on the V - Pc and C-Pt characteristics.
The simulation results obtained using the arterial model match
our experimentally observed data of decrease m peak compliance
with disease. The peak compliance was seen to decrease m
amplitude and shift left (towards decreasing transmural pressure) as
the artery got thicker with atherosclerotic disease. The V - Pc
characteristic, initially with a well defined peak, got flatter with
disease.
Our simulation results lead us to believe that the noninvasive
technique 1s sensitive enough to follow progressiOn of the
atherosclerotic disease. Morphometric and histochemical data were
collected subsequent to the sacrifice of the monkeys. Evaluation of these data and correlations with our compliance data will lead us to a more definitive statement on the method's sensitivity.
This however, is beyond the scope of this dissertation.
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