| Dr
DIMITRIOS – JAMES MANOS
10 November 2011
Definition
The classic definition by Spencer and
Denison of compliance (C) is the change in arterial
blood volume (ÄV) due
to a given change in arterial blood pressure (ÄP), i.e. C = ÄV/ÄP (5).
Arterial compliance, an index of
the elasticity of large arteries such as the thoracic aorta.
Arterial compliance is an important cardiovascular risk factor. Compliance
diminishes with age and menopause.
Arterial compliance is measured by ultrasound as a pressure
(carotid artery) and volume (outflow into aorta) relationship
(8).
Arterial
Compliance in simple words is an action in which an artery yields
to pressure or force without disruption. A measure of arterial compliance
is used as an indication of arterial stiffening. An increase in age and systolic pressure
are accompanied by a decrease of the arterial compliance (3).
Arterial compliance is the relation
between changes in transmural pressure and volume of an arterial
segment, where a high compliance signifies large changes in
volume per change in transmural pressure. The relation between changes in transmural
pressure and volume is far from linear as compliance
increases progressively with decreases in blood pressure. A
change in compliance could indicate static changes in arterial
wall composition, i.e.
the relation between elastic and collagen fibres and accumulation
of disease related deposits or dynamic changes caused by alterations
in muscular tone (13).
Arterial compliance and cardiovascular
risk
Evidence has been accumulating for
several years that protecting the endothelium is a key to reducing cardiovascular (CV) disease risk. Endothelial dysfunction results in reduced compliance
or increased arterial stiffness, particularly in the smaller
arteries. This abnormality is characteristic of patients with hypertension,
but may also be seen in normotensive patients before the appearance
of clinical disease. Reduced
arterial compliance is also seen in patients with diabetes and
in smokers, and is
part of a vicious cycle that further elevates blood pressure, aggravates
atherosclerosis (hardening
of the arteries), and leads to increased CV risk. Although other factors are involved, the damage to the endothelium results in reduced
secretion of nitric oxide, which influences smooth muscle growth,
migration, and contraction, as well as influencing inflammation
and clotting. Arterial compliance can be measured by
several techniques, most of which are invasive or otherwise
not clinically appropriate. Pulse contour analysis is a newly developed
noninvasive method that allows for easy, in – office measurement
of arterial elasticity to identify patients at risk for CV events
before disease becomes clinically apparent. Further research is needed to confirm whether this method
offers a means of improving risk stratification and therapeutic
decision making
(2).
Increasing arterial stiffness and
decreasing arterial compliance are now thought to occur at the
beginning of the hypertension disease process. Decreased
arterial compliance is associated with isolated systolic blood
pressure elevations. Many clinical trials
of isolated systolic hypertension provide indirect evidence
that improving compliance lowers the risk of cardiovascular
disease. The Systolic
Hypertension in the Elderly Program showed that antihypertensive
therapy in older patients with isolated systolic hypertension
lowers systolic blood pressure and narrows pulse pressure without
unduly lowering diastolic blood pressure. Treatment over 5 years
significantly reduced the incidence of stroke, nonfatal myocardial
infarction and coronary death, all cardiovascular events, and
all-cause mortality.
The Systolic Hypertension in Europe and Systolic Hypertension
in China trials showed similar effects on pulse pressure and
on clinical end points. The Heart Outcomes Prevention Evaluation was
a primary prevention study of the effect of an angiotensin-converting
enzyme (ACE) inhibitor in subjects who were not necessarily
hypertensive, but were at risk for cardiovascular events.
With minimal lowering of blood pressure, ramipril therapy provided significant
risk reduction in all major end points--overall mortality, stroke,
myocardial infarction, and cardiovascular death. ACE
inhibitors restore endothelial cell balance to improve arterial
compliance, thus they can provide benefits beyond lowering blood
pressure (11).
Abnormalities of peripheral arterial
compliance are clinically useful markers of atherosclerosis
and risk of vascular events. Local peripheral arterial compliance
can be easily and accurately assessed in the clinic by computer-controlled
pulse volume recordings (air plethysmography). A study
investigated the relationship between clinical cardiovascular
risk factors, a surrogate of atherosclerotic burden, and peripheral
arterial compliance in the thigh and calf determined by quantification
of local pulse volume recordings in patients undergoing coronary
angiography. Peripheral
arterial compliance in the thigh and calf was measured in 346
patients undergoing diagnostic cardiac catheterization at 4
centers. Demographic and cardiovascular risk factor data
were collected, and their relationship to local arterial compliance
examined using a new device that assesses maximal local arterial
volume change in an extremity segment. The results showed that
pulse
volume recordings detected decreased local arterial compliance
in the thigh associated with a history of hypertension, diabetes
mellitus, and hyperlipidemia. In the
calf, this arterial compliance measure was associated with a
history of hypertension and diabetes mellitus. Females had lower arterial compliance than
males in the thigh and calf.
Limited evidence of lower
arterial compliance in the thigh was found for those with obesity. This procedure also demonstrated that subjects
with multiple cardiovascular risk factors had lower arterial
compliance in the thigh than subjects with no or 1 risk factor. In conclusion, peripheral arterial compliance determined
by air plethysmography is strongly associated with standard
cardiovascular risk factors. The non-invasive measurement of
local arterial compliance by regional pulse volume recording
may be a useful adjunct for cardiovascular risk stratification
early in the course of the disease as well as for monitoring
vascular response to therapy (12).
Measurement of arterial compliance
Measurement of arterial compliance
is of interest in evaluating patients with atherosclerosis and
other diseases which affect the vessels. The most used method reflecting arterial compliance
is the measurement
of pulse wave velocity. However, the pulse wave velocity method
measures
compliance at ambient transmural pressures and is affected both
by the actual blood pressure and the rate of pressure change. Another commonly used method employs the echo-tracking technique to measure
the arterial diameter simultaneously with continuous blood pressure
monitoring. By this method it is possible to calculate
arterial compliance for continuous pressure values between the
diastole and the systole. The
volume-oscillometry method is based
on the fact that the artery can be made to collapse at the end
of the diastole by an occlusive cuff while it remains open in
a pressure dependent manner during the rest of the cardiac cycle.
Changes in the arterial volume is
transmitted to the cuff, where it induces a measurable change
in pressure, and hence the volume of the artery can be calculated
at different values of transmural pressures. Using this method on normal subjects has
shown that the arterial compliance decreases with increasing
age and that females have lower compliance than males primarily
due to a smaller diameter of their arteries. It has also been shown that patients
with essential (diastolic) hypertension have compliances which
are higher or equal to those of normal subjects, and that patients
with systolic hypertension have lower arterial compliances than
normal subjects. The former finding is in contrast
with pulse wave velocity measurements, where diastolic hypertension
was associated with low arterial compliance (13).
Seven classic and recently proposed
methods used for the estimation of total arterial compliance
have been evaluated for their accuracy and applicability in
different physiological conditions. The pressure and flow
data are taken from a computer model that provides realistic
simulations of the nonlinear-distributed systemic arterial tree.
The results showed that the
methods based on the two-element windkessel (WK) model are more
accurate than those based on the three-element WK model. The classic exponential decay and the diastolic
area method yield essentially similar results, and their compliance
estimates are accurate within 10% except at high heart rates. The later part of diastole (from
the time that the systolic pressure wave has reached all peripheral
beds) gives the best results. The newly proposed two-area and pulse
pressure methods, both based on the two-element WK model, are
accurate (errors
in general < 10%) and can be applied to other locations in
the arterial tree where the decay time and area method can’t. Methods
based on the three-element WK model consistently overestimate
total arterial compliance (> or = 25%). The errors in the methods based on the three-element
WK model arise from the fact that the input impedance in that
model deviates significantly from the true input impedance at
low frequencies. The strong dependence of compliance on pressure
(elastic nonlinearity) does not invalidate the compliance estimates (1).
Arterial compliance was determined in eight normal
subjects and 23 patients with hypertension and vascular disease
by two independent techniques: (a) with a plethysmographic
strain gauge (to measure pulsatile forearm volume changes as
representing intra-arterial volume changes) and an automated sphygmomanometric system (to measure pulse
pressure) and
(b) calculation from
the local pulse wave velocity and dimension of the brachial
artery measured by pulsed wave Doppler ultrasound. Arterial
compliance measured both by the plethysmographic technique
and calculated from the pulse wave velocity was
reduced in subjects with hypertension and vascular disease
as compared with normal subjects. The regression equation
between the compliance determined by the plethysmographic technique
(x) and that calculated (y) from the pulse wave velocity and
brachial arterial diameter was y = 3. 35x + 1.53, r = 0.77.
The good correlation appears to validate the
plethysmographic technique as a method of determining arterial
compliance (14).
Arterial compliance is measured by
ultrasound as a pressure (carotid artery) and volume (outflow
into aorta) relationship (8).
Abnormalities of peripheral arterial
compliance are clinically useful markers of atherosclerosis
and risk of vascular events. Local peripheral arterial compliance can
be easily and accurately assessed in the clinic by computer-controlled
pulse volume recordings (air plethysmography) (12).
Methods of attenuation
of the reduction on arterial compliance
A study assessed
whether a diuretic, or an angiotensin converting enzyme inhibitor
(ACEI) based treatment can reduce arterial wall hypertrophy
of a distal muscular medium-sized artery—the radial artery—and
the stiffness of a proximal large elastic artery—the common
carotid artery. Large-artery wall thickness and stiffness are increased during sustained
essential hypertension and contribute to the increased risk
of complications. In
the study, seventy-seven elderly hypertensive patients were
randomized to receive 9 months of double-blind treatment with
perindopril (2 to 8 mg/day) or the diuretic combination of hydrochlorothiazide
(12.5 to 50 mg/day) plus amiloride (1.25 to 5 mg/day) after a 1-month placebo washout period. If systolic blood pressure
remained at >160 mm Hg after 5 months, chlorthalidone or
atenolol was added, respectively. Arterial variables, including
radial artery mass and common carotid artery compliance, were
calculated from non-invasive measurements of internal diameter
and wall thickness with the use of high resolution echo-tracking
systems at baseline and after 5 and 9 months. The results showed
that during treatment,
blood pressure and arterial variables changed to the same extent
in both groups. After a 9-month treatment, systolic, diastolic and pulse
pressures and radial artery wall thickness, mass and thickness/radius
ratio decreased significantly, whereas carotid compliance increased. The decrease
in radial artery thickness/radius ratio after a 9-month treatment
was significantly related to the reduction in pulse pressure,
whereas the improvement in carotid compliance was related
to the reduction in mean arterial pressure. In healthy subjects and untreated hypertensive patients, radial artery
diameter, wall thickness and thickness/radius ratio and carotid
artery compliance did not change significantly during a 9-month
observation period. In conclusion, these results indicate that in
elderly hypertensive patients, both angiotensin-converting enzyme
inhibitor (ACEI) and diuretic combination based treatments can
reduce radial artery wall hypertrophy and improve carotid artery
compliance (4).
A reduction
in compliance of the large-sized cardiothoracic (central) arteries
is an independent risk factor for the development of cardiovascular
disease with advancing age. A study determined the role of habitual
exercise on the age-related decrease in central arterial compliance
by using both cross-sectional and interventional approaches. First they study assessed 151 healthy men
aged 18 to 77 years: 54 were sedentary, 45 were recreationally
active, and 53 were endurance exercise–trained. Central arterial
compliance (simultaneous
B-mode ultrasound and arterial applanation tonometry on the
common carotid artery) was lower in middle-aged and
older men than in young men in all 3 groups. There were no significant
differences between sedentary and recreationally active men
at any age. However, arterial compliance
in the endurance-trained middle-aged and older men was 20% to
35% higher than in the 2 less active groups. As such, age-related differences
in central arterial compliance were smaller in the endurance-trained
men than in the sedentary
and recreationally active men. Second, the study assessed 20 middle-aged and older (53±2 years) sedentary healthy men before and after a 3-month
aerobic exercise intervention (primarily walking). Regular exercise increased central arterial compliance
to levels similar to those of the middle-aged and older endurance-trained
men. These effects were
independent of changes in body mass, adiposity, arterial blood
pressure, or maximal oxygen consumption. The study concluded
that regular aerobic-endurance exercise attenuates age-related
reductions in central arterial compliance and restores levels
in previously sedentary healthy middle-aged and older men. This
may be one mechanism by which habitual exercise lowers the risk
of cardiovascular disease in this population (6).
A study aimed to determine the effects
of moderate resistance training as well as the combined resistance
and aerobic training intervention on carotid arterial compliance. Resistance training has become a
popular mode of exercise, but intense
weight training is shown to stiffen carotid arteries. In the study, thirty-nine young healthy men were assigned either to the moderate-intensity
resistance training (MODE), the combined resistance training
and endurance training (COMBO) or the sedentary control (CONTROL)
groups. Participants in the training groups underwent three
training sessions per week for 4 months followed by four additional
months of detraining. The results showed that all
training groups increased maximal strength in all the muscle
groups tested. Carotid
arterial compliance (via simultaneous carotid ultrasound and
applanation tonometry) decreased approximately 20% after moderate-intensity
resistance training (MODE) training (from 0.20 +/- 0.01 to
0.16 +/- 0.01 mm2/mmHg, P < 0.01). No significant changes in carotid
arterial compliance were observed in the combined resistance
training and endurance training (COMBO) (0.20 +/- 0.01 to 0.23 +/- 0.01 mm2/mmHg) and
control group (0.20 +/- 0.01 to
0.20 +/- 0.01 mm2/mmHg) groups. Following
the detraining period, carotid arterial compliance returned
to the baseline level. Peripheral
(femoral) artery compliance did not change in any groups. The
study concluded that simultaneously
performed aerobic exercise training could prevent the stiffening
of carotid arteries caused by resistance training in young healthy
men (9).
A double-blind,
placebo-controlled study
investigated the effects of dietary fish oil supplementation
on arterial wall characteristics in 20 patients with non-insulin-dependent
diabetes mellitus. Estimates
reflecting compliance values in the large arteries and more
peripheral vasculature, as measured by pulse-contour analysis,
improved significantly after 6 weeks of fish oil therapy
compared with values recorded at baseline and after 6 weeks’
administration of olive oil. The large-artery
compliance estimate increased from 1.50 mL/mm Hg at baseline
to 1.68 mL/mm Hg after fish oil administration. The oscillatory
compliance value increased from 0.015 mL/mm Hg at baseline to
0.022 mL/mm Hg after fish oil ingestion. No changes occurred
in arterial blood pressure, cardiac output, stroke volume, or
systemic vascular resistance with either intervention. The
improved compliance estimates with fish oil ingestion occurred
without altering fasting blood glucose and cholesterol concentrations.
These results support the hypothesis that fish
oils alter vascular reactivity and favorably influence arterial
wall characteristics in patients with non-insulin dependent
diabetes mellitus. These direct vascular effects, expressed
at the level of the vessel wall, may contribute to the cardioprotective
(protective for the heart) actions of fish oil in humans (7).
The possibility that the heightened
cardiovascular risk associated with the menopause can be reduced
by increasing dietary isoflavone intake was tested in 17 women
by measuring arterial
compliance, an index of the elasticity of large arteries such
as the thoracic aorta. Compliance diminishes with age and menopause. An
initial 3- to 4-week run-in period and a 5-week placebo period
were followed by two 5-week periods of active treatment with
40 mg and then 80 mg isoflavones
derived from red clover containing genistein, daidzein, biochanin,
and formononetin in 14 and 13 women, respectively, with 3 others serving
as placebo controls throughout. Arterial compliance, measured by ultrasound
as a pressure (carotid artery) and volume (outflow into aorta)
relationship, was determined after
each period; plasma lipids were measured twice during each period.
Urinary output of isoflavones was also determined. The results
showed that
arterial compliance rose by 23% relative to that during the
placebo period with the 80-mg isoflavone dose and slightly less
with the 40-mg dose. In the
three women receiving continuous placebo, compliance was .16
+/- .022, similar to that during the run-in period for the remaining
subjects. ANOVA showed a significant difference between treatments;
by Bonferroni multiple comparisons and by
paired t test, differences were significant between placebo
and 40- and 80-mg isoflavone doses. Plasma
lipids were not significantly affected. An important cardiovascular risk factor,
arterial compliance, which diminishes with menopause, was significantly
improved with red clover isoflavones. As diminished compliance
leads to systolic hypertension and may increase left ventricular
work, the findings indicate a potential new therapeutic approach
for improved cardiovascular function after menopause (8).
The possibility that the heightened cardiovascular
risk associated with the menopause, which is said to be ameliorated
by soybeans, can be reduced with soy
isoflavones was tested in 21 women. Although several
were perimenopausal, all have been included. A placebo-controlled
crossover trial tested the effects of 80-mg daily isoflavones
(45 mg genistein) over 5-
to 10-week periods. The results showed that systemic arterial
compliance (arterial elasticity), which declined with age in
this group, improved 26%, compared
with placebo. Arterial pressure and plasma lipids were unaffected.
The vasodilatory capacity of the microcirculation was measured
in nine women; high acetylcholine-mediated dilation in the forearm
vasculature was similar with active and placebo treatments.
LDL oxidizability measured in vitro was unchanged. The study concluded that one important
measure of arterial health, systemic arterial compliance, was
significantly improved in perimenopausal and menopausal women
taking soy isoflavones to about the same extent as is achieved
with conventional hormone replacement therapy (10).
REFERENCE:
1.http://ajpheart.physiology.org/content/268/4/H1540.short
2.http://www.sciencedirect.com/science/article/pii/S0895706101021549
3.http://www2.intota.com/experts.asp?strSearchType=all&strQuery=arterial+compliance
4.http://www.sciencedirect.com/science/article/pii/S0735109798000436
5.http://ajpheart.physiology.org/content/278/4/H1407.full#xref-ref-3-1
6.http://circ.ahajournals.org/content/102/11/1270.short
7.http://www.ncbi.nlm.nih.gov/pubmed/8068603
8.http://www.ncbi.nlm.nih.gov/pubmed/10084567
9.http://www.ncbi.nlm.nih.gov/pubmed/16915024
10.http://www.ncbi.nlm.nih.gov/pubmed/9437184
11.http://www.ncbi.nlm.nih.gov/pubmed/11728287
12.http://www.ncbi.nlm.nih.gov/pubmed/12799729
13.http://www.ncbi.nlm.nih.gov/pubmed/10977606
14.http://www.ncbi.nlm.nih.gov/pubmed/6734079
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