(Circulation. 1999;99:2677-2681.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
From the Service de Cardiologie (G.J., O.D., J.-P.B.) and Consultation Marfan (G.J.), Ambroise Paré, Boulogne, and the Department of Pharmacology and INSERM U 337 (P.B., P.L., B.L., S.L.), Hôpital Broussais, Paris, France.
| Abstract |
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Methods and ResultsTwenty patients with confirmed MFS and 20 age- and sex-matched control subjects were included in this cross-sectional, noninvasive study. Elastic properties of the abdominal aorta and common carotid, common femoral, and radial arteries were calculated from the pulsatile changes in arterial diameter and pressure. The ascending aorta diameter, measured with conventional echocardiography, was 37% larger in MFS than in control subjects (P<0.001). Arterial distensibility was 38% lower in MFS than in control subjects at the site of the abdominal aorta (P<0.01) but not at other sites (common carotid, common femoral, and radial arteries). Independently of age and body surface area, ascending aorta diameter was positively correlated with carotid PP in MFS (P<0.01) and negatively in control subjects (P<0.01) but was not correlated with brachial PP and mean blood pressure.
ConclusionsIn patients with MFS, local PP, estimated from carotid PP, was a major determinant of ascending aorta diameter, whereas brachial PP was not. Increased arterial stiffness was confined to the aorta.
Key Words: blood pressure arteries aorta Marfan syndrome
| Introduction |
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In MFS patients, the initial aortic size is an independent predictor of aortic dilatation, whereas brachial systolic and pulse pressures (PPs) are not.4 However, brachial PP may not accurately reflect local PP acting on the ascending aorta (AsAo), because increased wave reflections affect peripheral PP to a lesser extent than central PP.5 Thus, taking brachial PP instead of local PP into account may underestimate the role of pulsatile stress on aortic dilatation. We hypothesized that central PP, which takes wave reflections and aortic stiffness into account, is a better determinant of AsAo diameter than is brachial PP in MFS patients. In the present study, carotid PP, measured with applanation tonometry and calibrated with brachial blood pressure (BP), was used as a surrogate for aortic PP.
An increased wall stiffness of the thoracic aorta has been reported from invasive6 and noninvasive studies,7 8 9 10 in adults7 9 and children,8 10 and has been attributed to abnormalities of elastic fibers. Despite these studies, there is little information concerning the elastic properties of the various arterial segments downstream of the aortic root7 and the resulting PP at the site of the AsAo in patients with MFS.
Therefore, the objectives of the present noninvasive study were to determine, in MFS patients, (1) whether central PP, which takes into account wave reflections and aortic stiffness, was a better determinant of AsAo diameter than brachial PP and (2) whether arterial stiffness was increased at arterial sites downstream of the AsAo accessible to noninvasive investigation: the abdominal aorta (AbAo) and the common carotid (CCA), common femoral (CFA), and radial (RA) arteries.
| Methods |
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Twenty age- and sex-matched normal subjects constituted the normal control group. The study was approved by the institutional review committee of Broussais Hospital, and the subjects gave informed consent.
Cardiac Measurements
Echocardiographic examination was performed with
commercially available equipment (Apogée CX 200, Advanced
Technology Laboratory) according to the recommendations of the American
Society of Echocardiography. Aortic root diameter
was measured according to Roman et al12 at end
diastole, in the parasternal long-axis view, at 3 levels:
annulus (aortic valve insertion), sinuses of Valsalva, and supra-aortic
ridge. The largest diameter, always at the level of the sinuses of
Valsalva, was taken as the AsAo diameter for this study. Aortic arch
diameter was measured with the transducer placed on the suprasternal
notch. Left ventricular mean wall thickness, mass index,
and shortening fraction were calculated according to Devereux et
al.13
Arterial Measurements
CCA and RA pressure waveforms were recorded noninvasively
with a pencil-type probe incorporating a high-fidelity Millar
strain-gauge transducer (SPT-301, Millar Instruments).14
The CCA pressure wave was calibrated assuming that brachial and carotid
mean and diastolic BPs were equal, as previously
described.15 No calibration was required for RA PP because
of the very good agreement between radial and brachial
diastolic BP after adjustment to mean BP.
End-diastolic ID and pulsatile changes in diameter were measured on the right CCA and CFA and on the AbAo with a 7.5-MHz pulsed ultrasound echotracking system (Wall Track System, Neurodata) through the analysis of the radiofrequency signal obtained after B-mode echographic localization (Sigma 44 Kontron), as previously described.14 16 Wall thickness was measured at the site of the CCA but not at the CFA and AbAo because of insufficient repeatability. Measurements of RA ID and wall thickness were obtained on the right arm with a 10-MHz ultrasound system analyzing the radiofrequency signal (NIUS 02, SMH), as previously described.16
Wall Stress, Distensibility, and Compliance
Mean circumferential wall stress (
, kPa) was calculated
according to Lamé's equation as 
=MBP · Di/2h, where
MBP is mean BP, Di is mean ID, and h is wall thickness.5
Arterial compliance and distensibility were estimated
through the variations in arterial cross-sectional area
(
A) and BP (
P) during systole as previously
described,14 16 assuming the lumen to be circular. The
cross-sectional distensibility coefficient was calculated as
DC=
A/A ·
P, where A is the diastolic lumen
area,
A is the stroke change in lumen area, and
P is local PP,
and cross-sectional compliance coefficient as CC=
A/
P. Local CCA
and RA PPs, measured with applanation tonometry, were used in these
calculations. AsAo and CFA PPs were approximated by the brachial
PP,17 measured with the oscillometric recorder Dinamap
(Critikon).14
Statistics
Data are expressed as mean±SD. Quantitative variables were
compared by means of an unpaired Student's t test and
categorical variables by means of a
2
test. Multivariate regression
models18 were constructed in each group (control
subjects and MFS) and included mean BP and other variables (age,
sex, body surface area [BSA]), carotid PP, and brachial PP). A robust
multiple stepwise regression analysis was performed. A value of
P<0.05 was considered significant. The statistical
analysis was performed by means of an NCSS 6.0 package software
(J.L. Hintze, Kaysville, Utah).
| Results |
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Aortic Geometry and Cardiac Function
The diameter of the AsAo was significantly larger in MFS than in
control subjects at the site of the sinuses of Valsalva only (Table 2
). Left ventricular diameter
in systole or diastole, left ventricular mean
wall thickness, shortening fraction, and left ventricular
mass index were not significantly different between the 2 groups.
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Arterial Geometry and Function
The diameters of the AbAo, CCA, CFA, and RA were not statistically
different between the 2 groups (Tables 3
and 4
). Absolute and relative stroke
changes in AbAo diameter were lower in patients with MFS than in
control subjects, whereas those of the CCA, CFA, and RA were not
significantly different between the 2 groups. CCA and RA intima-media
thicknesses were not significantly different between the 2 groups.
Cross-sectional distensibility of the AbAo was lower in MFS than in
control subjects, whereas distensibility and compliance of the CCA,
CFA, and RA were not significantly different between the 2 groups.
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Carotid and RA PPs
Carotid PP, radial PP, carotid/brachial PP ratio, and
carotid/radial PP ratio were not significantly different between the 2
groups (Table 4
). Carotid PP was significantly correlated with
radial PP in MFS (r=0.74; P<0.01) and in control
subjects (r=0.52; P<0.05).
Influence of Local PP on AsAo Diameter
In patients with MFS, AsAo diameter was positively associated with
carotid PP, independently of aging and BSA, whereas in control
subjects, AsAo diameter was negatively associated with carotid PP
(Table 5
). Carotid PP alone explained 7%
of aortic diameter variance in MFS patients
(R2 increment, Table 5
).
Brachial PP, systolic BP, and mean BP were not significantly
related to AsAo diameter when introduced into the model instead of
carotid PP. Thus, in addition to age and BSA, carotid PP was a good
predictor of dilation of the AsAo in these patients, whereas brachial
BPs were not. Introducing the level of aortic
regurgitation (0, 1+, or 2+) into the model did not
change the relationship between AsAo diameter and carotid PP.
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In addition, we tested whether the ratio of carotid/brachial PP, an
index of pulse wave amplification, was a significant determinant of
AsAo diameter in MFS patients. In multivariate
analysis, carotid/brachial PP ratio represented
18% of the variance of AsAo diameter (r=0.45;
P<0.01) independently of age (8% of the variance) and BSA
(75% of the variance). No significant correlation between
carotid/brachial PP ratio and AsAo diameter was observed in control
subjects. To take into account the strong influence of BSA, AsAo
diameter was normalized to BSA for further calculations. Under these
conditions, we observed a positive univariate correlation
between the AsAo diameter/BSA ratio and the carotid/brachial PP ratio
in MFS patients (r=0.59; P<0.01) and no
significant correlation in control subjects (r=-0.26;
P=NS) (Figure
).
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| Discussion |
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AsAo Diameter and PP
Patients with MFS have a larger diameter of the AsAo than age- and
sex-matched control subjects, as previously
reported,4 7 9 10 but not larger AbAo, CCA, CFA, and RA
diameters. In MFS patients in the present study, carotid PP was a
significant determinant of AsAo diameter, independently of aging and
BSA, whereas brachial PP and radial PP were not. This is
consistent with the hemodynamic feature that
increased wave reflections affect brachial PP to a lesser extent than
central PP.5 The ratio of carotid/brachial PP, an index of
pulse-wave amplification, was also significantly and positively
associated with AsAo diameter, independently of age and BSA.
An explanation for the positive relationship between AsAo diameter and carotid PP may be suggested from biomechanical principles.19 Although to the best of our knowledge there is no direct evidence that the degree of dilation in MFS patients relates to the degree of elastin abnormality (ie, the extent of the primary disease), aortic dilation is most likely due to the failure by elastic fibers to sustain physiological hemodynamic stress, by analogy with aging. It is well accepted that the enlargement of elastic arteries of normal subjects with aging is associated with a progressive disorganization of elastic fibers, which show thinning and fragmentation.20 These changes are generally considered signs of mechanical failure and fatigue of biomaterials.19 That AsAo diameter was related to carotid PP but not mean BP suggests that cyclic stress (PP) plays a more important role than steady stress (mean BP). According to engineering principles, the fatiguing effect of cyclic stress is dependent on both the number of cycles and the amplitude of stress. Interestingly, in the present study, AsAo diameter of MFS was significantly and positively associated with aging and PP, which may be considered practical estimates of the number of cycles and the amplitude of stress, respectively.
A potential limitation of the study is that carotid PP was calibrated with brachial mean and diastolic BPs.15 The conditions for applying this method of calibration may not be fully reached in MFS patients, because the carotid/brachial PP ratio, which is influenced by the geometry of the arterial system and its stiffness, is probably different between MFS and control subjects. However, an alternative calculation using carotid PP not calibrated with brachial BP yielded similar results (ie, that AsAo diameter was independently and positively associated with carotid PP in MFS patients and negatively in control subjects).
The negative relationship between AsAo diameter and PP, observed in the control subjects of the present study, is most likely a result of the increase in aortic compliance, which parallels the increase in aortic diameter and reduces wave reflections and thus central PP.6
Arterial Stiffness of Aorta and Peripheral
Arteries
The increased arterial stiffness was confined to the
aorta in MFS: The distensibility of the AbAo was only half the value of
control subjects, but the distensibility of other arterial
sites (CCA, CFA, and RA) was not significantly different between MFS
and control subjects. These data thus provide additional information to
previous studies in MFS showing an increased stiffness of the whole
arterial tree7 by invasive methods and an
increased stiffness of the AsAo8 10 11 and
AbAo8 by noninvasive methods. The results of the
present study are consistent with the
histological finding that arterial
abnormalities in MFS are largely confined to the
aorta.4
Clinical Implications of Findings
Although the present study has a cross-sectional design and
included a small number of patients, it suggests that MFS patients with
high carotid PP are at high risk for AsAo dilation. Thus, carotid PP
could be useful, in preference to brachial PP and systolic BP,
for evaluating the risk for subsequent aortic complications.
The present finding that local PP but not mean BP was a determinant of AsAo diameter adds to earlier studies showing that reducing dP/dt (the rate of change in the central BP with respect to time) was far more protective against aortic dissection than reducing mean BP.21 Thus, in addition to reducing heart rate and dP/dt, a primary therapeutic goal should aim at reducing the amplitude of central PP.
In conclusion, in patients with MFS, local PP, estimated from carotid PP, was a major determinant of AsAo diameter, whereas brachial PP was not. Increased arterial stiffness was confined to the aorta.
| Footnotes |
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Received October 29, 1998; revision received March 9, 1999; accepted March 9, 1999.
| References |
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