(Hypertension. 1996;27:799-803.)
© 1996 American Heart Association, Inc.
Articles |
From the Departments of Internal Medicine (X.G., J.-J.M., M.S.) and Pharmacology (P.B., S.L.), Broussais Hospital, Paris; INSERM U337 (X.G., S.L., M.S.), Paris; and the Centre Hospitalier F.H. Manhes, Fleury-Mérogis (G.L.), France.
| Abstract |
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Key Words: arteries kidney hemodynamics blood flow velocity arteriovenous fistula
| Introduction |
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In human native vessels these phenomena have not been studied in great detail because of the lack of appropriate tools to measure arterial diameter and wall thickness in vivo. Recent advances in ultrasound methodology have made it possible to measure noninvasively and with a high accuracy the internal diameter and wall thickness of superficial arteries in humans.14 15 We designed the present study to further investigate the relationships between chronic changes in blood flow and the geometry of peripheral conduit arteries in humans. For this purpose, we studied the forearm circulation of patients with ESRD bearing an AVF for hemodialysis using a high-resolution echo-tracking system for internal diameter and wall thickness measurements coupled with a continuous Doppler system for blood flow velocity measurement. The objectives of the study were (1) to describe the geometry and hemodynamics of the radial artery feeding the AVF, (2) to compare these parameters with the contralateral radial artery taken as control, and (3) to compare the arterial parameters of patients with ESRD with parameters of a group of subjects matched by sex, age, and BP to determine whether the difference between cases and control subjects may be ascribed to the renal disease or hemodynamic pattern of the fistula.
| Methods |
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Among the 11 patients (mean age, 56±15 years), 9 received antihypertensive therapy, including calcium antagonists alone (n=2) or in combination with angiotensin-converting enzyme inhibitors (n=5). Two patients received a combination of three medications (calcium antagonist plus angiotensin-converting enzyme inhibitor plus ß-blocking agent).
The hemodynamic study was performed 24 hours after the midweek hemodialysis. Patients were dialyzed three times per week on AN 69 membranes (Hospal); the duration of hemodialysis was 4 to 5 hours. Dialysate was delivered by a system including bicarbonate delivery, adjustable sodium concentration, and controlled ultrafiltration.
Control subjects were matched as two controls for one case. In this control group, subjects had never received any antihypertensive treatment. No subject had signs or symptoms of hypertensive complications, and none had valvular heart disease, major arrhythmia, carotid artery stenosis, or diseases of noncardiovascular nature. The study was approved by the Ethics Committee of the Broussais Hospital, and all patients gave written informed consent.
Study Design
The investigation was performed in a controlled
environment at 22±2°C. Each patient was studied in the supine
position after at least 10 minutes of rest. Systolic and
diastolic pressures were determined automatically every 5
minutes at the non-AVF arm with a Dinamap 845 oscillometric BP
recorder. Systolic and diastolic pressures were
determined as the average of five measurements. Mean BP was determined
according to the usual definition (diastolic BP plus one
third pulse pressure).
During this period, measurements of radial artery parameters were obtained on both arms after the forearm was extended and secured comfortably on a splint. On the AVF side, measurements were performed on the radial artery at the wrist, a site at which the artery was close to the anastomosis but not included within the fistula. At that site, the radial artery was exposed to high blood flow because of the low-resistance shunt induced by the proximity of the fistula. Patients in whom high turbulent blood flow made blood flow velocity and diameter measurements impossible or artery calcifications made wall thickness measurements impossible were excluded from the study.
Measurements of Arterial
Parameters
Internal Diameter and Wall Thickness
The
ultrasound system used in the present study (NIUS 02,
SMH) has been previously
described14 15 16 and validated for
measurement of radial artery internal diameter and its
systolic-diastolic variations and measurement
of radial artery wall thickness in
humans.7 14 17 A
high-resolution pulse echo-tracking device was used to acquire
backscattered radiofrequency data from the radial artery at the wrist.
The probe consisted of a 10-MHz strongly focused piezoelectric
transducer (6-mm diameter, 11-mm focal length) operated in the
pulse-echo mode. The -10 dB beam width is 0.3 mm at the focal
point, and the depth of field at -10 dB is 5 mm. A
stereotactic arm permits motion of the transducer in
x, y, and z coordinates with
micrometric steps for placement of the probe perpendicular to the
arterial axis in its largest cross-sectional dimension.
The transducer is positioned so that its focal zone is located in the
center of the artery, and the backscattered echoes from both the
anterior and posterior walls can be visualized. A typical
radiofrequency signal is then displayed on a computer monitor
interfaced to the transducer system. Arterial diameter and
posterior wall thickness are measured when a "double peak"
radiofrequency ultrasound signal of the anterior and posterior walls is
obtained. These signals are visible only as the ultrasound beam crosses
the axis (center) of the vessel. They are characterized by a
high-amplitude signal followed by a relatively silent acoustic zone
and then a second high-amplitude signal. For measurement of
internal diameter and intima-media wall thickness of the posterior
wall, electronic trackers are positioned as follows: on the outer
radiofrequency line of the second peak of the anterior wall and on the
inner radiofrequency lines of the first and second peaks of the
posterior wall (Fig 1
). Their movements are
electronically tracked for 10 seconds, sampled at 100 MHz over 8 bits,
and stored at a 50-Hz repetition frequency on a 120-megabyte hard disk
for further data processing. Data are determined by computing the
analytic signal according to methods previously described and based on
Fourier transformation. Finally, diameter and wall thickness are
expressed in millimeters by multiplying time-of-flight
measurements by the approximate speed of sound in tissue (1.54
mm/µs). All data processing was performed with software developed by
Asulab installed in a 486, 25-MHz AT computer. The pulse length of this
10-MHz ultrasound system was 0.1 microseconds at 6 dB and corresponded
with a practical axial resolution of 0.16 mm for absolute internal
diameter or wall thickness measurements and 0.0025 mm for these
parameters during
systolic-diastolic
changes.14 15
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Systolic-diastolic variations in
internal
diameter and posterior wall thickness were recorded. For data
analysis, each patient was described by the average of
recordings of mean wall thickness (millimeters), calculated by
integrating the time course of the
systolic-diastolic variations, and
diastolic internal diameter (millimeters), over several
cardiac cycles. Radius-thickness ratio was calculated as
Di/(hx2), where Di is internal
diameter, and h is mean wall thickness. Because wall thickness is
influenced by the changes in distending BP (hence internal diameter),
WCSA is more appropriate for detecting arterial wall
hypertrophy. Because of the incompressibility of the
arterial mass,17 WCSA (millimeters squared)
can be calculated as
(Re2-Ri2),
where Re and Ri are the values of internal and
external radii, respectively. Mean circumferential wall stress
(kilopascals) is calculated as MBP·2h/Di, where
MBP is mean BP.
Blood Flow Velocity
Radial artery
blood flow velocity was measured by
continuous-wave Doppler (8-MHz transducer at a 60° angle,
Doptek 2002, Deltex), distal to the 10-MHz probe, at the site of
diameter measurements. No significant interference between the two
waves was noted throughout the experiment. Resting blood flow
(milliliters per minute) is the product of time-averaged mean
velocity and arterial lumen cross section. Mean shear
stress (dynes per centimeter squared) was approximated with a variation
of the Hagen-Poiseuille equation18 :
=4
Q/
Ri3, where
is the
viscosity of blood (0.035 poise), Q is the velocity of blood flow
(milliliters per second), and Ri is the vessel radius
(centimeters).
Statistical Analysis
Data are expressed as mean±SD.
Differences in
arterial parameters between the AVF side and
control side were analyzed with paired Student's t
test. The relationship between continuous variables was evaluated
by linear regression. Statistical significance was assumed at a value
of P<.05.
| Results |
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Comparison With Control Subjects
To determine whether the
geometric modifications of the radial
artery in ESRD patients were related to the renal disease or chronic
blood flow changes, we analyzed the relationship between
intima-media thickening and BP in ESRD patients and a group of 22
control subjects matched to the cases by sex, age, and BP
(136±22/81±9 mm Hg). In this group, the mean internal diameter
was
2.494±0.386 mm and the mean wall thickness was 254±56 µm.
Fig 2
shows the negative relationship between the
radius-thickness ratio and systolic BP in this group of 22
control subjects (y=-0.037x+10.1;
r=.752). For the following analysis, we referred to
this regression line and its 95% upper and lower confidence intervals.
For the radial artery of the AVF side, the radius-thickness ratio
was above the upper confidence interval line in 9 of 11 patients,
indicating an inappropriate thickening of the arterial wall
to the increased internal diameter for a given level of BP. By
contrast, for the radial artery of the normal forearm, 9 of 11 patients
with ESRD had values of the radius-thickness ratio inside the 95%
confidence interval (Fig 3
).
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| Discussion |
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Consideration of Methods
To study vascular geometry in vivo,
it is necessary to measure
internal diameter and wall thickness. Only recently has ultrasound
methodology made possible these noninvasive measurements in humans. In
the present study, we used a high-resolution echo-tracking
system previously validated for its ability to measure the wall
thickness of a medium-sized muscular artery such as the radial
artery14 and for the good reproducibility of these
measurements in a clinical setting.7 The coupling of this
apparatus with a continuous-wave Doppler system
made it possible for us to calculate blood flow variations along the
cardiac cycle and estimate wall shear stress. For newtonian fluids,
shear stress equals the local viscosity times the local wall shear
rate, which can be derived from the measures or estimates shape of the
instantaneous velocity across the lumen.18 With our
apparatus, it was impossible to determine the velocity
gradient near the wall. Thus, we estimated mean wall shear stress by
the Hagen-Poiseuille equation. Values obtained with this approach have
been described to underestimate the true value.18 However,
comparison of two different arteries in the same patient with the same
device minimizes the methodological reserves for the interpretation of
the results.
Exposure of an artery to pressure induces deformation in
each of the
three directions of space, resulting in an increase in circumference
and length as well as radial deformation, the latter inducing a
decrease in wall thickness. In opposition to these deformations, the
vessel wall generates increased stress in these three directions.
Stresses in the circumferential and longitudinal directions are
tensile, whereas stress in the radial direction is
compressive.2 In clinical studies performed in humans,
circumferential stress is usually calculated with Lamé's
equation (
=MBP·2h/Di). In the present
study, internal diameter (Di) and wall thickness (h) were
precisely measured at the site of each artery, but mean BP (MBP) was
estimated from the systolic and diastolic BPs
measured at the arm. Because the protocol was noninvasive, it was
impossible to measure mean BP intra-arterially at the
site of diameter measurement. However, in a previous study, we
demonstrated that mean BP was not significantly different between the
site of the AVF and the rest of the vasculature.19 Thus,
we considered that the calculation of circumferential wall stress with
the present methodology was reliable.
Interpretation of Findings
In vitro and in vivo studies have
demonstrated that an increase in
arterial blood flow and/or blood viscosity elicited
increased shear stress and endothelium-dependent
vasodilation.3 4 In the human radial artery, it was
recently demonstrated that nitric oxide but not prostacyclin is
essential for flow-mediated arterial dilation in vivo
under acute conditions.9 The present study indicates
that a chronic increase in arterial blood flow induces a
sustained and striking increase in arterial diameter. This
result confirms previous studies that have documented that the increase
in arterial diameter started shortly after the creation of
an AVF.10
In the present study, the large increase in radial artery diameter (+38% on the AVF side compared with the control side) suggests that structural modifications of the arterial wall contributed to the diameter enlargement in addition to a sustained phenomenon of flow-dependent dilation. Indeed, in response to an equivalent increase in blood flow occurring under acute conditions (reactive hyperemia), Joannides et al,9 using the NIUS apparatus, previously observed a 10% increase in radial artery diameter.
Despite high blood flow velocity on the AVF side, wall
shear stress
remained unchanged, probably because of diameter enlargement. Indeed,
wall shear stress on the AVF side was not significantly different from
control values. We must admit that the lack of statistical significance
could be due to the wide SD values on both sides, reflecting a high
intersubject variability. This high variability is mainly due to the
variability of blood flow velocity, thus mean blood flow, as reflected
by the high SD values shown in the Table
. Other researchers
have
reported similar SD values for mean blood flow.9 Another
argument favoring the consistency of our finding is that
the difference in wall shear stress between sides would be fourfold to
fivefold if no diameter changes were observed, indicating that shears
are undoubtedly reduced greatly by arterial expansion.
The original
finding of the present study is the
demonstration that the geometric adaptation of the arterial
wall to a chronic increase in blood flow does not result in wall
hypertrophy, despite the sustained increase in
circumferential wall stress. Indeed, one of the consequences of the
increased radial artery diameter is an increased parietal tension.
According to Lamé's equation, which states that circumferential
wall stress is directly proportional to intra-arterial
pressure and arterial radius and inversely proportional to
wall thickness, arterial wall hypertrophy
should develop to normalize wall stress. Thus, if normal wall stress is
to be maintained, with respect to an increased diameter and similar
pressure, a constant relationship between pressure and the
radius-thickness ratio should be observed. This was the case in
nonuremic subjects and ESRD patients on the normal arm side (Fig
3
) but
not in ESRD patients on the AVF side (Fig 2
).
The observed modifications of radial artery geometry in ESRD patients on the AVF side cannot be defined as true arterial wall hypertrophy because WCSA remained unchanged; rather, they must be defined as remodeling. The term remodeling has formerly been used for describing geometric changes of the small arteries in response to high BP.20 21 These changes are characterized by reduced internal and external diameters with an unchanged WCSA, indicating a rearrangement of the wall material around a smaller lumen. In the present study, remodeling may be used in a broader sense: an unchanged WCSA, whatever the lumen. Indeed, the radial artery on the AVF side had increased internal and external diameters, whereas WCSA was unchanged.
The reasons for this response are not clear. The influence of uremia per se is unlikely, because the relationship between BP and the radius-thickness ratio on the control artery of ESRD patients is superimposable on that of control subjects, indicating an adequate response of the arterial wall to the distending pressure in ESRD patients. The second possibility is that conduit arteries have a limited capacity to respond adequately to a combined flow and wall tension overload. In vein grafts subjected to separate mechanical factors, such as circumferential stretching and changes in blood flow velocity, Dobrin et al11 demonstrated that changes in shear stress influence intimal thickening, whereas medial thickening responds to changes in circumferential wall stress. Intimal thickening occurs in response to low flow velocity and decreased shear stress, whereas medial thickening occurs in response to increased parietal tension.
Furthermore, Kraiss et al22 demonstrated that an elevation in shear stress inhibited smooth muscle cell proliferation and neointimal thickening in polytetrafluoroethylene graft, and Rekhter et al23 demonstrated that cell proliferation associated with neovascularization was one of the major mechanisms of intimal hyperplastic lesions in the anastomosis region of the polytetrafluoroethylene graft material vein in human AVFs. However, Masuda et al24 suggested that when an artery is enlarged in response to increased flow, the intima may also thicken in addition to the media to provide wall thickness for restoration of normal wall tensile stress. Whatever the respective effects of shear stress and circumferential wall stress on intimal and medial thickening, the present results indicate that both stimuli, occurring simultaneously, result in unchanged intima-media thickening. Whether the increased shear stress inhibits the media thickening because of increased wall stress remains to be determined. This interpretation is limited by the current impossibility of noninvasive discrimination between the two layers of radial artery wall thickness. Further studies are needed for determination of the histological structure of high- and low-flow arteries in humans.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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| Footnotes |
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| References |
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A. M. Malek, S. L. Alper, and S. Izumo Hemodynamic Shear Stress and Its Role in Atherosclerosis JAMA, December 1, 1999; 282(21): 2035 - 2042. [Abstract] [Full Text] [PDF] |
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J. R. Linderman and M. A. Boegehold Growth-related changes in the influence of nitric oxide on arteriolar tone Am J Physiol Heart Circ Physiol, October 1, 1999; 277(4): H1570 - H1578. [Abstract] [Full Text] [PDF] |
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S. J. Marchais, F. Metivier, A. P. Guerin, and G. M. London Association of hyperphosphataemia with haemodynamic disturbances in end-stage renal disease Nephrol. Dial. Transplant., September 1, 1999; 14(9): 2178 - 2183. [Abstract] [Full Text] [PDF] |
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Y. Hayakawa, G. Takemura, J. Misao, M. Kanoh, M. Ohno, H. Ohashi, H. Takatsu, H. Ito, K. Fukuda, T. Fujiwara, et al. Apoptosis and Overexpression of Bax Protein and bax mRNA in Smooth Muscle Cells Within Intimal Hyperplasia of Human Radial Arteries : Analysis With Arteriovenous Fistulas Used for Hemodialysis Arterioscler Thromb Vasc Biol, September 1, 1999; 19(9): 2066 - 2077. [Abstract] [Full Text] [PDF] |
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C. Carallo, C. Irace, A. Pujia, M. S. De Franceschi, A. Crescenzo, C. Motti, C. Cortese, P. L. Mattioli, and A. Gnasso Evaluation of Common Carotid Hemodynamic Forces : Relations With Wall Thickening Hypertension, August 1, 1999; 34(2): 217 - 221. [Abstract] [Full Text] [PDF] |
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J. Blacher, B. Pannier, A. P. Guerin, S. J. Marchais, M. E. Safar, and G. M. London Carotid Arterial Stiffness as a Predictor of Cardiovascular and All-Cause Mortality in End-Stage Renal Disease Hypertension, September 1, 1998; 32(3): 570 - 574. [Abstract] [Full Text] [PDF] |
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M. E. Safar, G. M. London, R. Asmar, and E. D. Frohlich Recent Advances on Large Arteries in Hypertension Hypertension, July 1, 1998; 32(1): 156 - 161. [Abstract] [Full Text] [PDF] |
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J. Blacher, K. Demuth, A. P. Guerin, M. E. Safar, N. Moatti, and G. M. London Influence of Biochemical Alterations on Arterial Stiffness in Patients With End-stage Renal Disease Arterioscler Thromb Vasc Biol, April 1, 1998; 18(4): 535 - 541. [Abstract] [Full Text] [PDF] |
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J. L. Unthank, S. W. Fath, H. M. Burkhart, S. C. Miller, and M. C. Dalsing Wall Remodeling During Luminal Expansion of Mesenteric Arterial Collaterals in the Rat Circ. Res., November 1, 1996; 79(5): 1015 - 1023. [Abstract] [Full Text] |
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