(Hypertension. 1999;34:598-602.)
© 1999 American Heart Association, Inc.
Scientific Contributions |
From the Centro di Fisiologia Clinica e Ipertensione (L.M., A.A.M.), Cattedra di Cardioangiologia Medica (A.U.F.), Universitá di Milano, Ospedale Maggiore di Milano; Ospedale S. Gerardo (C.G., G.M.), Monza; Divisione di Cardioriabilitazione (A.U.F., L.M.), Az Osp Vimercate, Ospedale di Seregno (MI), Italy.
Correspondence to Prof Alberto U. Ferrari, MD, Centro di Fisiologia Clinica e Ipertensione, Via F. Sforza 35, 20122 Milan, Italy.
| Abstract |
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Key Words: sympathetic nervous system heart rate pacing, atrial ultrasonography arterial pressure rats
| Introduction |
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The mechanisms by which pacing-induced tachycardia reduces distensibility in some but not all arteries have never been investigated and clarified. A plausible hypothesis, however, is that the heart ratedependent stiffening of the arterial wall to a large extent is due to its viscous and thus inertial response to a distending force.2 3 4 Another plausible hypothesis is that this response is less apparent in vessels such as the femoral artery in which distensibility is tonically restrained by a high smooth muscle tone determined both by the abundance of smooth muscle tissue and by a more dense nerve supply of such tissue5 than in vessels such as the carotid artery, in which the influence of the much scantier smooth muscle tissue is less important.6 7 8 9
In the present study, we examined the effects of pacing-induced tachycardia on the common carotid and femoral arteries before and after sympathectomy (an intervention that markedly reduces vascular smooth muscle tone10 11 12 ).
| Methods |
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Arterial distensibility was measured according to the
method described and validated in previous reports.15 16
Briefly, an ultrasonic 10-MHz transducer (NIUS 01 System, Asulab)
connected to a pulsed Doppler device (Capital Medical Service) was
positioned over the common carotid and femoral arteries contralateral
to the cannulated vessel to track the movements of the proximal and
distal arterial wall over the entire
systolic-diastolic cycle. In the common carotid
artery, a high-quality signal was obtained transcutaneously, whereas in
the femoral artery, it was obtained after a superficial skin incision,
without severing muscle fasciae. In both vessels, sound transmission
was optimized by interposition of ultrasound gel. Care was taken to
avoid vessel deformation by the probe and to position it perpendicular
to the longitudinal axis and on the largest cross-sectional dimension
of the artery at a site corresponding to the tip of the contralateral
catheter. For the carotid or the femoral artery, the vessel wall
signals were acquired in continuum (at 300 Hz) and stored in the
computer used for acquisition of the blood pressure signal. The space
resolution of the ultrasonic system used in these experiments is
2.5 µm.17 In thin-walled vessels such as those of
the rat, no attempt is made to identify the echoes reflected from the
intimal and adventitial surface; either wall (proximal and distal) is
taken as a single point, and wall thickness is not considered. The
computer was programmed to calculate the diameter-pressure curve of the
vessel both for the increasing blood pressure values from
diastole to systole and for the decreasing blood pressure
values from systole to diastole, according to its fitting
with the arctangent model of Langewouters. This model is based on the
following
formula: S=
{
/2+tan-1[(P-ß)/
]}
where S is the cross-sectional area of the
vessel, P is the intravascular pressure, and
, ß, and
are 3 optimal parameters describing the spatial
position of the diameter-pressure curve. From this formula,
cross-sectional compliance
(C=
S/
P) was calculated as
follows: C=
/
{1+[(P-ß)/
]2}
and expressed as the compliance-pressure curve.18 Cross-sectional distensibility was calculated by normalization of compliance with respect to arterial diameter values. The area under the curve relating arterial distensibility to blood pressure was normalized with respect to pulse pressure and referred to as the "distensibility index."
Protocol and Data Analysis
The carotid and femoral arteries were studied in a random order
during a single experimental session. After the measuring devices were
positioned, a 10- to 20-minute period of stabilization was allowed.
With the rat in sinus rhythm, blood pressure and arterial
diameter data were acquired during 5 periods of 4 seconds each; each
data acquisition was separated from the next by an interval of 30
seconds. The atria were then paced in 5 randomly sequenced steps at the
rates of 280, 310, 340, 370, and 400 bpm. During each step, blood
pressure and arterial diameter data were acquired over 10
periods of 4 seconds each. After each pacing step, spontaneous sinus
rhythm was allowed to return and a series of 5 data acquisitions of 4
seconds each was again obtained.
Blood pressure, arterial diameter, and arterial distensibility were computed for each 4-second data acquisition period. Data were averaged for each sinus rhythm and each atrial pacing period. Results from individual rats were averaged to obtain values of mean±SEM separately for the sympathectomized and the intact groups.
Comparisons were made between values obtained during each pacing step and the corresponding sinus rhythm period. The statistical significance of the differences in mean values was assessed by the Student 2-tailed t test for paired observations. A value of P<0.05 was considered statistically significant.
| Results |
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Common Carotid Artery Mechanical Properties
In vehicle-treated rats, the diameter of the common carotid artery
was not substantially affected by pacing (Figure 2, top left), which, in contrast,
significantly reduced the arterial distensibility index at
each pacing step (Figure 2, bottom left). The reduction in
arterial distensibility tended to be greater as the pacing
rate increased, as was evident throughout the
systolic-diastolic pressure range (Figure 3, left). In sympathectomized rats,
carotid artery diameter was similar (Figure 2, top right)
whereas the arterial distensibility index was significantly
greater (Figure 2, bottom right) compared with vehicle-treated
rats. As in vehicle-treated rats, pacing-induced
tachycardia was associated with no significant change in
carotid artery diameter and with a significant reduction in the
arterial distensibility index. The reduction was evident
throughout the systolic-diastolic pressure range at
all pacing steps (Figure 2, right).
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Femoral Artery Mechanical Properties
In vehicle-treated rats, femoral artery diameter and
distensibility were not sizably affected by pacing (Figure 4, left). In sympathectomized rats with
spontaneous sinus rhythm, diameter was moderately and distensibility
index was markedly and significantly greater than in vehicle-treated
rats (Figure 4, right). Pacing-induced tachycardia
did not affect diameter but, at variance with intact rats, it markedly
reduced the distensibility index, the reduction being evident
throughout the systolic-diastolic pressure range at
all pacing steps (Figures 4 and 5).
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| Discussion |
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These data are consistent with the hypothesis that the rate-dependent decrease in arterial distensibility is primarily due to the viscous nature of the vessel wall and thus to its inertial response to changes in intravascular pressure; this is in agreement with the data of Boutouyrie et al19 that arterial wall viscosity is more strongly influenced by mechanical load than by smooth muscle tone. Our data are also consistent with the hypothesis that the rate-dependent arterial stiffening is less evident in muscle-type than in elastic-type arteries, because in the former, distensibility is already kept to remarkably low levels by the high smooth muscle tone that characterizes these vessels.20 21 22
Several other aspects of our study are worth comment. First, the femoral artery distensibility in sympathectomized rats was greater than that in intact rats during spontaneous sinus rhythm but not during pacing-induced tachycardia, whereas the carotid artery distensibility in sympathectomized rats was greater than that in intact rats irrespective of heart rate (ie, during both sinus rhythm and pacing-induced tachycardia). In other words, the stiffening influence of tachycardia was able to completely offset the antistiffening effect of sympathectomy in the femoral but not in the carotid artery.
Second, as mentioned above, after sympathectomy, arterial distensibility was increased both in the femoral and in the carotid artery, which indicates that sympathetic tone has a stiffening effect not only in arteries where muscle tone is prominent but also in arteries that have a predominantly elastic structure.10 11 12 23 This effect presumably is due to the fact that elastic-type arteries have a significant amount of smooth muscle tissue supplied by sympathetic nerve fibers24 (although to a lesser extent compared with muscle-type arteries,5 25 ); this supply is responsible for a contraction that elevates the elastic modulus of the vessel.
Third, in the carotid artery, the stiffening effect of tachycardia was similar in intact and sympathectomized rats, regardless of whether distensibility was smaller or larger. This indicates that sympathetic activity has a prominent stiffening effect on this vessel but also indicates that this effect is independent of the one due to tachycardia, with which it does not interact. On the other hand, the behavior of the femoral artery was in several ways at variance with that of the carotid artery; ie, in the former vessel, distensibility was also enhanced after sympathectomy, but this was only seen during sinus rhythm. Furthermore, the stiffening effect of tachycardia was evident in sympathectomized but not in intact rats. Thus, tachycardia and sympathetic nerve activity are important modulators of wall mechanics in muscle-type arteries; however, in these vessels, either factor alone is capable of a near-maximal effect, which prevents the influence of the other from manifesting.
Our study has some potential limitations. First, intact and sympathectomized animals have markedly different circulating levels of several substances, such as angiotensin, vasopressin, catecholamines,26 and possibly endothelin and natriuretic peptides, that have vasoactive properties and thus may affect arterial distensibility. However, it is unlikely that these differences were primarily involved in the acute effect of tachycardia on arterial distensibility due to the rapid (seconds) onset of the pacing-related alterations.
Second, our data were collected while the animals were anesthetized, a condition known to alter autonomic tone.27 28 Thus, our findings will have to be confirmed in conscious animals, in which measurement of arterial distensibility is technically demanding and less precise because it involves the disruption of periarterial tissue.
Third, our experiments cannot answer the question of which components of vascular smooth muscle tone are responsible for the differences in the behavior of the carotid versus the femoral artery; ie, whether the neurally dependent or intrinsic tone of the vessel is responsible. However, it is reasonable to propose that both components are involved: a role for the former factor is suggested by the observation that sympathectomy had more prominent antistiffening effects on the femoral than on the carotid artery; on the other hand, a role for the latter is indicated by the fact that even after sympathectomy, the femoral artery remained much stiffer than the carotid artery.
Fourth, caution needs to be exerted as to the clinical implications of our findings, because pacing-induced tachycardia may be different from everyday tachycardias, which are mediated by neural mechanisms that may concurrently alter vascular smooth muscle tone. In a recent study in a large number of human subjects, overall arterial compliance was shown to be inversely related to heart rate.29 This may be due to the fact that overall compliance depends to a major degree on elastic-type vessels.
In conclusion, our experiments in anesthetized normotensive rats provided evidence to support the notions (1) that in predominantly elastic-type arteries, the stiffening effect of tachycardia is exerted independently of sympathetic modulation of the vessel wall properties and (2) that in predominantly muscle-type arteries, removal of sympathetic influences unmasks the stiffening effect of tachycardia; this effect is not directly demonstrable in the intact animal, presumably because of an already prominent stiffening effect of sympathetically mediated smooth muscle tone in this vessel type.
Received April 14, 1999; first decision May 4, 1999; accepted May 31, 1999.
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