(Circulation. 1995;91:2669-2678.)
© 1995 American Heart Association, Inc.
Articles |
Effect of Vasa Vasorum Flow on Structure and Function of the Aorta in Experimental Animals
From the Department of Cardiology, Hippokration Hospital, University of Athens, Greece, and the Department of Cardiology, Ohio State University, Columbus (H.B.).
Correspondence to Christodoulos Stefanadis, MD, 9 Tepeleniou St, Paleo Psychico, Athens 154 52, Greece.
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Abstract |
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Background It is known that vasa vasorum flow contributes substantially to the nutrition of the outer layers of the thoracic aorta. This investigation was undertaken to test the hypothesis that impairment of vasa vasorum flow would alter the structure of the aortic wall and change the elastic properties of the aorta.
Methods and Results The periaortic fat that contains the vasa vasorum for the ascending aorta was removed in seven anesthetized dogs, and the results were compared with those obtained from six weight-matched sham-operated control dogs. Aortic pressures, aortic diameters, and aortic distensibility were obtained before and 30 minutes and 15 days after removal of the periaortic vasa vasorum network. Aortic pressures were measured directly with a fluid-filled catheter. Aortic diameters were measured simultaneously with aortic pressures with an elastic, air-filled ring connected to a transducer. Aortic distensibility was calculated by the formula 2xpulsatile change in aortic diameter/(diastolic aortic diameterxpulse pressure). Histology was performed in transverse blocks of aortic wall at the end of the experiment in both groups. The efficacy of the technique for the interruption of vasa vasorum blood supply to the aortic wall was demonstrated by histology in four additional animals that were killed without removal of vasa vasorum (two animals) and immediately after vasa vasorum removal (two animals). At baseline, heart rate, aortic pressures, aortic diameters, and aortic distensibility were similar in the two groups. A significant decrease in aortic distensibility was observed 30 minutes and 15 days after removal of the vasa vasorum in the experimental group (baseline, 3.453±1.023; 30 minutes, 2.521±0.760; 15 days, 1.586±0.488 10-6 · cm2 · dyn-1; F=9.532, P<.001). No changes were observed in aortic distensibility in the control group during the experiment. Histology of the aorta revealed medial necrosis, alterations of the elastin fibers, and a trend (P=.055) for altered collagen-to-elastin ratio in a region occupying more than the one (outer) half of the media of the experimental group animals. No changes were observed in the control group.
Conclusions The findings of the present study demonstrated that interruption of vasa vasorum flow led to an acute decrease in the distensibility of the ascending aorta. Moreover, structural changes of the aortic wall and further deterioration of the elastic properties of the aorta occurred 15 days after vasa vasorum removal.
Key Words: aorta • elasticity • microcirculation • ischemia • structure
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Introduction |
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It is known that vasa vasorum flow supplies the adventitia and most of the media of the thoracic aorta with nutrients. Vasa vasorum flow, therefore, may play an important role for aortic wall structure and function.1 2 3 4 5 6 7 8 9 10
Previous studies from our laboratory showed that aortic distensibility was decreased significantly 30 minutes after the removal of vasa vasorum in experimental animals compared with baseline.1 In the same study, preliminary observations suggested that medial necrosis may occur after vasa vasorum removal. The aim of the present study was to assess chronic changes in aortic distensibility after vasa vasorum removal and to determine a possible relation between these changes and structural abnormalities of the aortic wall. For this purpose, aortic distensibility was measured both 30 minutes and 15 days after vasa vasorum removal, and extensive histological studies of the aortic wall were performed.
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Methods |
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Experimental Animals and Procedures
Seventeen healthy mongrel dogs of either sex, weighing between 13 and 21 kg, were studied. Of these, 13 were used for the study of aortic distensibility and structure and 4 for validation of the vasa vasorum removal technique.
The 13 animals in which aortic distensibility and structure were studied were premedicated with diazepam (0.25 mg/kg IV) and atropine (0.005 mg/kg IV). Anesthesia was initiated with sodium pentobarbital (30 mg/kg IV). To provide a steady anesthetic state, supplementary doses were given throughout the experiment. After intubation, the animals were connected to a volume respirator (Harvard) and were mechanically ventilated with room air. To obtain measurements, respiration was interrupted for short intervals.
A left thoracotomy was performed through the fourth intercostal space. The pericardium was opened longitudinally and the heart suspended in a pericardial cradle.
This investigation was approved by the review committee of our institution and was performed in accordance with the guiding principles of the American Physiological Society.
Aortic Pressure Measurements
A 4F fluid-filled catheter was
advanced through the left
subclavian artery to the ascending aorta under fluoroscopic control and
connected through a pressure transducer (Statham P23Db) to a DR-8
recorder (Electronics for Medicine Inc).
Aortic Diameter Measurements
A special air-filled ring,
manufactured in our research and
development laboratory and sensitive in detecting changes of the aortic
diameters, was placed around the aortic root 2 cm distal to the
coronary arteries in each animal. This low-compliance device consisted
of an elastic latex air-filled ring connected through a rigid tube to a
transducer as described previously from our laboratory.1
Fluctuations in diameter of the aorta due to the pulsatile flow
produced changes in the pressure of the air inside the ring and were
recorded as a waveform. Rings of appropriate sizes were used to fit
into the aortic root of each individual animal. The use of the
air-filled ring was preferred in the present study because this
device is capable of estimating global changes in aortic diameter
rather than changes in the distance between two points of the aortic
wall.11 12 13 14 15 16
Before each experiment, the system was calibrated against known increments of diameter as follows: The air-filled ring was placed around an elastic cylinder, the cylinder was subsequently filled with normal saline at a temperature of 37°C, and its diameter was increased gradually from 1.2 to 2.8 cm and correlated with the level of the signal recorded.1 Thus, from the waveform amplitude, the actual diameter of the cylinder could be measured.
The characteristics of the device have been described.1 In brief, they are as follows. Within the range of diameters encountered in this study, the calibration of the various devices used in the experiments was essentially linear, and the sensitivity ranged from 1.8 to 2.3 cm of recorder deflection per millimeter of diameter at maximum amplification. Frequency response of the system was found to be uniform (±5%) to 20 cycles per second. An essentially linear phase lag–versus–frequency relation of approximately 5° per cycle occurred through this frequency range.
Experimental testing regarding the mechanical impedance that the device might offer to the motions of the aorta showed that the device does not alter the stiffness of the aortic wall.1 Moreover, when measurements of diameters by the device were compared with high-resolution echocardiography in in vitro testing, there was no difference between the diameters obtained by the two methods, and an excellent correlation was found between them.1
Calculation of Aortic Distensibility
The distensibility of
the ascending aorta was calculated from
the formula: Aortic distensibility=2x
d/(dxP),
where d is
diastolic aortic diameter, d is systolic minus diastolic aortic
diameter (pulsatile change in aortic diameter), and P is systolic
minus diastolic aortic pressure (pulse
pressure).11 17
Experimental Protocol
The air-filled ring was initially
placed around the ascending
aorta through a small tunnel in the periaortic fat. Extreme care was
taken to avoid, as far as possible, damage of the penetrating vasa
vasorum.1 When the hemodynamic condition of the animal was
considered stable, the filling pressure of the ring was adjusted to be
60 mm Hg at diastole; this filling pressure was in every case below
the aortic diastolic pressure of the animal, so that the ring would be
in gentle contact with the aortic wall. At systole, the filling
pressure of the ring increased by approximately 5 mm Hg because of the
distension of the aorta (the exact value depended on the amount of
distension of the aorta). Conditions of contact between the ring and
the aortic wall were stable throughout the experiment. After placement
of the ring, changes of the aortic diameter were recorded
simultaneously with the ECG and the aortic pressures at a paper speed
of 50 mm/s. To evaluate the data of each experimental subject, 10
consecutive pressure and diameter complexes from the simultaneous
recordings were measured and averaged.1
The 13 dogs in which aortic distensibility and structure were studied were then separated into two groups: Group A, the control group, consisted of 6 animals (weight, 17.6±2.8 kg); and group B, the experimental group, consisted of 7 animals (weight, 17.3±2.9 kg).
In group A, after the first recordings, the ring was removed and the animals remained in a stable anesthetic state; no further manipulations were performed on the ascending aorta. The ring was repositioned around the aorta 30 minutes later, and the same recordings were obtained.
In group B, after the first recordings, the ring was removed and all the perivascular fat containing the vasa vasorum network was carefully dissected and removed from the coronary arteries to the left subclavian artery. Bleeding was controlled with the application of gauzes moistened with normal saline at a temperature of 37°C. No diathermy was used.1 The ring was repositioned around the aorta 30 minutes later, and all recordings were repeated.
During the 30-minute interval from the first to the second recordings, dressings moistened with normal saline at a temperature of 37°C were applied on the ascending aorta of all animals of both groups. All manipulations were performed with extreme care to avoid damage of the aortic wall.
After the recordings, in all animals of both groups, the air-filled ring was removed, the pericardium and chest were closed, and the pneumothorax was treated. The catheter for the aortic pressures was then removed. All dogs recovered quickly from the operative procedure and were actively ambulatory and apparently in good health until they were killed.
Fifteen days after the initial procedure, all animals were premedicated, anesthetized, mechanically ventilated, and thoracotomized as described above. A 4F fluid-filled catheter was advanced through the left subclavian artery to the ascending aorta under fluoroscopic control, and the air-filled ring was again placed around the aorta. Recordings of aortic diameters and aortic pressures were obtained under conditions identical to those of the acute phase of the experiment.
After the final recordings, the animals were killed. In group A, transverse blocks of the wall of the ascending aorta, together with the adjacent periaortic fat, were cut to include the entire circumference. In group B, transverse blocks to include the entire circumference were cut from the region of the aortic wall supplied by the removed vasa vasorum; additional blocks were cut from a level beyond the area supplied by the removed vasa vasorum.
Validation of Vasa Vasorum Removal Technique
To test that the
technique interrupted circulation to the aortic
wall effectively, transverse blocks of aortic wall were cut from the
area of interest to include the entire circumference in 4 animals. In 2
animals, the periaortic fat containing the vasa vasorum network was not
removed. The animals were killed, and samples of the ascending aorta
together with the intact adjacent periaortic fat were taken. The
remaining 2 animals were anesthetized and operated on as for animals in
group B and then killed after removal of periaortic fat containing the
vasa vasorum network. Samples of the aortic wall and the removed
periaortic fat were taken.
Histological Studies
All removed tissues were placed in
formaldehyde 10% fixation
fluid, where they remained for at least 24 hours. The blocks of tissues
were then embedded in paraffin, sliced in 5-µm sections, and stained
with hematoxylin-eosin, Verhoeff's elastica, Gomori, Masson's
trichrome, and Sirius red (0.1% Sirius red F3BA dissolved in saturated
picric acid, pH 2.0) stains. The sections were examined under a Zeiss
light microscope.
Morphometric Studies
Medial thickness, considered to be the
perpendicular distance
between the innermost and the outermost medial elastic lamellae, was
measured on microscopic sections by means of an eyepiece micrometer.
Measurements were made at four positions, the first randomly chosen and
the others at approximately 45°, 90°, and 135° around the vessel
perimeter; the average of the four measurements was taken as medial
thickness. The number of medial elastic lamellae at each of the
measurement positions was counted; the average of the four counts was
considered to be the total number of medial elastic lamellae.
Total elastin and collagen content was quantified by image analysis in the necrotic region in the experimental group and in the corresponding region in the control group by image analysis as previously described18 in appropriately stained (with Verhoeff's stain for elastin and with Sirius red stain for collagen) cross sections. The equipment included a video camera system fitted to the light microscope, a host computer (Olivetti PC 240), and an image analyzer (Optomax V, Analytical Measuring Systems Inc). Sections were studied with a x16 magnifying lens and projected onto a video camera as a monochromatic microscopic image at a final magnification with a calibration factor of 0.72 µm/pixel. In this monochromatic microscopic image, components of interest (elastin and collagen) were displayed in gray tones from dark gray to nearly black, whereas the other tissue components were displayed in gray tones from light gray to nearly white. By computer analysis, a 256-level gray-level histogram was obtained. In this histogram, the frequency of occurrence of pixels at each particular gray level was depicted at the ordinate and expressed as a percentage of the total area. By systematic scanning of a glass slide, a sequence of 30 nonoverlapping, randomly selected images from each area of interest was digitized. Subsequently, a composite histogram was obtained. The percentage of the area of the component of interest (elastin and collagen) for the particular histological section was obtained by calculating the integral of the part of this histogram above a certain threshold. This threshold was defined as the minimum of the gray-level histogram in histograms showing a bimodal distribution or as the change in slope of the distribution along its ascending portion for distributions without two clear modes.
Statistical Analysis
Values are expressed as mean±SD.
For changes within each group
regarding heart rate, aortic diameters, pressures, and distensibility,
ANOVA for repeated measurements was used. In morphometric studies,
comparisons between the two groups were performed with an unpaired
t test.
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Results |
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Baseline Characteristics
Body weight, heart rate, aortic pressures, aortic diameters, and aortic distensibility were similar in the two groups (Tables 1
and 2).
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Changes in Heart Rate, Aortic Diameters and Pressures, and Aortic
Distensibility Within Each Group
In group A, the control group, there
were no statistically
significant changes in any of the measured parameters throughout the
study period (Table 1
, Fig 1A).
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In group
B, the experimental group (Table 2), heart rate, systolic
aortic pressure, diastolic aortic pressure, and pulse pressure did not
change significantly throughout the study period. A significant
decrease in pulsatile change of the aortic diameter was observed in
this group (F=11.364, P<.001). Compared with baseline,
pulsatile change of the aortic diameter was lower both 30 minutes after
vasa vasorum removal and 15 days after vasa vasorum removal
(P=.05 and P<.001, respectively). Additionally,
15 days after vasa vasorum removal, pulsatile change in aortic diameter
was significantly lower compared with 30 minutes after vasa vasorum
removal (P<.05).
A significant decrease in aortic
distensibility was observed in the
experimental group (F=9.532, P<.001, Fig
1B). Compared with
baseline, aortic distensibility was lower both 30 minutes and 15 days
after vasa vasorum removal (P<.05 and P<.01,
respectively). Additionally, aortic distensibility 15 days after vasa
vasorum removal was significantly lower compared with 30 minutes after
vasa vasorum removal (P<.05).
Histological Examination
The periaortic vasa vasorum network
is shown in Fig 2A and 2B. In the animals that
were killed immediately
after vasa vasorum removal for validation of the technique for
interruption of blood supply to the aortic wall, the vasa vasorum
network was found to be removed completely. Neither hemorrhage nor
trauma of the smooth muscle cells or the elastic lamellae was observed
(Fig 2C).
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In all the group A animals, all structures
were apparently normal (Figs 3A and 4A).
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In all group B animals, necrosis (infarct) of the media had developed
in the region that corresponded to the removed periaortic vasa vasorum
network. At most levels, a uniform band of necrosis of the outer layers
of the media more than one half its width completely encircled the
entire circumference. In the necrotic zones, there was complete loss of
smooth muscle cells. These zones bordered sharply on the viable inner
medial layers (Figs 3B and 4B). In the necrotic
zones, elastin fibers
and their sheaths were preserved; however, they appeared uncoiled,
dislodged, somewhat thinned, and focally fragmented (Figs 5 and
6). In areas of both the necrotic
and the viable zones located on both sides of the borderline of
necrosis, close apposition of the elastic lamellae was observed.
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In other sections bordering the central area of vasa vasorum removal, arclike areas with complete loss or reduced numbers of smooth muscle cells were found. In these sections, a close apposition of the elastic lamellae was frequently observed. In sections approximating the large vessels of the neck or the origin of the aorta, midzonal foci of the same necrotic lesions were also observed.
A proliferation of fibroblasts was observed in the adventitia, in which evidence of collagen formation was noted with Masson's trichrome and Sirius red stains. There was no leukocytic infiltration to the necrotic zone.
In the sections taken from the part of the aorta beyond the area of vasa vasorum removal, no alterations were observed, and all structures were apparently normal.
Morphometric Analysis
The thickness of the media was not
different between the two
groups at the time the animals were killed (group A, 1.39±0.33 versus
group B, 1.44±0.31 mm, P=NS).
The total number of elastic lamellae did not differ between the two groups. Of interest, however, is the observation that although the inner viable zone of the media in the vasa vasorum removal group had a variable width depending on the total width of the media of the animal, the number of elastic lamellae in this zone was constant and equal to 28.4±2.6 elastic lamellae.
Collagen and Elastin Content
The percentage of collagen was not different in the two groups
(group A, 22.15±1.96% versus group B, 20.59±2.48% of total area,
P=NS, Fig 7). Elastin showed a trend,
although it was not statistically significant, to be less in the
experimental group compared with the control (42.77±6.15% versus
49.29±4.52% of total area, respectively, P=.055, Fig
5).
Likewise, the collagen-to-elastin ratio showed a trend, although
it was not statistically significant, to be higher in the experimental
group compared with control (0.483±0.028 versus 0.450±0.027,
respectively, P=.055).
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Discussion |
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The present study confirmed previous observations from our laboratory1 according to which a decrease in the distensibility of the ascending aorta was noted 30 minutes after the removal of the vasa vasorum. Moreover, the present study demonstrated that aortic distensibility was decreased further 15 days after the removal of the vasa vasorum. Along with these functional changes, significant structural abnormalities of the aortic wall were observed 15 days after the vasa vasorum removal. Lack of blood supply to the outer part of the aortic wall is most likely to have accounted for these findings.
Vasa Vasorum Flow of the Aorta
Thin-walled aortas are
supplied with nutrients by diffusion from
the lumen or from the adventitial vessels. In the thoracic aorta of
large mammals, such as humans and dogs, the media exceeds a critical
thickness, defined as 29 elastic lamellae, and thus nutrition of the
aortic wall is supplemented by blood flow through small vessels, the
vasa vasorum, that form a perivascular network and penetrate into the
medial
layers.3 4 6 7 8 9
Our findings support this concept,
since the inner viable zone of the media, which was not affected by the
interruption of vasa vasorum supply and was presumably nourished by
diffusion from the lumen, had an average width of 28.4 elastic
lamellae. For the ascending aorta, the vasa vasorum originate from the
coronary arteries; for the aortic arch, mainly from the great vessels
of the neck and their proximal branches; and for the descending
thoracic aorta, from the intercostal arteries.2 8
Flow
through vasa vasorum is subject to active
regulation5 7 10
and provides a considerable amount of blood supply to the aortic
wall.
Vasa Vasorum Flow and Medial Necrosis
Previous studies from
our laboratory1 demonstrated a
mild degree of edema in the wall of the ascending aorta 30 minutes
after the removal of the vasa vasorum. The present study
demonstrated ischemic necrosis, alterations in the elastic tissue of
the aortic wall, and a trend for altered collagen-to-elastin ratio 15
days after the removal of the vasa vasorum. Although they are less
vulnerable than smooth muscle cells, as the gravity of the changes
indicates, elastin fibers were preserved, yet they appeared to a
certain extent thinned, flattened, disarranged, and focally fragmented
in the areas corresponding to the vasa vasorum removal. These findings
are in agreement with previous observations according to which necrosis
of the middle third of the media and changes in the elastica of the
thoracic aorta were observed after ligation of intercostal arteries in
dogs.8 Similar changes secondary to ischemia were found in
the carotid arteries of experimental animals.19
Previous investigators have also suggested that decreased blood flow of the aortic wall during acute hypertension might contribute to aortic media necrosis.7 Our findings correlate well with studies in which an aortic infarction lesion similar to the lesion described in the present report was observed in the central zone of the media of the ascending aorta in patients with dissection. The authors suggested that this lesion is related to lack of sufficient blood supply.20
Vasa Vasorum Flow and Functional Abnormalities of the Aorta
A
progressive decrease in the distensibility of the ascending
aorta after the removal of the vasa vasorum was observed in the
present study.
Elastic properties of the aortic wall are determined principally by the elastic components of the media. Elastin, collagen, collagen-to-elastin ratio, and smooth muscle cells, as well as composition and amount of extracellular matrix, thickness of the aortic wall, and aortic diameter, play an important role in the determination of the aortic elastic properties.21 22 23 24 25 26 27
Removal of vasa vasorum led to sustained ischemia of the aortic wall, as suggested by the ischemic nature of necrosis observed 15 days after the procedure. Ischemia leads to acute alterations of the elastic components of the aortic wall. The muscle layers, composed of metabolically active cells, are dependent on a steady flow of nutrients for survival and normal function. Indeed, previous studies from our laboratory1 showed a mild degree of interstitial edema in the aortic wall, suggesting possible smooth muscle cell ischemia, 30 minutes after removal of the vasa vasorum. Experimental studies have shown deterioration in left ventricular distensibility after 30 minutes of ischemia of the myocardium.28 Moreover, an early loss of smooth muscle cells, as well as dislodging and disarranging of some elastin fibers in the aortic media of experimental animals, 1 day after ligation of the intercostal arteries was found by other investigators.8 Therefore, it seems reasonable to suggest that inadequate blood supply of the aortic wall leads to acute changes in the elastic components of the aortic wall and alters its elastic properties acutely.
According to the findings of the present study, necrosis of the smooth muscle cells and changes in the elastic fibers of the aortic wall occurred in the media of the ascending aorta 15 days after the removal of the vasa vasorum. Moreover, the collagen-to-elastin ratio, to which more functional significance is attached than to the absolute percentage of each constituent, showed a trend to shift toward higher values, indicating a stiffer vessel. These structural changes, secondary to interruption of vasa vasorum flow, are most likely to have accounted for the late reduction in the aortic distensibility observed in the present study.
Specific Comments: Limitations of the Study
Change in blood
pressure is a factor that could have an effect on
aortic distensibility. However, changes in blood pressure were not
observed throughout the experiment; consequently, this factor could not
have accounted for the findings of the present study.
The lack of
changes in blood pressure indicates that the deterioration
in aortic elasticity was due to nonpassive changes in aortic elastic
properties and that the slope of the elasticity line (plot of pressure
versus diameter) of the individual animal has been substantially
modified. Although elasticity indexes, such as vessel distensibility,
when measured in vivo are reliable means for gaining insight into the
elastic properties of the vessel, they describe only a specific part of
the elasticity line, the one corresponding to the range of pressures
encountered in the particular study. As documented by our results, the
vessel becomes less distensible and the slope of the elasticity line
steeper within the range of pressures encountered in our study (between
diastolic and systolic pressure) after vasa vasorum removal. However,
because of lack of information regarding the total configuration of the
elasticity line, we cannot determine whether the vessel becomes more or
less distensible in low pressures after the removal of the vasa vasorum
(Fig 8). Nevertheless, regardless of the configuration
of the curve below a certain point, the significance of our results
remains intact because they indicate that under in vivo conditions
(without regard to what happens under theoretical conditions), the
aorta becomes stiffer, with obvious clinical implications.
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Factors related to manipulations of the experiment other than the removal of vasa vasorum could have an effect on the findings of the present study. This seems unlikely, however, because except for the removal of the periaortic fat network, the animals in the two groups were otherwise manipulated in an identical manner for the same length of time. Moreover, no histological changes were observed in areas beyond the area supplied by the removed vasa vasorum in the experimental group.
Removal of periaortic fat probably removes nerves to the aortic wall, which might reduce smooth muscle cell tone and influence the findings of the present study. The smooth muscle cell contribution to the elastic properties of large arteries is a controversial topic. Some investigators29 30 31 found a decrease in the distensibility of large arteries by smooth muscle cell excitation, whereas others32 33 34 observed a paradoxical increase. This difference in interpretation is due to conditions under which the measurements are made.35 36 37 Nevertheless, as discussed extensively in the literature, the physiological condition when the artery is in situ and the muscle is largely contracting isometrically is reflected by a decrease in the incremental distensibility by smooth muscle cell contraction.29 35 37 38 Thus, reduction of smooth muscle cell tone in our experiments would be expected to increase aortic distensibility. However, any such effect was overcome by the effect of ischemia, which finally resulted in a decrease of aortic distensibility. Nevertheless, any possible contribution of this factor is confined to the acute phase of the experiment.
Removal of a part of the adventitia, per se, could have an effect on the changes in aortic distensibility documented in our study. The contribution of the adventitia to the mechanical properties of the aortic wall is small. However, removal of an additional layer of the aortic wall, no matter how small its contribution to the stiffness of the aortic wall might be, would theoretically be expected to increase and not to decrease distensibility.
Periaortic fibrosis was noted in the group with vasa vasorum removal, and because of the relatively high elastic modulus of collagen, this could contribute to the decrease of the distensibility 15 days after the procedure. However, proliferation of fibroblasts could be induced either by the surgical stripping of the periaortic fat or by the ischemia itself, thus constituting an expression of the underlying abnormal process.
The absence of inflammatory cells in the area of necrosis can be explained by the interruption of circulation to the aortic wall due to the removal of vasa vasorum. The same finding was observed in similar lesions in experimental8 and human studies.20
Vasa Vasorum Flow: Aortic Structure and Function: Clinical
Implications
Structural Changes
Since the structural
changes demonstrated in our study resemble in
many ways those observed in aging
aortas,8 39 40 it can be
speculated that the degenerative changes that occur with advancing age
may be related to impaired nutrition of the aortic wall.
The findings of the present study support the hypothesis that impaired vasa vasorum flow of the ascending aorta may play an important role in the development of medial degeneration in humans.8 Interestingly enough, vasa vasorum are most abundant in the ascending aorta and arch, precisely the segments most susceptible to the development of dissecting aneurysm. Since muscle lesions tend to increase with age, especially after 40 years of age, and in patients with hypertension, they correlate well with the known occurrence of dissecting aneurysms.41 On the other hand, the infarction of the aortic media with histological features very similar to those observed in our study that was found in dissected human aortas was attributed to disruption of the vasa vasorum by the dissection and consequent interruption of vasal supply.20 Nevertheless, even if aortic infarction follows rather than precedes dissection, it can be speculated that the necrotic zone would constitute a nidus for continuing or future dissections.20 In the same studies, the necrotic zone was more extended when thrombus or atheromata were obstructing the false lumen, which, after the dissection, maintained the media bordering the dissection tract by diffusion of the nutrients. Accordingly, although this was not described in these studies, one might suppose that extensive atherosclerotic lesions of the inner aortic wall would disturb diffusion of nutrients from the true lumen and induce necrosis.
With regard to aortic dilatation, it can be suggested that the weakening of the media by these structural changes would result in disturbed absorption of the hemodynamic forces that act on the wall, which in turn will eventually lead to aortic dilatation. Further, this process may in turn set up a vicious circle of events (including the occurrence of dissection) because, according to Laplace's law, wall tension increases proportionally with arterial radius.42
It is also possible that the lesions observed in the aortic media in syphilitic aortitis are produced by the impairment of blood supply through the thickened, narrowed, and often obstructed vasa vasorum.43
Functional Changes
Changes
in vasa vasorum flow may occur in several
conditions,7 10 such as hypertensive crisis,
decreased
cardiac output or shock, congestive heart failure,
vasoconstriction or vasodilation due to neurohumoral activation,
and administration of pharmacological agents. Vasa vasorum flow may
also be compromised after cardiac surgery.
Studies from our14 16 44 45 and other46 47 48 laboratories have shown that aortic distensibility is unfavorably affected in the presence of coronary artery disease. Apart from the mechanical effect of aortic wall atheromata, abnormal nutrition of the aortic wall could be another possible explanation for this finding, since the vasa vasorum for the ascending aorta originate from the coronary arteries.
It has been demonstrated15 49 that aortic distensibility is reduced in hypertensive patients, and it has also been shown that chronic hypertension significantly decreases the vasodilatory capacity of vasa vasorum to the thoracic aorta.5 Accordingly, it is reasonable to speculate that a possible contributing mechanism to the reduced aortic elasticity of hypertensive patients might be reduced vasa vasorum flow.
Thus, under certain conditions, alterations of the elastic properties of the aorta may occur after acute or chronic changes in vasa vasorum flow.
Aortic Distensibility
Several studies have
shown that aortic distensibility is an
important factor determining left ventricular
performance50 51 52 and coronary blood
flow.46 53 Coronary flow, and especially
subendocardial
flow, is reduced in cases of decreased distensibility.53
Moreover, decreased aortic distensibility may contribute to left
ventricular dysfunction and dilatation in patients with aortic
regurgitation.54
Present technology allows accurate evaluation of the elastic properties of the aorta.13 14 15 16 44 45 46 47 48 49 54 55 Thus, abnormal elastic properties of the aorta may provide an indication of early disease process, and changes of the elastic properties of the aorta may provide information for the natural history of the disease.54 In addition, measurements of aortic distensibility after therapeutic interventions15 16 may further our understanding of the function of the aorta in disease states.
In conclusion, removal of the vasa vasorum network led to an acute decrease in the distensibility of the ascending aorta. In addition, structural changes of the aortic wall and further deterioration of the elastic properties of the aorta were observed 15 days after removal of the vasa vasorum.
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Acknowledgments |
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This investigation was supported by a grant from the Hellenic Heart Foundation. The authors gratefully acknowledge the advice and constructive comments of Anton E. Becker, MD, in the histological evaluation.
Received November 1, 1994; accepted December 13, 1994.
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References |
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1. Stefanadis C, Karayannacos P, Boudoulas H, Statos C, Vlachopoulos C, Dontas I, Toutouzas P. Medial necrosis and acute alterations in aortic distensibility following removal of the vasa vasorum of canine ascending aorta. Cardiovasc Res. 1993;27:951-956.
2. Robertson HF. Vascularization of the thoracic aorta. Arch Pathol. 1929;8:881-890.
3. DeSousa A, Alvares L. Microangiographic study of the vasa-vasorum of the thoracic aorta. Radiology. 1960;75:91-97.
4.
Wolinsky H, Glagov S. Nature of species
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