(Stroke. 1997;28:1216-1224.)
© 1997 American Heart Association, Inc.
Articles |
In Vivo Angioplasty Prevents the Development of Vasospasm in Canine Carotid Arteries
Pharmacological and Morphological Analyses
From the Department of Surgery, Division of Neurosurgery (J.F.M., J.M.F.), Department of Pharmacology (B.V., D.A.C.) and Surgical Medical Research Institute (M.H.C.), University of Alberta, Edmonton, Alberta, Canada.
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Abstract |
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Background and Purpose To study the effects of in vivo transluminal balloon angioplasty (TBA) on the structure and function of the arterial wall, a canine model of hemorrhagic cerebral vasospasm of the high cervical internal carotid artery (ICA) was used. This model was also used to determine whether TBA performed before clot placement could prevent the development of vasospasm.
Methods Twelve dogs underwent surgical exposure of both
distal cervical ICAs, followed by baseline angiography. One randomly
selected ICA in each dog was then subjected to in vivo TBA and repeated
angiography. Both distal ICAs were then surrounded with blood clots
held by silicone elastomer sheaths. Seven days later angiography was
repeated, and all animals were killed. The ICAs in four animals were
perfusion-fixed in situ for morphological analysis by electron
microscopy, and the arteries in the remaining eight animals were
removed and immediately immersed in oxygenated Krebs'
solution. Contractile responses of isolated arterial rings
from each ICA were recorded after treatment with KCl,
noradrenaline, serotonin, and
prostaglandin F2
, while relaxations in
response to the calcium ionophore A23187 and papaverine were
recorded after tonic contraction to noradrenaline had
been established. The morphology and pharmacological responses of ICAs
that had been exposed to blood with or without prior TBA were compared
with data obtained from control arterial segments of
intact, more proximal regions of the ICAs from each animal.
Results TBA resulted in immediate angiographic enlargement
of the ICA lumen that was still evident 7 days later despite the
placement of clotted blood around the artery. Scanning and transmission
electron microscopy demonstrated flattening of the intima and internal
elastic lamina in these dilated arteries, associated with patchy losses
of endothelial cells. In contrast, ICAs that had been
exposed to clotted blood but had not undergone prior TBA developed
consistent angiographic and morphological vasospasm. In
comparison with control vessels and nondilated vasospastic vessels,
vessels dilated with TBA and then exposed to clotted blood showed
significantly diminished responses to all compounds tested, with the
exception of prostaglandin F2.
Conclusions These results indicate that in vivo TBA results in a degree of functional impairment of vascular smooth muscle that persists for at least 7 days. This result is consistent with previous observations of the acute effects of TBA in isolated arteries. Furthermore, these results support the hypothesis that normal smooth muscle function is required for the development of vasospasm. Finally, these results indicate that TBA performed before the onset of vasospasm prevents its development.
Key Words: angioplasty • subarachnoid hemorrhage • vasospasm • dogs
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Introduction |
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The development and maintenance of delayed-onset cerebral vasospasm after aneurysmal SAH is believed to represent a combination of sustained smooth muscle constriction and structural change, both of which cause narrowing of the arterial lumen diameter. The mechanisms involved have been subjected to intensive experimental investigation.1 Severe vasospasm causing symptomatic cerebral ischemia can now be treated with TBA, which is the mechanical dilatation of vasospastic arteries by means of inflatable microballoons attached to the tips of percutaneously inserted arterial catheters.2 3 4 5 6 7 8 While it has been recognized that TBA results in efficient arterial dilatation and that improvement in cerebral blood flow leads to clinical recovery in a high percentage of patients, the morphological and functional effects of TBA on the cerebral arterial wall have only recently been reported.9 10 11
Previous work in this laboratory has demonstrated that in vitro TBA of both normal and vasospastic canine basilar arteries causes an immediate and profound functional impairment of vascular smooth muscle, as well as providing structural evidence of arterial wall stretching.9 However, determination of longer-term effects of TBA requires an animal model of cerebrovascular spasm in which the affected artery is accessible to consistent and safe in vivo TBA. We have thus developed a model of vasospasm using placement of clotted blood around the distal cervical ICA.
While it is clear that TBA can dilate spastic vessels, it is not known whether prior TBA can prevent the development of vasospasm. This question is of more than academic interest; if it is possible to alter the function of normal arteries by stretching, and this then renders them insensitive to agents that would normally produce vasospasm, the observation would have important implications about the mechanism by which vasospasm develops. It would also have clinical implications, in that if prior TBA can prevent the development of cerebrovascular spasm, it would support early TBA to halt the progression of the condition. Finally, long-term effects of TBA remain uncertain, and the use of the model and the experimental protocol outlined below would enable additional information on this area to be obtained.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Twelve mongrel dogs of either sex weighing between 16 and 25 kg were used in this study. The protocol was evaluated and approved by the University of Alberta Animal and Ethics Review Committee, and experiments were conducted with strict adherence to the standards of the Canadian Council on Animal Care.
Model of Blood Clot–Induced Vasospasm in the Canine High Cervical
Carotid Artery
This model is an adaptation of models used by others in the rat
femoral artery12 and in the rabbit cervical carotid
artery.10 11 13 Prior studies, performed in 20 animals in
our laboratory, have shown that placement of a blood clot held in place
by a silicone elastomer sheath around the canine ICA results in typical
angiographic and structural vasospasm 7 days later, while placement of
a silicone elastomer sheath alone does not significantly affect the
vessel wall. Furthermore, the time course of vasospasm development and
resolution in this model is consistent with that observed in
human cerebral arteries. For this study the following surgical protocol
was used:
Animals were anesthetized with sodium pentobarbital (0.5 mL/kg) and intubated on day 0 for angiography, TBA, and blood clot placement. Adequate anesthesia was maintained by administration of intravenous boluses of pentobarbital (0.05 mL/kg). The animals were allowed to breathe room air spontaneously, and arterial blood gases were determined on the first four animals to confirm that the protocol maintained normocarbia. A midline cervical incision was made, and both cervical ICAs were exposed and punctured with a 24-gauge angiocatheter for angiography (performed with 5 mL of iothalamate meglumine, injected at a rate of 0.75 mL/s). With the operating microscope used for magnification, 60-mm sections of both ICAs were then dissected free of adjacent tissues.
One of the ICAs was chosen by block randomization to undergo TBA, and a silicone elastomer balloon angioplasty catheter was introduced into this ICA lumen through a small arteriotomy proximal to the area of study. The balloon, which was 7 mm in diameter and 12 mm in length when inflated (303.9 to 506.5 kPa pressure), was used to dilate the ICA twice for 10 seconds, each time to approximately 150% of its original diameter along a length of 50 mm. Angiography was then repeated on this artery. Both ICAs were then covered along their dissected segments with 10 mL of autologous arterial clotted blood. The blood clot was placed within a 50-mm segment of silicone elastomer tubing sheath with a 10-mm inner diameter. The silicone elastomer tubing was secured with three silk ties along its length. Small cotton pledgets positioned at either end of the tube contained the clot.
The animals were cared for in the usual fashion for 7 days with daily neurological monitoring. There was no mortality or severe morbidity related to the procedures outlined above, and no animal developed a neurological deficit. On day 7 the animals were again anesthetized, the cervical incision was reopened, and angiography was repeated on both ICAs. The animals were then killed with sodium pentobarbital (30 mg/kg).
Four animals, designated as group A, underwent immediate in situ perfusion fixation of both ICAs with the use of 2.5% glutaraldehyde in 0.12 mol/L Millonig's buffer solution (pH 7.2). In the remaining group of 8 animals (group B), both ICAs were immediately removed and placed in Krebs' solution of the following composition: 120 mmol/L NaCl, 5 mmol/L KCl, 1.5 mmol/L CaCl2, 1 mmol/L KH2PO4, 1 mmol/L MgSO4, 25 mmol/L NaHCO3, 55 mmol/L dextrose, aerated with 95% O2/5% CO2 and maintained at 37°C. These arterial segments were used for pharmacological studies. In addition, a segment of ICA proximal to the original surgical field and the previously dissected ICA segment was removed on one side from each animal to provide a control preparation for both the morphological and pharmacological studies. Two rings were cut from each isolated ICA obtained from animals in group B, thus enabling us to examine the pharmacological properties of dilated and nondilated blood-coated ICAs and normal control ICAs. The inner diameters of the segments were measured with a micrometer under magnification before pharmacological analysis. The remainder of the arterial segments were prepared for morphological analysis.
Angiographic Measurement of Vasospasm and the Effects of
TBA
For all angiograms the diameter of the carotid artery in
millimeters was determined by direct measurement of the angiogram, at
the point corresponding to the midpoint of the blood-filled silicone
elastomer sheath. For the nondilated group, the degree of angiographic
vasospasm was determined by comparing vessel diameters on day 0 and on
day 7 and calculating the percent change in vessel diameter. For the
dilated group, the initial magnitude of the TBA-induced dilatation was
estimated by determining the percent change in vessel diameter on day
0, before and after angioplasty. After 7 days, the persistence of the
dilatation induced on day 0 was estimated by calculating the percent
change in vessel diameter between that seen on the day 7 angiogram and
that seen on the day 0 preangioplasty angiogram.
Pharmacological Studies
Responses of arterial rings were recorded
isometrically with force-displacement transducers connected to a
polygraph. Rings of cerebral arteries were suspended between two
stainless steel hooks, under a resting tension of 1 g in organ
baths of 10 mL working volume containing Krebs' bicarbonate solution
maintained at 37°C and bubbled with 95% O2/5%
CO2. After an equilibration period of 1 hour, during which
the Krebs' solution was changed every 15 minutes, the response to KCl
(60 mmol/L) was recorded, and preparations were washed until
resting tension was again obtained. Cumulative dose-response curves for
noradrenaline (10-9 to
10-5 mol/L), 5-HT
(10-9 to 10-5 mol/L),
and PGF2 (10-9 to
10-6 mol/L) were then recorded for each
arterial ring. Tonic contraction of preparations was
established with the use of noradrenaline
(10-5 mol/L) before vasorelaxation studies in
which cumulative dose-response curves to the calcium ionophore A23187
(10-8 to 10-5 mol/L)
were obtained. The response to papaverine
(5x10-4 mol/L) was also recorded after
tonic contraction with the use of noradrenaline
(10-5 mol/L) had been established.
Preparations were washed until the resting tension had been restored,
before another agent was tested. The ring preparations were studied in
the organ baths for approximately 8 to 10 hours. At the end of each
experiment the ring preparations were tested with KCl (60 mmol/L)
to confirm that the responses were not different from the initial
responses obtained to KCl at the beginning of the experiment.
Electron Microscopy
Segments from the three groups of vessels were examined with
transmission electron microscopy and scanning electron microscopy.
Cross sections of the vessel wall were examined with transmission
electron microscopy. The intact vessel wall (luminal and
cross-sectional aspects) was examined with scanning electron
microscopy.
All specimens were prefixed in 2.5% glutaraldehyde in 0.12 mol/L Millonig's buffer solution (pH 7.2) overnight at room temperature. After samples had been washed three times for 15 minutes each in Millonig's buffer, they were postfixed with 1% osmium tetroxide in the same buffer for 2 hours. These samples were briefly washed in distilled water and dehydrated in a graded series of ethanol solutions (50%, 60%, 70%, 80%, and 90%; 10 to 15 minutes each grade) before the final two 10-minute rinses with absolute ethanol. From this point onward, preparation for scanning electron microscopy and transmission electron microscopy samples differed.
For the scanning electron microscopy study, samples in absolute ethanol were dried in a CO2 critical-point dryer at 31°C for 5 to 10 minutes and then mounted on aluminum stubs. All samples were sputter-coated with gold before examination under the scanning electron microscope.
For the transmission electron microscopy study, absolute ethanol bathing the samples was replaced with propylene oxide, which was changed three times at 10-minute intervals. Samples were then embedded in Araldite CY212 mixture/propylene oxide for 1 hour and subsequently in complete Araldite CY212 mixture overnight in a vacuum desiccator. The mixtures embedding the samples were allowed to polymerize at 60°C for 2 days before sectioning. Sections were stained with 4% uranyl acetate for 30 minutes and lead citrate for 5 minutes. Photomicrographs of samples were taken at 75 kV with a transmission electron microscope.
We assessed the morphological appearance of nondilated and dilated groups separately; that is, nondilated arteries and dilated arteries were compared with normal arteries. Based on electron micrographs, a pairwise semiquantitative comparison of morphological differences between nondilated and normal arteries and dilated and normal arteries for each dog was performed by three independent researchers blinded to specimen identification. Specific features in scanning electron micrographs of the intact vessel wall were identified as follows: degree of luminal narrowing, wall thickness, and corrugation of intima and IEL. For transmission electron microscopy of vessel cross sections, observations included the degree of thinning of the entire vessel wall and its component layers; stretching and breakage of the IEL; straightening, stretching, and surface rippling of smooth muscle cells; and the amount of endothelial changes such as cell loss and flattening of the luminal surface.
Statistical Analysis
For the pharmacological study, comparisons between groups at
each concentration for each vasoconstrictor or vasodilator were
assessed with one-way ANOVA, followed by a Scheffé's test of
multiple comparisons if a significant probability was reached. Data
were expressed as mean±SE of the mean. A level of P<.05
was considered significant.
Equipment and Supplies
Force-displacement transducers (model FT.03) and a polygraph
(model 7D) were obtained from Grass Instrument Co. The critical-point
dryer was manufactured by Seevac, Inc. The sputter-coater, model S150B,
was manufactured by Edwards Vacuum. The scanning electron microscope
(model S-2500) and the transmission electron microscope (H-7000) were
obtained from Hitachi Ltd.
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Results |
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Angiographic Measurement of Vessel Diameter
In the absence of arterial dilation with TBA, the placement of a clot of autologous blood around the artery resulted in a highly significant constriction of the ICA. On day 7 after clot placement, the angiographic vessel diameters in the nondilated group were 48% to 89% of their original size, with a mean reduction to 63±3% (P<.05) (Fig 1B
). Dilation by TBA
had an immediate effect on arterial diameter. On day 0,
immediately after TBA and before clot placement, the angiographic
vessel diameters in the dilated group were 148% to 200% of their
original size, with a mean of 166±6% (P<.05) (Fig 2B). On day 7 after angioplasty and clot placement, the
angiographic vessel diameters in the dilated group showed that they
remained dilated, with a diameter of 125% to 203% of their original
size, giving a mean of 164±9% (P<.05) (Fig 2C). Similar
results were obtained when the actual vessels were measured with a
micrometer. On day 7 after clot placement, the measured
vessel diameters in the nondilated group were 51% to 97% the size of
normal vessels, with a mean of 69±2% (P<.05), while
vessels that had been dilated with TBA on day 0 had a diameter of 120%
to 163% of control, with a mean of 144±2% (P<.05).
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Thus, these results show that periarterial clot placement around segments of canine cervical ICAs results in significant reduction of vessel diameter after 7 days and that angioplasty of arterial segments immediately before the clot placement results in a vasodilatation that is sustained for 7 days even in the presence of surrounding clot.
Pharmacological Effects of Vasoconstrictor Agents
Responses to a single dose of KCl (60 mmol/L) and cumulative
dose-response curves for noradrenaline, 5-HT, and
PGF2 were recorded for the three groups of vessels:
normal, nondilated, and dilated.
KCl
Dilated vessels showed significantly diminished responses in
comparison to nondilated and normal vessels when exposed to KCl at
60 mmol/L. The response of nondilated vessels in comparison to
normal vessels was diminished, but the results did not reach
statistical significance. These results are shown in Fig 3A.
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Noradrenaline
Dilated vessels showed significantly diminished responses in
comparison to nondilated and normal vessels when exposed to
noradrenaline at concentrations of
10-8 to 10-5 mol/L.
The responses of nondilated vessels in comparison to normal vessels
were slightly diminished at all concentrations, but the diminution did
not reach statistical significance at any point. These results are
shown in Fig 3B
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5-HT
Dilated vessels showed significantly diminished responses in
comparison to nondilated and normal vessels when exposed to 5-HT at
concentrations of 10-7 to
10-5 mol/L, while at the lower concentrations
of 10-8 and 10-9
mol/L the diminution was present but only significant when the
comparison was between dilated and nondilated vessels. As in the case
of noradrenaline, the responses of nondilated vessels in
comparison to normal vessels were slightly diminished at concentrations
of 10-7 to 10-5
mol/L, but not at a statistically significant level. These results are
shown in Fig 3C.
PGF2
The responses of vessels in all study groups was very small. There
were no responses of any arterial rings at the lower
concentrations of 10-9 and
10-8 mol/L, nor was there any significant
difference between normal vessels and those exposed to blood with prior
TBA at any concentration. Vessels in vasospasm appeared to be more
responsive than control or dilated vessels, and this reached
statistical significance at a concentration of
10-6 mol/L. These results are shown in Fig 3D.
Summary
Although the responses to PGF2 differ from those to
other vasoconstrictor agents in that they are smaller, and vasospasm
seems to result in enhanced responsiveness to PGF2 in
this model, the observation that TBA pretreatment reduces the
reactivity of arteries surrounded by perivascular clot is common to all
vasoconstrictors tested. The observation that the reduction in response
of nondilated (vasospastic) arteries compared with normal arteries
(when tested with KCl, noradrenaline, and 5-HT) did not
reach statistical significance is consistent with other models
of vasospasm.9
Pharmacological Effects of Vasodilators
After tonic contraction with noradrenaline
(10-5 mol/L) had been established, cumulative
dose-response curves for the calcium ionophore A23187 and the response
to a single dose of papaverine (5x10-4 mol/L)
were recorded for the three groups of vessels. In most cases,
pretreatment with noradrenaline produced a large enough
tonic contraction in dilated vessels to allow comparative percent
relaxation data to be obtained.
Calcium Ionophore A23187
Dilated vessels showed significantly diminished relaxations in
comparison to nondilated and normal vessels when exposed to the calcium
ionophore A23187 at concentrations of 10-6 and
10-5 mol/L. The responses of normal vessels in
comparison to nondilated vessels were slightly diminished at all
concentrations, but not at a statistically significant level. This is
consistent with other models of vasospasm.9 These
results are shown in Fig 4.
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Papaverine
All vessel groups showed 100% relaxation after exposure to
papaverine (Fig 5).
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Summary
These studies suggest that for the calcium ionophore A23187
(10-7 to 10-5 mol/L),
an endothelium-dependent vasorelaxant, vessels treated
with periarterial blood but first dilated with TBA have an
impaired response compared with vessels treated similarly but not
dilated on day 0. However, endothelium-independent
relaxation to papaverine is preserved even in dilated vessels.
Changes Observed With Scanning Electron Microscopy
Photomicrographs of normal, dilated, and nondilated vessels are
shown in Fig 6. Scanning electron microscopy of
nondilated vasospastic vessels showed moderate diminution of the vessel
lumen, decreased luminal diameter/wall thickness ratio (Fig 6B, bottom), corrugation of the IEL, and folding of the
endothelial surface (Fig 6B, top). Similar observations
of dilated vessels showed moderate enlargement of the vessel lumen (Fig 6C, bottom), patchy endothelial denudation, and
straightening and thinning of the IEL (Fig 6C, top). Scanning electron
micrographs of normal vessels are shown in Fig 6A (top and bottom).
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Changes Observed With Transmission Electron Microscopy
Photomicrographs of normal, nondilated, and dilated vessels are
shown in Fig 7. Results were consistent with
those obtained by scanning electron microscopy; nondilated vasospastic
vessels showed corrugation of the IEL, folding of the
endothelial surface, and thickening of the vessel wall,
especially the tunica media (Fig 7B). There was some swelling and
vacuolation of endothelial cells, with cellular
rounding and some cell separation. Rounding of smooth muscle cells with
surface rippling and occasional breaks in the IEL were also noted.
Transmission electron microscopy of dilated vessels showed flattening
of the endothelial cells; straightening, thinning, and
occasional rupturing of the IEL; and straightening and crowding of
smooth muscle cells in the tunica media (Fig 7C). A transmission
electron micrograph of a normal vessel is shown in Fig 7A.
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Discussion |
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Use of TBA for Cerebral Vasospasm
Conventional treatment for symptomatic cerebral vasospasm has usually involved the use of hypervolemic and hypertensive therapy. However, recent advances in interventional neuroradiology have allowed for the development of newer therapeutic modalities. One technique that has gained favor since its first description by Zubkov et al2 in 1984 is TBA. It is successful in reversing the clinical effects of vasospasm in approximately 75% of selected patients4 5 and generally produces results that are immediate and long-lasting. Aneurysmal repair is a prerequisite for the use of TBA. The technique is not without complications, including vessel rupture and occlusion.6 At present TBA is generally reserved for cases of moderate to severe symptomatic vasospasm that have been refractory to conventional treatment measures. However, it is not clear that these are the ideal conditions under which TBA should be employed, and earlier implementation may optimize patient outcomes. In a retrospective study, Coyne et al7 concluded that the best results of angioplasty occurred when it was performed within a short time of the onset of symptoms. Little benefit was seen in patients of poor clinical grade or when a new neurological deficit had become established. They suggested that angioplasty be considered if hypervolemic, hypertensive therapy did not reverse the symptoms of vasospasm within 6 to 12 hours.
Models of Vasospasm
To perform prospective, time-course studies of vasospasm, in vivo
animal models have been developed. These have involved the use of
cerebral arteries in animals including the cat,14
dog,9 15 and monkey.16 The advantages of
these models include the presence of a cisternal space into which
autologous blood can be introduced and retained and the fact that they
use intracranial cerebral arteries, thus facilitating comparison with
SAH in humans. However, because of their small size and intracranial
location, it is sometimes difficult to access these arteries in the
course of endovascular techniques such as TBA. This has led to the
development of models of vasospasm in which extracranial arteries such
as the femoral artery of the rat12 and the cervical
carotid artery of the rabbit10 11 13 are used. These
models can contribute valuable information, but it is important to
remember that there are morphological and pharmacological differences
between extracranial and intracranial arteries17 18 and
that some caution must be used when results obtained from experiments
involving these arteries are extrapolated to human cerebral arteries.
Despite our concerns about these limitations, we have developed a model
of vasospasm in the canine high cervical carotid artery. As
demonstrated by the angiograms and vessel caliber measurements outlined
earlier, the approach produces angiographic and morphological vessel
constriction after 7 days in a consistent and reproducible
fashion. The vessels under study are of approximately the same caliber
as the larger cerebral arteries at the base of the human brain and,
most important from the perspective of this study, the vessels are
suitable for endovascular manipulation.
In Vivo Mechanism of Action of TBA
There is accumulating evidence, both in
extracranial10 11 19 20 and in
intracranial9 21 arteries, that TBA results in sustained
arterial dilatation through a mechanism of smooth muscle
cell injury and paralysis. This is consistent with other
evidence that vasospasm represents active smooth muscle cell
contraction22 and that it can be acutely reversed in the
presence of vasodilators such as papaverine.23 24 The
results of our study suggest that in vivo balloon angioplasty performed
immediately before the induction of vasospasm prevents vasoconstriction
after SAH and produces a functional impairment in vascular reactivity
that is sustained for at least 7 days. These results are
consistent with our previous report on the effects of immediate
in vitro TBA9 and those of others investigating in vivo
TBA performed after SAH.10 11 21
High millimolar concentration of KCl produces smooth muscle cell
contraction through electromechanical coupling mechanisms with
depolarization of the sarcolemma,25 whereas
noradrenaline, 5-HT, and PGF2 all act
through pharmacomechanical coupling mechanisms with activation of
second messengers such as 1,2-diacylglycerol and inositol
1,4,5-trisphosphate.25 26 27 The results of the present
experiment indicate that both of these mechanisms are affected. After 7
days, normal arteries subjected to TBA showed responses to KCl,
noradrenaline, and 5-HT that were markedly attenuated
compared with vasospastic and normal arteries. This suggests a
sustained functional impairment of smooth muscle cell function that
does not reverse rapidly. The precise nature of this impairment cannot
be determined from these experiments but may involve altered calcium
homeostasis that prevents the intracellular signaling leading to
contraction.
The minimal response of all vessel groups to the presence of
PGF2 is in contrast to previous results found in
preparations of other canine cerebral arteries.9 It is
possible that the canine cervical carotid artery expresses fewer
PGF2 receptors than the canine basilar artery. The
persistence of vasospasm in arteries with a rather weak response to
PGF2 would appear to provide additional evidence against
a pivotal role for this compound in cerebrovascular spasm.
The calcium ionophore A23187 produces vasorelaxation through an endothelium-dependent mechanism. This mechanism requires the elevation of intracellular calcium within endothelial cells. This leads to the stimulation of NOS, which converts L-arginine to NO, a potent vasodilator. The NO then rapidly diffuses both within the endothelial cells and across membranes to nearby smooth muscle cells, where it interacts with soluble guanylyl cyclase, to cause relaxation.28 29 30 The calcium ionophore A23187 directly promotes entry of calcium across the endothelial cell membrane.29 In the present experiments the response of dilated vessels to the calcium ionophore was attenuated in comparison to the responses of vasospastic and normal arteries. This may be because the patchy endothelial damage seen with scanning and transmission electron microscopy after TBA reduces the amount of NO generated and/or because there is diminished NO synthesis and release in the remaining endothelium. Also, the responsiveness to NO of the guanylyl cyclase in the smooth muscle cells of dilated vessels may be reduced.
Endothelium-independent relaxation to papaverine is preserved after TBA. This agent has rapid and profound effects on most smooth muscle cells, and it is not surprising that the residual response to vasoconstrictors was reversed by papaverine.
It has been suggested that vasospastic arteries may remain dilated after TBA because angioplasty disrupts arterial wall components such as smooth muscle cells, myofibroblasts, or the extracellular matrix, thereby causing a mechanical impairment of contraction.31 The results of this experiment indicate that TBA causes alterations in the IEL, tunica intima, and tunica media, but that it does not cause frank disruptions in those layers. These results are generally consistent with previous in vitro and in vivo studies.9 10 11 21 32 33 However, although no gross mechanical disruptions of the vessel wall were seen, it is possible that the smooth muscle cell functional impairment observed represents a cellular or subcellular mechanical disruption. Indeed, it may be that after TBA the artery consists of different cell populations, some of which may be approximately normal in function, while others have gross impairment of normal function.
A number of questions remain to be answered. The work reported here involved a considerable dilatation of the artery. This was done because initially it was thought that the higher balloon pressures were required to dilate the thicker walled carotid artery and because it was possible to actually observe the process of dilatation through the operating microscope. The results were entirely successful in preventing the development of cerebrovascular spasm, but it is certainly possible that a less forceful dilatation would have been equally effective and perhaps safer. Studies of varying the effects of TBA with various levels of distension or duration have important clinical implications. Early results of work from our laboratory, in which lower balloon dilatation pressures (101.3 to 202.6 kPa) were used, are similar to those reported here. The protocol described here did not contain a study group in which TBA was performed on arteries without subsequent blood clot placement. Current experiments in our laboratory include such a group, and preliminary results indicate a pharmacological impairment similar to that observed in dilated arteries surrounded with blood clot. Finally, we do not yet know long-term effects in normal arteries that have been distended by TBA, and the 7-day period studied here should be extended to examine the effects weeks or months later, to determine whether normal function returns.
In conclusion, this canine in vivo model of vasospasm using the ICA produces consistent and reproducible vasospasm, as demonstrated by both angiography and direct measurement of vessel caliber. This region of the cerebral circulation is readily accessed by catheter and is thus suitable for studies of TBA. In this model, TBA performed before the induction of vasospasm prevents its development, lending experimental support to the growing clinical evidence that balloon angioplasty may have optimal benefit when performed early in the course of symptomatic vasospasm. Examination of the pharmacological and gross morphological changes present 7 days after TBA indicates a greater impairment in vasoreactivity than structural disruption of the vessel wall. Balloon dilatation and stretching of the arterial wall, which can be observed morphologically on scanning electron microscopy, result in an alteration of smooth muscle contraction and vasodilation that is sustained for at least 7 days. Further studies are required to determine whether and when vessels subjected to angioplasty return to normal contractile behavior.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This study was supported by a grant from the Department of Surgery, Division of Neurosurgery, University of Alberta.
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Footnotes |
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Reprint requests to J. Max Findlay, University of Alberta, 2D1.02 W.C. Mackenzie Health Sciences Centre, 8440-112 St, Edmonton, Alberta, Canada T6G 2B7.
Presented in part at the 21st International Joint Conference on Stroke and Cerebral Circulation, San Antonio, Tex, January 25-27, 1996, and the 1996 American Association of Neurological Surgeons Meeting, Minneapolis, Minn, April 27-May 2, 1996.
Received December 6, 1996; revision received February 17, 1997; accepted March 12, 1997.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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1. Findlay JM. Vth International Conference on Cerebral Vasospasm: an overview. In: Findlay JM, ed. Cerebral Vasospasm. Proceedings of the Vth International Conference on Cerebral Vasospasm. Amsterdam, Netherlands: Elsevier Science Publishers BV; 1993:1-3.
2. Zubkov YN, Nikiforov BM, Shustin VA. Balloon catheter technique of dilatation of constricted cerebral arteries after aneurysmal SAH. Acta Neurochir (Wien).. 1984;70:65-79.[Medline] [Order article via Infotrieve]
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