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1985;57;105-112 Circ. Res.V Fuster

PM Steele, JH Chesebro, AW Stanson, DR Holmes, Jr, MK Dewanjee, L Badimon and a pig model

Balloon angioplasty. Natural history of the pathophysiological response to injury in

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105

Balloon AngioplastyNatural History of the Pathophysiological Response to Injury in a Pig

Model

Peter M. Steele, James H. Chesebro, Anthony W. Stanson, David R. Holmes, Jr.,Mrinal K. Dewanjee, Lina Badimon, and Valentin Fuster

With the technical assistance of Holly B. Lamb, James M. Byrne, and Brenda I. Wendland

From the Division of Cardiovascular Diseases and Internal Medicine, Department of Diagnostic Radiology, and Section of DiagnosticNuclear Medicine, Mayo Clinic and Mayo Foundation, Rochester, Minnesota

SUMMARY. The restenosis or occlusion that frequently follows balloon angioplasty is poorlyunderstood. Thus, the pathophysiological response to angioplasty of the common carotid arteryin 38 heparinized normal pigs was investigated by quantification of the lnIn-labeled plateletdeposition and histological and electron microscopic examination from 1 hour to 60 days afterangioplasty. At 1 hour, the following findings were noted: complete endothelial denudation in allarteries, marked platelet deposition (44.7 ± 20.7 X 10'/cm2), mural thrombus in seven of 10 pigs,and a medial tear extending through the internal elastic lamina in nine of 18 arteries. All ninearteries with tears had associated mural thrombus and severe platelet deposition (76 x 10'/cm2);in contrast, the nine arteries without a tear had no mural thrombus and much lower plateletdeposition (6 X 106/cm2). Necrosis of medial smooth muscle cells was evident at 24 hours. Plateletdeposition remained high at 24 hours (40.5 ± 20.6 X 106/cm2), but was markedly reduced at 4days (4.4 ± 1.5 X 106/cm2), coincident with partial regrowth of endothelium or periluminal liningcells. No significant platelet deposition was noted at 7 days, when the endothelial cell type ofregrowth was largely complete. Intimal proliferation of smooth muscle cells was mild and patchyat 7 days, significantly greater and more uniform at 14 days, and unchanged at 30 and 60 daysafter angioplasty. Complete thrombotic occlusion occurred in four (11%) of the 38 pigs. Asignificant stenosis present at 30 days after angioplasty was shown by histological examinationto be due to organization of mural thrombus. Thus, balloon angioplasty produced extensivearterial damage and was a potent stimulus for acute platelet-thrombus deposition and subsequentintimal hyperplasia. These results may have important clinical implications in the prevention ofrestenosis and occlusion after angioplasty. (Circ Res 57: 105-112, 1985)

PERCUTANEOUS transluminal coronary angio-plasty (PTCA) has rapidly gained acceptance as atherapeutic option in the treatment of obstructivecoronary artery disease (Gruntzig et al., 1979; Kentet al., 1982), even in the treatment of multivesseldisease (Vlietstra et al., 1983; Hartzler, 1983). Themechanism by which angioplasty improves the pat-ency of atherosclerotic arteries probably involvesplaque fracture and expansion of the external di-ameter of the artery (Castaneda-Zuniga et al., 1980;Block et al., 1981; Sanborn et al., 1983). Endothelialdenudation, platelet deposition, and mural throm-bus formation have been observed in dogs afterangioplasty (Pasternak et al., 1980). The naturalhistory of platelet-thrombus deposition is unknown.Platelet-thrombus deposition may be an importantcause of acute occlusion (Block et al., 1981) andrestenosis (Essed et al., 1983), which occurs within6 months in 25%-35% of patients (Holmes et al.,1984; Dangoisse et al., 1982).

We have established a model of arterial angio-plasty in the pig and observed the pathophysiolog-

ical response up to 60 days after the procedure, withparticular reference to the relationship of the struc-tural injury and repair of the artery to the plateletdeposition and mural thrombus formation.

Methods

AnimalsThe study animals, obtained from local farmers, were

normal pigs (average weight, 35 kg) of the Babcock 4-waycross stock (mixture of Landrace, Yorkshire, Hampshire,and Duroc breeds). All pigs were fed a normal chow diet.

Experimental Procedure and Angioplasty TechniqueThe pigs were sedated with ketamine, 300 mg, intra-

muscularly (Ketaject, Bristol), intubated after inhalation ofa small amount of ether (ether USP, J.T. Baker), andmechanically ventilated with room air (Harvard respirator)mixed with 0.5% halothane (Fluothane, Ayerst) to main-tain anesthesia. Continuous electrocardiographic and ar-terial pressure monitoring was performed with a Honey-well multichannel recorder. All pigs were fully heparin-ized immediately after insertion of the catheter, with a

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106 Circulation Research/Vo/. 57, No. 1, July 1985

TABLE 1Pathophysiological Response to Angioplasty over Time

Tune ofsacrifice

1 h r24 hr4 days7 days

14 days30 days60 days

Pigsno.

10654544

Mural thrombus(no./total)

7/103/6(1)J1/5 (1»2/4

V5 (1»2/4 (1»0/4

Platelets*no. (x 10*/cm2)

44.7 ± 20.740.5 ± 20.64.4 ± 1.51.2 ±0.60.2 ± 0.00.2 ±0.10.1 ±0.0

Endothelialfregrowth

_-±++++

Thickness ofintima

(mm x 10"3)

62.4 ± 6.585.6 ± 15.980.0 ± 22.884.8 ± 19.2

• Mean ± SE.| Endothelium or periluminal lining cells: —, absent; +, complete; ±, incomplete.j Complete thrombotic occlusion.

bolus of 100 USP units/kg given intravenously. The hep-arin was not reversed at the end of the procedure, and nofarther doses of heparin were administered.

Through a right femoral cutdown, the balloon catheter(Meditech 8 mm X 3 cm, polyethylene balloon) was ad-vanced into the left and right common carotid arteriesunder fluoroscopic visualization. The balloon was inflatedto 6 atmospheres (Meditech pressure manometer) for 30seconds (these balloons inflate to a maximal diameter of8 mm). Five inflations at 60-second intervals were per-formed on both common carotid arteries (except for twopigs, killed at 1 hour, in which only one artery was dilatedand the other served as a control). The average diameterof the common carotid artery was 5-6 mm. Selectivecarotid spot films were obtained before and immediatelyafter the dilation procedure by the manual injection of 6ml of diatrizoate meglumine and diatrizoate sodium (Ren-ografin-76, Squibb), and plain spot films were obtainedduring dilation. Measurements taken from the spot filmstaken before, during, and after balloon inflation demon-strated that the diameter of the balloon inflated withinthe artery was only 9.1 ± 6.1% (mean ± SD) more thanthe diameter of the artery on the films obtained beforedilation.

Angioplasty was performed on 38 pigs that were killedfrom 1 hour to 60 days after the procedure (Table 1).Immediately after a lethal dose of sodium pentobarbital,the carotid arteries were perfused in situ with 2% glutar-aldehyde and 1% paraformaldehyde in 0.1 M cacodylate(pH 7.25) at a pressure of 100 mm Hg for 15 minutes. Thearterial tissues then were removed, cleaned of all adven-titia, and prepared for analysis.

Quantification of Platelet DepositionAutologous platelets were labeled with '"In (tropo-

lone)3 (Dewanjee et al., 1981) by a modification of ourpreviously described method (Badimon et al., 1983), inwhich the platelets were labeled in plasma instead of acidcitrate dextrose-saline. The platelets were injected 18-24hours before sacrifice, in doses of 300-400 jtCi. The onlyexceptions were pigs that were killed 24 hours after an-gioplasty; in these cases, the platelets were labeled andinjected 48 hours before sacrifice. The platelet depositionon each dilated artery (X 106/cm2) was calculated fromplatelet counts and In activity on the arterial wall andin blood by a method previously described by us (Dewan-jee et al., 1984).

Tissue AnalysisThe location of the dilated portion of the fixed artery

was identified easily because of the in situ fixation thatshowed the almost invariable vasospasm proximal anddistal to the involved area. This finding was confirmed invivo by the films, as described previously. The dilatedportion was divided into two segments (1-1 Vi cm), and asegment of similar size was taken from the distal unin-volved artery. The tissue segments were examined underlow-power magnification (x2 lens, Sunnex Laboratories)for the presence of mural thrombus formation.

From each arterial segment, 2- to 3-ring sections wereremoved and stained with hematoxylin and eosin andLason's elastic-Van Gieson stain. The histological sectionswere examined by two investigators and a consensusevaluation was made. Histological sections were photo-graphed. From these pictures, the thickness of the intimafrom the surface to the internal elastic lamina was meas-ured in six equidistant sections that were again dividedinto 10 equal sections for a total of 60 measurementsaround the circumference of the intima.

Two longitudinal specimens were cut from each seg-ment, coated with carbon and gold-palladium alloy, andexamined with a scanning electron microscope (ETECAutoscan). Representative areas were photographed andevaluated by at least two investigators, and a consensusreading was made.

Selected specimens were thin-sectioned (600-700 A),mounted on a 200-mesh copper grid, and stained withuranyl acetate and lead citrate. Sections were examinedwith a Philips 201 transmission electron microscope.

StatisticsStudent's f-test or the x2 test w a s used to determine

statistical significance.

Results

Arterial Injury and RepairBalloon angioplasty produced complete endothe-

lial denudation in the dilated area (Figs. IB and 2A).Four days after the procedure, there was partialregrowth (Fig. 1C); by 7 days, the endothelium orperiluminal lining cells had largely regrown (Fig.

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Steele et al. /Balloon Angioplasty in a Pig Model 107

FIGURE 1. Scanning electron micro-scopic views of the luminal surface ofthe common carotid artery in the pig(original magnification, l,000x). PanelA: normal endothelium. Panel B: com-pletely denuded endothelium replacedby layer of adherent platelets 1 hourafter angioplasty. Panel G partial re-growth of endothelium or periluminallining cells with a few platelets adheringto uncovered subintima 4 days after an-gioplasty. Panel D: endothelium or per-iluminal lining cells largely regrownwith no adherent platelets 7 days afterangioplasty.

ID), except in focal areas overlying mural thrombus.The nuclei of the smooth muscle cells in the mediafrequently adopted a corkscrew shape (Castaneda-Zuniga et al., 1980) 1 hour after angioplasty (Fig. 3).By 24 hours, there often was necrosis of the smoothmuscle cells and damage to the normal architectureof the elastic fibers.

Of the 18 arteries examined at 1 hour after angio-plasty, nine (50%) showed a tear that extendedthrough the internal elastic lamina and into themedia to a variable depth (Fig. 4). The dilated arter-ies without a tear were thinned and showed less ofa corkscrew pattern and less necrosis of the smoothmuscle cells than did the arteries with tears.

The reparative process in the media of the dilatedarteries was evident at 14 days after angioplastywith connective tissue remodeling in an attempt torestore the luminal contour to normal. Elastic stainsof histological sections showed loss or distortion ofnormal elastic fibers with fibrocellular ingrowth intoareas of necrosis and into defects devoid of elasticfibers that were probably created by a previous acutetear.

Platelet-Thrombus DepositionOne hour after angioplasty, there was heavy

platelet deposition on the subintima (Figs. IB and2B) that persisted to 24 hours (Table 1). The plateletdeposition in an artery at an untouched site distalto the dilation was <0.5 X 106/cm2, which is thesame value obtained from normal arteries in 10control pigs (unpublished data). One hour after theangioplasty, 50% of the dilated arteries had a mac-roscopic mural thrombus (Table 2) in addition to themicroscopic layer of platelets. A mural thrombusand marked platelet deposition was seen only inthose arteries that had a medial tear (Figs. 2B and4). Conversely, all arteries with tears had a muralthrombus (Table 2).

At 4 days after angioplasty, the platelet depositionwas significantly reduced (P < 0.05); this decreasecoincided with the partial regrowth of an endothelialcell type (Fig. 1C; Table 1). No significant plateletdeposition (Fig. ID) was noted at 7 days, at whichtime the endothelial cell type of regrowth (perilu-minal lining cells) was largely complete, as seen by

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108 Circulation Research/Vol. 57, No. 1, July 1985

FIGURE 2. Transmission electron micrographs from site of dilation1 hour after angwplasty. Panel A: monolayer of degranulated plateletsspread across subintima adjacent to internal elastic lamina (I) (originalmagnification 10,5OOx). Panel B: thrombus (T) composed mainly ofaggregated and degranulated platelets attached to underlying medialconnective tissue and smooth muscle cells within a tear. Internalelastic lamina is absent (original magnification, 3,0O0x.).

both scanning and transmission electron micros-copy. Whether smooth muscle cells contribute inpart to this endothelial cell type of layer is uncertain.

In four (11%) of the 38 pigs, the mural thrombusprogressed to complete thrombotic occlusion, theearliest of which was evident at 24 hours after theprocedure (Table 1).

The mural thrombi that were detected at 7 daysor later showed evidence of organization and evensimulated a significant stenosis by a fibrous plaque(Fig. 5, A and B).

Intimal Proliferation of Smooth Muscle Cells(Table 1)

At 7 days after angioplasty, intimal proliferationof smooth muscle cells was seen, but it was patchy,and the thickness of the intima (62.4 ± 6.5 mm X10~3) was significantly more (P < 0.05) than inundamaged control arteries (31.0 ± 1.0 mm X 10~3).By 14 days, intimal proliferation was significantlymore (P < 0.05) than at 7 days, and was moreuniform around the circumference of the artery. At30 and 60 days, the thickness of intimal proliferationwas not significantly greater than at 14 days.

DiscussionPTCA, particularly of the coronary arteries, has

made a significant impact in a short time on the

FIGURE 3. Sections of the common carotid artery in the pig at the siteof dilation. One hour after angioplasty, many nuclei of the smoothmuscle cells in the inner half of the media demonstrate characteristiccorkscrew appearance. Note absence of endothelium and presence ofintact internal elastic lamina 0) (hematoxylin and eosin; originalmagnification, 20OX).

FIGURE 4. Sections of common carotid artery at site of dilation. Tear(arrows) extending through internal elastic lamina (I) and almostthrough entire media in association with mural thrombus (T) at1 hour after angioplasty (hematoxylin and eosin; original magnifica-tion, X64).

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Steele et al. /Balloon Angioplasty in a Pig Model

TABLE 2Platelet-Thrombus Deposition According to Presence or

Absence of Medial Injury

Medial tear*

+

• +, present;

No. ofarteries

99

—, absent

Mural thrombus(no./total)

9/90/9

Platelets(x 107cm2)

Mean no. Range

76 7-2976 3-8

treatment of stenotic arterial disease (Gruntzig et al.,1979; Kent et al., 1982; Vlietstra et al., 1983; Hart-zler, 1983). Despite this development, many basicquestions concerning the pathophysiological eventsafter balloon injury remain unanswered. This studyaddressed some of these questions.

Animal Model

It is extremely difficult to find an appropriateanimal model for experimental angioplasty. Rabbits(Faxon DP et al., 1982; Sanborn et al., 1983) or dogs(Castaneda-Zuniga et al., 1980; Pasternak et al.,1980) have been used in previous studies. Neitheranimal develops atherosclerosis naturally, althougha high-cholesterol diet (2%) after endothelial denu-dation in the rabbit produces intimal thickening witha large number of foam cells (Faxon et al., 1982;Sanborn et al., 1982). The diet also induces veryhigh serum levels of cholesterol (1,476 ± 467 mg/dl) (Sanbom et al., 1982). Thus, although a stenoticlesion is produced by this model, there could bemajor limitations in relating this pathophysiologicalresponse to angioplasty to man because of the un-natural situation of an excessive amount of lipid inthe arterial wall. Our study used pigs, animals thatspontaneously develop atherosclerosis without cho-lesterol feeding (French et al., 1965; Fuster et al.,1982b) and whose platelet-coagulation system ismore closely related to humans' (Folts and Rowe,1983; Leach and Thorbum, 1982). Because theywere young, the pigs in this study did not haveatherosclerotic lesions, nor were they given a high-cholesterol diet. The normal lipid levels may havereduced the platelet-vessel wall interaction and de-gree of smooth muscle cell proliferation. Pigs of thisage have a serum cholesterol of 101 ± 13 (SD) mg/dl and triglycerides of 53 ± 19 mg/dl (Fuster et al.,1982). The disadvantage of this model is the lack ofan atherosclerotic lesion to dilate. However, becauseof the lack of lesion bulk, it might be expected toresult in a lesser expansion of the external diameter(<10% average increase in diameter compared withprediction) and lesser degTee or frequency of deeparterial wall damage [tears into media in approxi-mately 50% of normal arteries (Lam et al., 1985)compared with nearly inevitable splitting of theatherosclerotic plaque in a successful angioplasty(Castaneda-Zuniga et al., 1980; Block et al., 1981)].This decreased damage could decrease the stimulus

109

for and the amount of platelet-thrombus deposition.Thus, this model probably underestimates the re-sponse to angioplasty compared to the clinical situ-ation. In addition, although continuous heparin in-fusion in rats after air injury to the endothelium hasinhibited smooth muscle cell proliferation (Guytonet al., 1980), the single dose of heparin used in thisstudy would not be expected to affect the smoothmuscle cell proliferation in these pigs.

Relation of Injury to Mechanism of Angioplasty

Balloon angioplasty produced endothelial denu-dation in all arteries examined acutely, and majormechanical damage to the arterial wall. The mostimpressive feature was that a medial tear (Fig. 4)was noted in 50% of arteries. These tears appearedsimilar to the disruptive lesions of the media and toatherosclerotic plaque reported in patients and inatherosclerotic rabbits after angioplasty (Block et al.,1981; Sanborn et'al., 1983; Essed et al., 1983). Aspreviously suggested (Castaneda-Zuniga et al.,1980; Block et al., 1981; Sanborn et al., 1983), sucha tear or fracture of the intimal plaque and adjoiningmedia is usually necessary for a successful proce-dure.

The frequent finding of smooth muscle necrosisin the media at 24 hours after angioplasty is ofinterest. This effect may be very desirable, essen-tially making the artery an inert conduit (incapableof vasospasm). In none of the arteries in theseanimals did spasm occur in the dilated segment.However, coronary spasm is usually multifocal;thus, angioplasty would not be an appropriate treat-ment, even in certain resistant patients with angio-graphically documented coronary artery spasm.

Platelet-Thrombus Deposition

There is little information and no quantitative dataabout platelet deposition after angioplasty. Thescanning electron microscopic appearance soon afterthe procedure in a small number of dogs has beendescribed (Pasternak et al., 1980), but there is noreported data about the natural history or determi-nants of platelet-thrombus deposition.

This study confirmed the suspicion that angio-plasty was a potent thrombogenic stimulus (Table1). Endothelial regrowth seemed to protect againstde novo platelet deposition both quantitatively (Ta-ble 1) and qualitatively (Fig. 1). Therefore, in thismodel, the critical period for platelet-thrombus dep-osition would include at least the first 4 days afterthe procedure. This finding suggests that futureintervention studies should particularly involvetherapy that is started before the procedure in orderto achieve adequate blood levels of antithrombotictherapy to prevent platelet-thrombus deposition thatbegins during the procedure. In this model, therapyshould be most potent for the first 4-7 days afterthe procedure, but this duration may differ in hu-humans with atherosclerotic disease.

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110

As has been observed in other models of endo-thelial injury (Fishman et al., 1975; Groves et al.,1979; Fuster et al., 1983), the platelets were depos-ited on the subinrima as a thin layer (usually amonolayer). In addition, a mural thrombus wasnoted in half of the arteries from pigs killed withinan hour, was related to the presence of a medialtear, and contributed greatly to the measured plate-let deposition (Fig. 2B; Table 2). The exposed mediaseemed to be an overwhelming stimulus for plateletdeposition and thrombus formation, even in fullyheparinized animals. We would expect that the tearor fracture of a plaque produced by angioplasty inhumans (Block et al., 1981) would be no less throm-bogenic.

The reasons that deeper arterial injury caused agreater thrombotic stimulus are not clear but may inpart be related to both the enhancement of throm-bosis with the exposure of fibrillar collagen foundpredominantly in the media and adventitia (Baum-gartner and Haudenschild, 1972) and the destruc-tion of two nahiral antithrombotic systems presentin intact endothelial and subendothehal arterial lay-ers, namely, the prostaglandin system with prosta-cyclin production and the fibrinolyric system(Kwaan and Astrup, 1967; Moncada et al., 1977;Cragg et al., 1983). Prostacyclin production variesby the site in the arterial wall; as the arterial walllayers approach the luminal surface, the prostacyclinproduction increases (Moncada et al., 1977). Loss ofthe antithrombotic mechanisms noted above is alsoaccompanied by a greater thrombotic stimulus fromthe exposure of collagen to circulating blood. Expo-sure of platelets, to the basement membrane alonemay result in platelet spreading without release(Baumgartner et al., 1976), whereas platelet adher-ence to fibrillar collagen results in both spreadingand release of proaggregating substances fromwithin the platelet granules (Baumgartner et al.,1976; Kinlough-Rathbone et al., 1980). In addition,thrombin generation via the intrinsic or extrinsiccoagulation pathways promotes both platelet aggre-gation and the production of fibrin which stabilizesplatelet thrombi (Nemerson and Nossel, 1982). Theexposure of collagen to circulating factor XII or factorXI can activate the intrinsic pathway (Wilner et al.,1968; Walsh, 1972) and, thus, stimulate thrombingeneration. Likewise, the acutely damaged area ofthe arterial wall can generate tissue thromboplastin,which activates the extrinsic pathway and thus pro-motes thrombin. generation and further thrombosis(Nemerson and Pitlick, 1972). Cells from the deeperlayers of the arterial wall, fibroblasts and smoothmuscle cells, generate greater peak thromboplasticactivity than the cells at the luminal surface, theendothelial cells (Maynard et al., 1977). All of thesefactors may contribute to the greater frequency andseverity of platelet-thrombus deposition with deeperarterial wall injuiy.

In 11% of the pigs, the mural thrombus pro-gressed to complete thrombotic occlusion (Table 1).Although the timing of this occlusion is uncertain,

Circulation Research/Vo/. 57, No. 1, July 1985

since no observation was possible before the time ofsacrifice, the histological appearance was that of oldand organizing thrombus, suggesting that thisstarted during the acute phase. In a larger series ofcontrol pigs, we observed acute occlusion within1 hour of angioplasty in two of 48 (4%) arteries(Lam et al., 1985). It is uncertain whether contin-uation of heparin beyond the acute period wouldhave influenced this development. No thrombi wereapparent on the monoplane angiograms obtainedsoon after the angioplasty; however, angiography isnot sensitive for the detection of mural thrombi. Ourpreliminary studies with in vivo imaging using n 'In-labeled platelets suggest that this is a much moresensitive method of detecting mural thrombi in ar-teries (Steele et al., 1984a). Thus, the process ofmural thrombus formation probably starts duringthe procedure, and may be present immediately afterPTCA in man [similar to observations in iliofemoralangioplasty (Ezekowitz et al., 1983)] but may not beevident angiographically.

Possible Mechanisms of RestenosisRestenosis remains a challenging clinical problem

that affects 25%-35% of patients within 6 monthsafter PTCA (Dangoisse et al., 1982; Holmes et al.,1984). Although some clinical features are associatedwith an increased risk of restenosis (Holmes et al.,1984), the cause is unknown. The data from thisstudy suggest that restenosis may occur via twomechanisms, both of which are instrumental in thedevelopment and progression of atherosclerosis(Ross and Glomset, 1976; Fuster and Chesebro,1982).

First, platelet deposition on the damaged arterialwall can form a mural thrombus, which mayundergo organization and cause restenosis. In thisstudy, we have shown that a medial tear is a verypotent stimulus for thrombus formation. Muralthrombi can organize over a period of weeks andresult in an obstructive lesion (Fig. 5) that eventuallyis indistinguishable from a fibrous plaque (Jorgensenet al., 1967). In our study, the frequency of muralthrombi immediately after angioplasty despite fullheparinization suggests that these thrombi may bea very important mechanism for restenosis in man.

Second, the platelets deposited early after angio-plasty may, by the release of platelet-derived growthfactor, induce the smooth muscle cells in the mediato migrate to the intima and proliferate. This intimalproliferation of smooth muscle cells may contributeto restenosis. This mechanism is essentially the re-sponse to injury hypothesis of atherogenesis (Rossand Glomset, 1976). We observed this intimal pro-liferation in pigs, and it also has been reportedrecently in a patient who died 5 months after angio-plasty (Waller et al., 1983).

The contribution of these mechanisms to resten-osis may vary, depending on a number of features:composition and distribution of the atheroscleroticplaque, blood lipid levels, balloon size, and pressure

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Steele et al./Balloon Angioplasty in a Pig Model 111

FIGURE 5. Artery at site of dilation 30days after angioplasty. Panel A- macro-scopic appearance of significant stenoticlesion. Panel B: histological section ofsame lesion showing that obstruction isdue to organization of mural thrombus(Lason's elastic-Van Gieson; originalmagnification, 64x).

and duration of inflation, presence and degree ofexposed media, degree of smooth muscle cell necro-sis, platelet deposition and thrombus formation, re-coil and degree of fibrous tissue ingrowth in thearterial wall, and vasospasm. Despite the multiplic-ity of possible contributing factors, this studystrongly suggests that the early platelet-thrombusdeposition after angioplasty plays a pivotal role inthe pathophysiological response to balloon injuryand may be closely involved with the process ofrestenosis (Fig. 5). Thus, although this model doesnot reproduce the human situation of dilating ahigh-grade lesion and probably underestimates theclinical response to angioplasty, it may provide abasis for screening potential antithrombotic andplatelet-inhibitor agents for use in future drug trialsfor the prevention of acute occlusion and restenosisafter arterial angioplasty (Steele et al., 1984b, 1984c),as suggested by the negative response to low molec-ular weight dextran in our animal model and thenegative response in patients (Steele et al., 1984b;Swanson et al., 1985).

We appreciate the secretarial expertise of Carol A. Stocker.Doctor Steele was supported by the National Heart Foundation of

Australia. His current address is Cardiac Clinic, Royal AdelaideHospital, Adelaide, South Australia.

Doctor Fuster's current address is Division of Cardiology, MountSinai Medical Center, New York, New York 10029.

Address for reprints: James H. Chesebro, M.D., Mayo Clinic, 200First Street, S.W., Rochester, Minnesota 55905.

Received August 30, 1984; accepted for publication April 4, 1985.

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INDEX TERMS: Angioplasty • Platelet deposition • Thrombus• Endothelial damage • Smooth muscle cell proliferation • Arterialinjury

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