quantitative angiographic morphology of coronary artery … · coronary angiography was performed...

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Quantitative Angiographic Morphology of theCoronary Artery Lesions at Risk of

Thrombotic OcclusionYves Taeymans, MD; Pierre Theroux, MD;

Jacques Lesperance, MD; and David Waters, MD

Background. Coronary angiography in acute myocardial infarction has revealed complicatedatherosclerotic plaque and a high rate of thrombotic occlusion. However, the characteristics oflesions at high risk of subsequent occlusion are not well known.Methods and Results. In the present study, the qualitative and quantitative angiographic

features of38 coronary artery lesions that occluded within 3 years to cause an acute myocardialinfarction were compared with 64 control segments from the same patients that did not occlude.Compared with control lesions, the lesions that occluded were more likely to have a divisionbranch originating within the stenosis (76% versus 52%, p<0.05). The percent lumen diameterreduction was more severe (47.5±17.8% versus 41±12.5%, p<0.05) and the inflow (21±+100versus 16±7°, p<0.05) and outflow (20910° versus 16+8°, p<0.05) angles of the stenosis weresteeper. Time to myocardial infarction after the angiogram interacted with the importance ofthese features (p< 0.02). Thus, paired analysis of the lesions that occluded within 3 months andof the most severe control lesion from each patient showed percent lumen diameter reductionof 62.1±11.5% and 46.4+11.4%, respectively (p<0.001). The length of the stenosis, itsasymmetry, and the irregularity of the contours did not help differentiate occlusive from controlsegments.

Conclisions. Coronary artery lesions at high risk of thrombotic occlusion share commoncharacteristics that favor higher shear stress and flow separation. (Circulion 1992;85:78-85)

C haracterization of the morphology of the cor-onary artery lesions associated with acutemyocardial infarction and unstable angina

have improved our understanding of the underlyingpathophysiological mechanisms.'-4 The major role ofcoronary artery thrombus formation as the cause ofthe acute obstruction has been recognized. Themechanism triggering thrombus formation appears tobe local and related to rupture of an atheroscleroticplaque.5,6 However, most of our observations arebased on studies performed after coronary occlusionhas occurred, so the characteristics of the lesions athigh risk for thrombotic occlusion are in large partunknown. Identification of such lesions would helpexplain the pathophysiological mechanisms trigger-ing thrombosis and identify stenoses for which amore aggressive treatment would be warranted. Ef-forts made in this direction have been limited7-9 andhave suggested that stenosis severity could be less

From the Montreal Heart Institute, Montreal, Canada.Address for reprints: Pierre Th6roux, MD, Montreal Heart

Institute, 5000 East Belanger Street, Montreal, Quebec, H1T 1C8,Canada.

Received June 2, 1988; revision accepted August 20, 1991.

important7'8 and morphological features of theplaque9 could be more important than previouslyrecognized. The present study was designed to inves-tigate further whether high-risk lesions could berecognized using qualitative and quantitative angio-graphic analyses of coronary lesions that later oc-cluded to provoke an acute myocardial infarction.These lesions were compared with control lesionsfrom the same patients that did not occlude.

MethodsStudy PopulationThe study population was selected from a pool of

patients catheterized during the acute phase of myo-cardial infarction. Mean time to catheterization afterthe acute infarct was 15.1 +±12 days. The clinicaldiagnosis of myocardial infarction was based on thepresence of chest pain more than 30 minutes induration with diagnostic enzyme elevation, evolvingischemic ST-T changes, and new Q wave formation.The patients were considered for the study if theyhad had a coronary angiogram performed in ourinstitution within the previous 3 years and they had

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Taeymans et al Coronary Lesions at Risk of Occlusion 79

been without intervening coronary events such asbypass surgery, coronary angioplasty, congestiveheart failure, unstable angina, or myocardial infarc-tion. Of approximately 1,400 patients catheterizedduring acute infarction, 86 qualified for the study onthe basis of these selection criteria. The selectionprocess was then extended to require unequivocalidentification of a well-defined coronary artery seg-ment responsible for the infarction by the demonstra-tion of a complete occlusion or of an intracoronarylumen defect suggestive of thrombus and corre-sponding to the electrocardiographic site of the newQ wave, usually with a new wall motion abnormalityon the left ventriculogram. The 38 patients respond-ing to these entry criteria formed the study popula-tion. Excluded were 28 patients with previous myo-cardial infarction and diffuse coronary artery lesionsprecluding exact identification of the culprit lesion,four patients without a coronary lesion in a controlsegment, and 16 patients with angiograms technicallyunsuitable for quantitative analysis.

Coronary AngiographyCoronary angiography was performed using stan-

dard techniques,10 including special angulatedviews.1' Left ventriculography was obtained in the 300right anterior oblique position. The angiogram per-formed after the acute infarct was used as a referenceto establish the site of the new occlusion and localizethe preexisting lesion. For this purpose, the twoangiograms for each patient were projected simulta-neously and visualized by an experienced radiologistand a cardiologist to identify the occlusive and con-trol segments to be analyzed. Overall, a total of 217coronary lesions were identified in the 38 patients onthe first angiogram. Excluded were 29 lesions show-ing either complete or partial occlusion related toprevious myocardial infarction and 86 lesions withpoor definition or image superposition. This firstcoronary angiogram, preceding the infarct, was rean-alyzed at a later time by visual inspection as well as bya computer-assisted procedure using the Cardiovas-cular Angiography Analysis System (CAAS) (PieData Medical, B.V.).12 This latter analysis of the 38segments that subsequently occluded to cause thequalifying myocardial infarction and of the 64 dis-eased segments that did not represent the goal of thepresent study and was performed completely blindly;the second angiogram was not available, and theanalysis involving cine frames was identified by num-bers alone.The qualitative description of the first angiogram

included identification of the location and the sever-ity of all coronary artery narrowings and of thepresence or absence of a division branch originatingwithin them. Severity of stenoses was expressed aspercent lumen diameter reduction. Morphologicalfeatures of each lesion noted were regular or irreg-ular contours, symmetry of the stenosis,3 and pres-ence of intraluminal defects.

The quantitative analysis was performed on eachof the coronary artery narrowings suited for such ananalysis, excluding control segments with suboptimalcontrast opacification, stenoses with overlapping ves-sels, and occlusions or stenoses related to a previousmyocardial infarction. Measurements were made in asingle projection. The cine frame showing the steno-sis at its most severe in a projection as perpendicularas possible to the axis of the radiographic beam wasselected. Lesions of the right coronary artery werebest visualized in the left or right anterior obliqueprojection, lesions of the left anterior descendingcoronary artery were best visualized in the sameprojections with cranial angulation, and lesions of thecircumflex coronary artery were best visualized in theright anterior oblique position with or without caudalangulation. Analyses of the frames were performedat the time of best filling, during diastole wheneverpossible.Minimal diameter, length of stenosis, percent di-

ameter reduction, asymmetry index, and inflow andoutflow angles were measured. Contours of the arte-rial segment were determined automatically by thecomputer system and corrected interactively by theuser when they seemed inappropriate, as might occurwhen division branches are present in the area ofinterest. The percent diameter reduction was com-puted from the minimal diameter of the diameterfunction and the mean normal diameter at a user-indicated reference position, avoiding areas of post-stenotic dilatation. This method was preferred to thesemiautomated method, which requires a more idealstenosis with a proximal normal segment. The asym-metry index was measured from the predefined cen-terline of the vessel and given a value between 0 and1, with 1 representing a perfectly concentric lesionand 0 representing the most severe case of eccentric-ity. The inflow angle of stenosis was defined by theaverage slope of the diameter function over thesection bounded by the position of the minimaldiameter and the proximal boundary of the stenoticsegment. The outflow angle represented the samemeasurement from the minimal diameter to thedistal boundary of the stenotic segment.12 Figure 1illustrates a typical example and the methods usedfor the various measurements. The intraobserver andinterobserver variabilities of these methods of mea-surement are very small.13"14

Statistical AnalysesThe characteristics of the 38 coronary artery seg-

ments that subsequently occluded and of the 64coronary artery segments that remained patent werecompared using SYSTAT 4.0 30 and BMDP softwares. x2analyses were performed for discrete variables andan analysis of variance with unbalanced design forthe continuous variables. The patients were stratifiedin three groups according to the time between thefirst coronary angiogram and the myocardial infarc-tion: 3 months or less, 3-18 months, and more than18 months. A paired analysis of variance for repeated



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80 Circulation Vol 85, No 1 January 1992

User-Defined Method Interpolated Method

D

6 IF~~~~~~~~~~~PB 08

Proximal Stenotic Segment DistalFIGURE 1. Illustrative example of a computer-analyzed artery stenotic segment and of the derived computations. Top left panel:User-defined method. Reference diameter (RD) is manually identified by operator. Top right panel: Interpolated method. RD iscalculated by computer by incorporating diameters above and below stenosis and reconstructing presumed diameter of vesseLDiameterfunction (D-Function) was directly derived in this study by computerfrom serial diameter measurements obtained betweentwo edges ofanalyzed coronary artery segment (upper curve). Function could also be measured by videodensitometry (lower curve).Proximal boundary (PB) and distal boundary (DB) of stenotic segment are automatically defined by computer on D-functiondisplay as points where diameters deviate significantly from RD. Bottom panel: Magnification of D-function showing PB, DB,minimal diameter (MD), and interpolated RD (ID). Inflow (IF) and outflow (OF) angles of coronary obstruction are defined byaverage slopes of D-function between MD and PB and between MD and DB, respectively. Asymmetry index measures maximaldistance from each side of detected contour to RD. It is expressed as ratio of minimum to maximum distance and is given a valuebetween 0 and 1, with 1 representing most concentric lesion and 0 representing most eccentric.

measures was done with lesions being the repeatedfactor and time from baseline to infarction being thegrouping factor. In a very conservative analytic ap-proach, the patient's control lesion showing thegreater amount of abnormality was used for thisanalysis. Discrete data were compared by the McNe-mar's test, selecting at random one of the patient'scontrol lesions for the paired comparison with theocclusive lesion and repeating the random procedure20 times; the number of experiments with a signifi-cant difference was reported as x divided by 20.

ResultsClinical EvaluationThe study included 38 patients (30 men and eight

women). Age ranged from 35 to 65 years, with amean of 53 years. A previous myocardial infarctionhad occurred in 14 patients. The interval between thetwo angiograms was 10.3±11.2 months, with a rangeof 0.12-36 months; the interval was 3 months or lessin 21 patients, between 3 and 18 months in 11patients, and more than 18 months in 10 patients.The qualifying myocardial infarction involved theanterior wall in half the patients and the inferior wallin the other half. The mean peak creatine kinase

elevation was 1,717±1X213 IU/l (range, 167-5,000IU/l). Ejection fractions were 58.5+13% at the firstangiogram and 52±7.5% at the second. Thirteen ofthe 38 segments with thrombotic occlusion werelocated in the right, eight in the left circumflex, and17 in the left anterior descending coronary artery; theocclusion was complete in 26 segments, subtotal ineight, and greater than 70% in the remainder. Thedistribution of the control lesions was similar, with 27in the right, 16 in the left circumflex, and 21 in the leftanterior descending coronary artery.

Qualitative AssessmentThe results of the morphological evaluation of the

lesions are shown in Table 1. A division branch origi-nating within the stenosis was present in 29 of the 38lesions (76%) that occluded and in 33 of the 64 controllesions (52%, p<cO.OS). All 20 paired comparisons se-lecting a control lesion at random had a probabilityvalue of less than 0.05. This higher frequency waspresent irrespective of the time interval after the firstangiogram, but it was more conspicuous during the first18 months (Figure 2A). Lesions with irregular contoursand more asymmetric lesions were observed slightlymore frequently in the control group (p=NS) and, in



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TABLE 1. Qualitative and Quantitative Evaluation of Lesions at Risk

Occlusive Controllesions lesions

No. of lesions (n=38) (n=64) pQualitative features

Division branch 29 (76%) 33 (52%) <0.05Irregular contours 18 (47%) 43 (67%) 0.05Asymmetry 24 (57%) 41 (64%) NSIntraluminal defects 0 (0%) 0 (0%)Percent stenosis (visual analysis) 53.7+21 43.6±+19.4 <0.01

Quantitative measurementsMinimal diameter (mm) 1.55+±0.6 1.76+0.6 NSLength of stenosis (mm) 7.16±3.1 7.09±3.1 NSLumen diameter reduction (%) 47.5±17.8 41.0±12.5 <0.05Asymmetry index 0.46±0.28 0.44±0.29 NSInflow angle (0) 21±10 16±7 <0.05Outflow angle (°) 20±10 16±8 <0.05

the paired random analysis, were more frequent in theocclusive segment in only two of the 20 comparisons.Intraluminal defects on the first angiogram were notdetected in this series of patients. The severity ofstenosis estimated visually at the time of the firstangiogram was 53.6±21.3% for the occluded segmentsand 43.6±19.4% for the controls (p<0.01).

Quantitative AssessmentThe minimal diameter of the stenosis, its length,

and the asymmetry index could not differentiatesegments that subsequently occluded from thosethat did not. However, percent stenosis and inflowand outflow angles of the stenosis significantlydiscriminated between the two groups. The valuesfor the various quantitative measurements arelisted in Table 1.

Percent stenosis was 47.5±17.8% in the occludedsegments and 41±12.5% in the controls (p<0.05).The paired analysis of variance showed a significantinteraction between occluded and control segmentsstratified as a function of time with more severelystenosed segments occluding earlier (p<0.001) (Fig-ure 2). All 11 coronary artery segments with a 60% orgreater stenosis that occluded did so in the first year,as did 17 of the 19 stenoses with a 50% or greaterstenosis. Among 17 cases with infarction within thefirst 3 months, only one control lesion was moresevere than the occlusive lesion and two were ofcomparable severity; overall, 19 of the 36 stenoseswere 50% or greater occluded, and seven of the 21had stenoses with 50-60% occlusion.The inflow angle discriminated between the oc-

cluding and the control segments (p<0.05) and alsohad a significant interaction with time (p<0.001)(Figure 2). Of the 21 lesions with inflow angle of 250or greater, 14 occluded, 12 of the 14 during the firstyear. The outflow angle was also significantly steeperin segments that occluded (200 versus 160, p<0.05),with some interaction with the time of follow-up.

DiscussionIn the present morphological and quantitative an-

giographic analysis of coronary artery stenoses athigh risk for thrombotic occlusion that will cause anacute myocardial infarction, three predictors of sub-sequent occlusion could be identified: the presenceof a division branch originating within the stenosis,the severity of the stenosis as assessed by percentlumen diameter reduction, and the steepness of thestenosis inflow and outflow angles. These character-istics share the common property of directly influenc-ing blood flow rheology, in particular, shear stressand flow separation.The main determinant of higher shear stress is the

severity of the stenosis; other geometric factors of thestenosis play a less important role.15-17 In the presentstudy, the length of the stenosis, its asymmetry index,and its irregularity did not influence outcome. Shearstress increases as percent stenosis increases to reacha maximal value near the site of maximal stenosis.15Wall stress drops sharply at the point of widening ofthe stenosis, resulting in deceleration of flow, recoveryof pressure, and flow separation.15,18 Experimentalstudies indicate that flow separation can occur even ina relatively mild constriction, below that required tolimit coronary flow under resting conditions.16-'9 Ele-vated plasma and whole blood viscosity, found in somepatients with coronary artery disease, may also con-tribute to higher shear stress and an enhanced ten-dency to thrombosis.20 Branching vessels are sites ofpredilection for atherosclerosis2' and modify flow sep-aration in a complex way, depending on many factors,including flow velocity, angle of the division branch, itscurvature, and its diameter.'8'22-24 These branchingpoints have also been shown to be more sensitive toacetylcholine-mediated vasoconstriction.25

Extrapolation of in vitro and in vivo experimentalresearch to diseased coronary arteries is furthercomplicated by any active vasomotor tone that ispresent in the epicardial arteries proximal and distal



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80-Presenceof divisionbranch(% patients) 4 0

0O

80

Lumendiameterreduction(%) 4

0

Inflowangle(degree)

4 o

Outflowangle(degree)

20-

00 -3

NS

3 181

18 - 36

Time between coronary anglography and myocardialinfarction (months)

FIGURE 2. Bar graphs of interaction between time period from coronary angiography to myocardial infarction and angiographicfeatures associated with a higher rsk of acute thrombotic coronary artery occlusion. Myocardial infarction had occurred in 17patients within the first 3 months, in 11 between 3 and 18 months, and in 10 patients between 18 and 36 months. Paired statisticsfor quantitative data were obtained by comparing in each patient the occlusive lesion with the control lesion showing the most severeabnonnality.

to the stenosis as well as, to varying degrees, at thesite of the stenosis.26-28 This dynamic aspect may inpart explain some of the variability in the results andwhy outcome was more directly related to percentstenosis than to absolute minimum diameter.29 Also,percent stenosis in part accounts for blood flowvelocity in the diseased segment, and minimal diam-

eter does not (e.g., a 0.4-mm minimal diameter willnot cause much turbulence in a 2-mm artery [20%stenosis] but will in a 0.5-mm artery [80% stenosis]).The statistically significant association between se-verity of the stenosis, the inflow angle, and thepresence of a collateral branch strongly suggest thatshear stress and flow separation may be the most

p H0.05 [ Occlusion lesionsm Control lesions

p -0.05

-Ii40



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important determinants of outcome of the plaque.These two factors are associated with trauma to thevascular wall,30 development and progression of ath-erosclerosis,31,32 platelet deposition,33 and thrombusformation.23,34

Previous angiographic studies have described spe-cific lesions associated with the acute phase of myo-cardial infarction and unstable angina. Thus, a highincidence of coronary artery thrombus is found inpatients catheterized acutely1 to decrease subse-quently, presumably because of spontaneous lysis.Later, lesions appear to be eccentric with a narrowneck or irregular borders, representing either a dis-rupted atherosclerotic plaque or a partially occlusiveor lysed thrombus.4 Quantitative morphological anal-ysis of these lesions shows a high degree of lumenirregularity, suggesting plaque ulceration and disrup-tion35,36 in accordance with the pathological findingof a high incidence of thrombus formation andplaque fissure in unstable angina,5 sudden death,6and myocardial infarction.37These angiographic and pathological observations

have all been obtained after the acute event, and it isnot known to what extent they are the consequencerather than the cause of the acute occlusion. Theabsence of eccentric and irregular lesions beforeocclusion in the present study suggests that thesefeatures do not play an important etiologic role.Furthermore, the findings do not determine whetherplaque fissuring and thrombus formation are randomprocesses or are associated with specific characteris-tics of the plaque.

Studies performed before the occurrence of acutecoronary events have yielded more controversial re-sults. The distribution and extent of coronary arteryobstructive lesions have been shown to be weakpredictors of ischemic events.38,39 In a 5-year fol-low-up of patients with one-vessel coronary arterydisease, the anatomic location of the lesion did notinfluence the rate of ischemic events,40 and theseverity of the stenosis was a weak predictor ofsubsequent infarction. In a report from the CoronaryArtery Surgery Study (CASS) registry of 118 patientswith left anterior coronary artery disease, the 3-yearrisk of anterior infarction was 2% with stenosis of lessthan 50%, 6% with stenosis of 50% or greater, and11% with two or more stenoses of 50% or greater.Stenoses of 90-98% had a 15% 3-year risk.41

In a further analysis of the morphology of coro-nary lesions that caused myocardial infarction, theCASS investigators matched the 118 patients to acontrol group without infarction and the sameextent of left anterior descending coronary involve-ment. A multivariate analysis revealed edge rough-ness as the most potent predictor of subsequentinfarction; the length of the stenosis and the pres-ence of a branching point at the site of the stenosisduring the first year were also predictive.9 The poorreproducibility of the method for edge roughnessmeasurement in their study (72% intraobserveragreement, p=0.07) and the absence of angiogra-

phy to document the site of occlusion limit thevalidity of their conclusions. In the present study,edge roughness, defined as irregular contours, wasdiagnosed in 47% of the infarct lesions and in 67%of the control lesions compared with 16.1% and3.5%, respectively, in the aforementioned study.This large difference suggests important interob-server differences in the evaluation of this qualita-tive parameter and might explain the absence ofpredictive value in our analysis. In a study by Littleet al,8 an angiogram performed 706±685 days be-fore an acute infarct was not predictive of the site ofinfarction as determined by repeat angiographyshortly after the infarct. Ambrose et al,7 also usingangiographic confirmation of the site of infarct,reported more frequent myocardial infarction withproximal stenoses of 50% or more in severity;percent stenosis, left anterior descending coronaryinvolvement, and type 2 eccentric lesions were notpredictive.The observations made in the present study docu-

ment that thrombotic occlusion of a coronary arteryat the site of an atherosclerotic plaque is not acompletely random process but is influenced by he-modynamic disturbances related to the geometry ofthe plaque and to local blood flow. These observa-tions should stimulate a more in-depth analysis ofcoronary atherosclerotic lesions in an attempt topredict which lesions at higher risk of occlusionwould benefit from more aggressive treatment.The sensitivity and specificity of the plaque's geomet-

ric features to predict subsequent occlusion are, how-ever, relatively low, suggesting that other critical char-acteristics of the plaque not detected by angiographycould be operative and possibly also the state of acti-vation of platelets and of the blood factors. Thus,plaques that undergo disruption tend to be softer, withhigh concentrations of cholesterol and cholesterol es-ter.42 Fissuring is frequently observed at the lateralmargin of plaques containing an eccentric pool ofextracellular lipid in the intima43 and in plaques withcaps infiltrated with macrophages.44

Study LimitationsAs in all retrospective analyses, case selection may

have introduced biases so that the results of thepresent study may not accurately reflect reality. Thecomputer-assisted measurement method may intro-duce further bias because unmeasurable stenoseswere excluded. Another limitation of the presentstudy is that the characteristics of the plaque wereanalyzed only once before the infarction; an evolu-tion of the lesion between the time of angiographyand the time of the myocardial infarction cannot beruled out. A history of worsening angina beforeinfarction is reported by many patients and maycorrespond to this process.Many other parameters not analyzed in these stud-

ies, more specifically, clinical factors, could also haveconsiderably influenced the outcome of the plaque.Thus, risk factors such as smoking, hypertension, lipid



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disorders, and other blood coagulation or platelet ab-normalities could have been determinants of occlusionfor some plaques.41,45 The design of the present studymay miniimize the importance of these factors becauselesions that occluded were compared with lesions thatdid not occlude in the same patients. However, differ-ent lesions in the same patient can behave differently byconstricting or dilating in response to the same stimu-lus.25 Interrelations between clinical factors, plaquegeometry and characteristics, and outcome representan exciting field for further investigation, with thechallenge to better identify high-risk lesions.

AcknowledgmentThe authors thank Luce Begin for her excel-

lent secretarial work.

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KEY WORDS * thromboses * arteriography * myocardialinfarction



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Y Taeymans, P Théroux, J Lespérance and D Watersthrombotic occlusion.

Quantitative angiographic morphology of the coronary artery lesions at risk of

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quantitative angiographic morphology of coronary artery … · coronary angiography was performed...

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