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Circulation. 2003;108:1664-1672
doi: 10.1161/01.CIR.0000087480.94275.97
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(Circulation. 2003;108:1664.)
© 2003 American Heart Association, Inc.

Review: Current Perspective

From Vulnerable Plaque to Vulnerable Patient

A Call for New Definitions and Risk Assessment Strategies: Part I

Morteza Naghavi, MD; Peter Libby, MD; Erling Falk, MD, PhD; S. Ward Casscells, MD; Silvio Litovsky, MD; John Rumberger, MD; Juan Jose Badimon, PhD; Christodoulos Stefanadis, MD; Pedro Moreno, MD; Gerard Pasterkamp, MD, PhD; Zahi Fayad, PhD; Peter H. Stone, MD; Sergio Waxman, MD; Paolo Raggi, MD; Mohammad Madjid, MD; Alireza Zarrabi, MD; Allen Burke, MD; Chun Yuan, PhD; Peter J. Fitzgerald, MD, PhD; David S. Siscovick, MD; Chris L. de Korte, PhD; Masanori Aikawa, MD, PhD; K.E. Juhani Airaksinen, MD; Gerd Assmann, MD; Christoph R. Becker, MD; James H. Chesebro, MD; Andrew Farb, MD; Zorina S. Galis, PhD; Chris Jackson, PhD; Ik-Kyung Jang, MD, PhD; Wolfgang Koenig, MD, PhD; Robert A. Lodder, PhD; Keith March, MD, PhD; Jasenka Demirovic, MD, PhD; Mohamad Navab, PhD; Silvia G. Priori, MD, PhD; Mark D. Rekhter, PhD; Raymond Bahr, MD; Scott M. Grundy, MD, PhD; Roxana Mehran, MD; Antonio Colombo, MD; Eric Boerwinkle, PhD; Christie Ballantyne, MD; William Insull, Jr, MD; Robert S. Schwartz, MD; Robert Vogel, MD; Patrick W. Serruys, MD, PhD; Goran K. Hansson, MD, PhD; David P. Faxon, MD; Sanjay Kaul, MD; Helmut Drexler, MD; Philip Greenland, MD; James E. Muller, MD; Renu Virmani, MD; Paul M Ridker, MD; Douglas P. Zipes, MD; Prediman K. Shah, MD; James T. Willerson, MD

From The Center for Vulnerable Plaque Research, University of Texas—Houston, The Texas Heart Institute, and President Bush Center for Cardiovascular Health, Memorial Hermann Hospital, Houston (M. Naghavi, S.W.C., S.L., M.M., A.Z., J.T.W.); The Leducq Center for Cardiovascular Research, Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, Mass (P.L., M.A.); Department of Cardiology and Institute of Experimental Clinical Research, Aarhus University, Aarhus, Denmark (E.F.); Experimental Cardiology Laboratory, Vascular Biology of the University Medical Center in Utrecht, the Netherlands (G.P.); Ohio State University (J.R.); the Zena and Michael A. Wiener Cardiovascular Institute, Mount Sinai Medical Center, New York, NY (Z.F.); Cardiac Catheterization Laboratory at the VA Medical Center, University of Kentucky, Lexington (P.M.); Cardiovascular Division, Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, Mass (P.H.S.); Division of Cardiology, New England Medical Center, Boston, Mass (S.W.); Department of Medicine, Section of Cardiology, Tulane University School of Medicine, New Orleans, La (P.R.); Department of Cardiovascular Pathology, Armed Forces Institute of Pathology, Washington, DC (A.B., A.F., R.V.); Department of Radiology, University of Washington, Seattle (C.Y.); Stanford University Medical Center Stanford, Calif (P.J.F.); Cardiovascular Health Research Unit, University of Washington, Seattle (D.S.S.); Department of Cardiology, Athens Medical School, Athens, Greece (C.S.); Catheterization Laboratory, Thorax Center, Erasmus University, Rotterdam, the Netherlands (C.L.d.K.); Division of Cardiology, Department of Medicine, University of Turku, Finland (K.E.J.A.); Institute of Arteriosclerosis Research and the Institute of Clinical Chemistry and Laboratory Medicine, Central Laboratory, Hospital of the University of Münster, Munich, Germany (G.A.); Department of Clinical Radiology, University of Münster, Munich, Germany (C.R.B.); Mayo Clinic Medical School, Jacksonville, Fla (J.H.C.); Department of Medicine, Division of Cardiology, Emory University School of Medicine, Atlanta, Ga (Z.S.G.); Bristol Heart Institute, Bristol University, Bristol, United Kingdom (C.J.); Cardiology Division, Massachusetts General Hospital and Harvard Medical School, Boston, Mass (I.-K.J.); Department of Internal Medicine II, Cardiology, University of Ulm, Ulm, Germany (W.K.); University of Kentucky, Lexington, Ky (R.A.L.); R.L. Roudebush VA Medical Center, Indianapolis, Ind (K.M.); School of Public Health, University of Texas—Houston, Houston, Texas (J.D.); Division of Cardiology, University of California Los Angeles, Los Angeles, Calif (M. Navab); Fondazione Salvatore Maugeri, University of Pavia, Pavia, Italy (S.G.P.); Department of Cardiovascular Therapeutics, Pfizer Global Research and Development, Ann Arbor Laboratories, Ann Arbor, Mich (M.D.R.); Paul Dudley White Coronary Care System at St. Agnes HealthCare, Baltimore, Md (R.B.); Center for Human Nutrition, University of Texas Health Science Center, Dallas (S.M.G.); Lenox Hill Hospital, New York, NY (R.M.); Catheterization Laboratories, Ospedale San Raffaele and Emo Centro Cuore Columbus, Milan, Italy (A.C.); Human Genetics Center, Institute of Molecular Medicine, Houston, Tex (E.B.); Department of Medicine, Baylor College of Medicine, Houston, Tex (C.B., W.I.); Minneapolis Heart Institute and Foundation, Minneapolis, Minn (R.S.S.); Division of Cardiology, University of Maryland School of Medicine, Baltimore, Md (R.V.); Karolinska Institute, Center for Molecular Medicine, Karolinska Hospital, Stockholm, Sweden (G.K.H.); Section of Cardiology, University of Chicago, Ill (D.P.F.); Vascular Physiology and Thrombosis Research Laboratory of the Atherosclerosis Research Center, Cedars-Sinai Medical Center, Los Angeles, California (S.K.); Cardiology Department, Hannover University, Hannover, Germany (H.D.); Department of Medicine, Feinberg School of Medicine, Northwestern University, Chicago, Ill (P.G.); UCLA School of Medicine and Cedars-Sinai Medical Center, Los Angeles, Calif (P.K.S.); Massachusetts General Hospital, Harvard Medical School and CIMIT (Center for Integration of Medicine and Innovative Technology), Boston, Mass (J.E.M.); Cardiovascular Division, Division of Preventive Medicine, Brigham and Women’s Hospital, Boston, Mass (P.M.R.); and Indiana University School of Medicine, Krannert Institute of Cardiology, Indianapolis (D.P.Z.).

Correspondence to Morteza Naghavi, MD, Association for Eradication of Heart Attack, 2472 Bolsover, No. 439, Houston, TX 77005. E-mail mn{at}vp.org

Abstract

Atherosclerotic cardiovascular disease results in >19 million deaths annually, and coronary heart disease accounts for the majority of this toll. Despite major advances in treatment of coronary heart disease patients, a large number of victims of the disease who are apparently healthy die suddenly without prior symptoms. Available screening and diagnostic methods are insufficient to identify the victims before the event occurs. The recognition of the role of the vulnerable plaque has opened new avenues of opportunity in the field of cardiovascular medicine. This consensus document concludes the following. (1) Rupture-prone plaques are not the only vulnerable plaques. All types of atherosclerotic plaques with high likelihood of thrombotic complications and rapid progression should be considered as vulnerable plaques. We propose a classification for clinical as well as pathological evaluation of vulnerable plaques. (2) Vulnerable plaques are not the only culprit factors for the development of acute coronary syndromes, myocardial infarction, and sudden cardiac death. Vulnerable blood (prone to thrombosis) and vulnerable myocardium (prone to fatal arrhythmia) play an important role in the outcome. Therefore, the term "vulnerable patient" may be more appropriate and is proposed now for the identification of subjects with high likelihood of developing cardiac events in the near future. (3) A quantitative method for cumulative risk assessment of vulnerable patients needs to be developed that may include variables based on plaque, blood, and myocardial vulnerability. In Part I of this consensus document, we cover the new definition of vulnerable plaque and its relationship with vulnerable patients. Part II of this consensus document focuses on vulnerable blood and vulnerable myocardium and provide an outline of overall risk assessment of vulnerable patients. Parts I and II are meant to provide a general consensus and overviews the new field of vulnerable patient. Recently developed assays (eg, C-reactive protein), imaging techniques (eg, CT and MRI), noninvasive electrophysiological tests (for vulnerable myocardium), and emerging catheters (to localize and characterize vulnerable plaque) in combination with future genomic and proteomic techniques will guide us in the search for vulnerable patients. It will also lead to the development and deployment of new therapies and ultimately to reduce the incidence of acute coronary syndromes and sudden cardiac death. We encourage healthcare policy makers to promote translational research for screening and treatment of vulnerable patients.


Key Words: coronary disease • plaque • myocardial infarction • atherosclerosis • death, sudden

Cardiovascular disease has long been the leading cause of death in developed countries, and it is rapidly becoming the number one killer in the developing countries.1 According to current estimates, 61 800 000 Americans have one or more types of cardiovascular disease.2

Every year, >1 million people in the United States and >19 million others worldwide experience a sudden cardiac event (acute coronary syndromes and/or sudden cardiac death). A large portion of this population has no prior symptom.3 There is considerable demand for diagnosis and treatment of the pathologic conditions that underlie these sudden cardiac events. This consensus document proposes new directions to prevent infarction and sudden cardiac events.4

Underlying Causes of Sudden Fatal and Nonfatal Cardiac Events
Figure 1 delineates the underlying causes of acute cardiac events. The first branch point of the tree indicates patients who lack significant atherosclerosis or related myocardial damage, that is, those who have no ischemic heart disease (see The Nonischemic Vulnerable Myocardium). This leaves the patients with atherosclerosis, some of whom also have a hypercoagulable state (see Vulnerable Blood).



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  		Figure 1. Proposed diagram of the potential underlying pathology of acute coronary syndrome, (ie, unstable angina, acute myocardial infarction and sudden cardiac death).

The next branch point involves the presence or absence of an occlusive or subocclusive thrombus. A thrombus identifies a culprit plaque that may be ruptured or nonruptured.

Plaque rupture is the most common type of plaque complication, accounting for {approx}70% of fatal acute myocardial infarctions and/or sudden coronary deaths (Figure 2). Several retrospective autopsy series and a few cross-sectional clinical studies have suggested that thrombotic coronary death and acute coronary syndromes are caused by the plaque features and associated factors presented in Table 1.5–7 Most techniques for detecting and treating vulnerable plaque are devoted to rupture-prone plaque. This type of plaque has been termed a "thin-cap fibroatheroma."8



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  		Figure 2. Different types of vulnerable plaque as underlying cause of acute coronary events (ACS) and sudden cardiac death (SCD). A, Rupture-prone plaque with large lipid core and thin fibrous cap infiltrated by macrophages. B, Ruptured plaque with subocclusive thrombus and early organization. C, Erosion-prone plaque with proteoglycan matrix in a smooth muscle cell-rich plaque. D, Eroded plaque with subocclusive thrombus. E, Intraplaque hemorrhage secondary to leaking vasa vasorum. F, Calcific nodule protruding into the vessel lumen. G, Chronically stenotic plaque with severe calcification, old thrombus, and eccentric lumen.


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  		TABLE 1. Underlying Pathologies of "Culprit" Coronary Lesions

In some cases, a deep plaque injury cannot be identified despite a careful search. The thrombus appears to be superimposed on a de-endothelialized, but otherwise intact, plaque. This type of superficial plaque injury is called "plaque erosion."9 Other types of culprit plaques also exist (Figure 2). In cases involving nonruptured plaques, plaque erosion or nodular calcification usually accompanies the luminal thrombus.5 Other forms of thrombosis in nonruptured plaques may be described in the future. In all cases that involve a superimposed thrombus, the underlying lesion may be stenotic or nonstenotic. However, nonstenotic lesions are far more frequent than stenotic plaques and account for the majority of culprit ruptured plaques.10


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  		TABLE 2. Descriptions Used by Pioneers for Culprit Plaques93,94

In cases of sudden cardiac death without thrombosis, we hypothesize that coronary spasm, emboli to the distal intramural vasculature, or myocardial damage related to previous injury may account for a terminal arrhythmic episode.

The Challenge of Terminology: Culprit Plaque Versus Vulnerable Plaque
Culprit Plaque, a Retrospective Terminology
Interventional cardiologists and cardiovascular pathologists retrospectively describe the plaque responsible for coronary occlusion and death as a culprit plaque, regardless of its histopathologic features. For prospective evaluation, clinicians need a similar term for describing such plaques before an event occurs. Plaque rupture was reported sporadically by pathologists in the early 20th century; it became a focus of attention of pioneering scientists in the 1960s (Table 2) and was later documented further by others.11–15

Since the 1970s, scientists have been seeking the mechanisms responsible for converting chronic coronary atherosclerosis to acute coronary artery disease.11–15,17 As insights into this process have evolved, the relevant terminology has been continually updated. In the 1980s, Falk11 and Davies and Thomas15 used "plaque disruption" synonymously with "plaque rupture." Later, Muller and colleagues18,19 used "vulnerable" to describe rupture-prone plaques as the underlying cause of most clinical coronary events. When this functional definition was proposed, the plaque considered responsible for acute coronary events (based on retrospective autopsy studies) had a large lipid pool, a thin cap, and macrophage-dense inflammation on or beneath its surface (Figure 3).



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  		Figure 3. Schematic figure illustrating the most common type of vulnerable plaque characterized by thin fibrous cap, extensive macrophage infiltration, paucity of smooth muscle cells, and large lipid core, without significant luminal narrowing.

Over the past several years, "vulnerable plaque" has been used sometimes to denote this concept and at other times to denote the specific histopathologic appearance of the above-described plaque. This dual usage is confusing, particularly as plaques can have other histologic features (see Figure 2) that may also cause acute coronary events.5

Vulnerable Plaque, a Future Culprit Plaque
The term "vulnerable" is defined by English dictionaries as "susceptible to injury or susceptible to attack,"20 as in "We are vulnerable both by water and land, without either fleet or army" (Alexander Hamilton). It denotes the likelihood of having an event in the future. The term vulnerable has been used in various reports in the medical literature, all of which describe conditions susceptible to injury. In this regard, the term "vulnerable plaque" is most suitable to define plaques susceptible to complications. An alternative term, "high-risk plaque," has been recently proposed.18 The term "high-risk" is often used to describe the high-risk patient groups with acute coronary syndromes. However, our intention is to provide a terminology to identify apparently healthy subjects at risk of future events. Therefore, the term vulnerable seems to be more appropriate. Also, because "vulnerable plaque" has already been widely adopted by investigators and clinicians, we recommend that the existing usage of this term be continued. We advise that the underlying morphological features be described broadly enough to include all dangerous plaques that involve a risk of thrombosis and/or rapid progression.

To provide a uniform language to help standardize the terminology, we recommend "vulnerable plaque" to identify all thrombosis-prone plaques and plaques with a high probability of undergoing rapid progression, thus becoming culprit plaques (Table 3). A proposed histopathologic classification for different types of vulnerable plaque is presented in Figure 2. A list of proposed major and minor criteria for defining vulnerable plaques, based on autopsy studies (culprit plaques), is presented in Table 4.


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  		TABLE 3. Interchangeable Terms Used to Denote Vulnerable Plaque


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  		TABLE 4. Criteria for Defining Vulnerable Plaque, Based on the Study of Culprit Plaques

A large number of vulnerable plaques are relatively uncalcified, relatively nonstenotic, and similar to type IV atherosclerotic lesions described in the American Heart Association classification.21 However, as depicted in Figure 3, different types of vulnerable plaque exist. Although Table 1 shows the relative distribution of ruptured and nonruptured culprit plaques, the exact prevalence of each type of vulnerable plaque is unknown and can only be determined in prospective studies.

Pan-Coronary Vulnerability
Several investigators have noted the presence of more than one vulnerable plaque in patients at risk of cardiovascular events. Mann and Davies22 and Burke et al23 in cardiac autopsy specimens, Goldstein et al24 in angiography studies, Nissen25 and Rioufol et al26 with intravascular ultrasound, and Buffon et al27 measuring neutrophil myeloperoxidase found multiple rupture-prone or ruptured plaques in a wide range of cardiovascular patient populations. A most recent series of publications on vulnerability reiterated the importance of going beyond a vulnerable plaque and called for evaluating the total arterial tree as a whole.28–30

Silent-Plaque Rupture
Thrombotic complications that arise from rupture or fissure (small rupture) of a vulnerable plaque may be clinically silent yet contribute to the natural history of plaque progression and ultimately luminal stenosis.31,32

Beyond the Atherosclerotic Plaque
It is important to identify patients in whom disruption of a vulnerable plaque is likely to result in a clinical event. In these patients, other factors beyond plaque (ie, thrombogenic blood and electrical instability of myocardium) are responsible for the final outcome (Figure 4). We propose that such patients be referred to as "vulnerable patients." In fact, plaques with similar characteristics may have different clinical presentations because of blood coagulability (vulnerable blood) or myocardial susceptibility to develop fatal arrhythmia (vulnerable myocardium). The latter may depend on a current or previous ischemic condition and/or a nonischemic electrophysiological abnormality.



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  		Figure 4. The risk of a vulnerable patient is affected by vulnerable plaque and/or vulnerable blood and/or vulnerable myocardium. A comprehensive assessment must consider all of the above.

Definition of a Cardiovascular Vulnerable Patient
The term "cardiovascular vulnerable patient" is proposed to define subjects susceptible to an acute coronary syndrome or sudden cardiac death based on plaque, blood, or myocardial vulnerability (for example, 1-year risk >=5%). Extensive efforts are needed to quantify an individual’s risk of an event according to each component of vulnerability (plaque, blood, and myocardium). Such a comprehensive risk-stratification tool capable of predicting acute coronary syndromes as well as sudden cardiac death would be very useful for preventive cardiology (Figure 4).

Diagnosis of Vulnerable Plaque/Artery
A number of issues have hampered establishment of ideal criteria for defining vulnerable plaque: (1) the current body of evidence is largely based on cross-sectional and retrospective studies of culprit plaques; (2) robust prospective outcome studies based on plaque characterization have not been done (due to the lack of a reproducible, validated diagnostic technique); and (3) a lack of a representative animal model of plaque rupture and acute coronary syndrome/sudden death.

On the basis of retrospective evidence, we propose that the criteria listed in Tables 4 and 5Down be used to define a vulnerable plaque. The sensitivity, specificity, and overall predictive value of each potential diagnostic technique need to be assessed before entering clinical practice.


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  		TABLE 5. Markers of Vulnerability at the Plaque/Artery Level

Major Criteria
The following are proposed as major criteria for detection of a vulnerable plaque. The presence of one or a combination of these factors may warrant higher risk of plaque complication. Techniques for detection of vulnerable plaque based on these criteria are briefly summarized here. A detailed discussion of advantages and disadvantages are reviewed elsewhere.33

1. Active Inflammation
Plaques with active inflammation may be identified by extensive macrophage accumulation.13 Possible intravascular diagnostic techniques34,35 include thermography (measurement of plaque temperature),36,37 contrast-enhanced (CE) MRI,38,39 fluorodeoxyglucose positron emission tomography,33,40 and immunoscintigraphy.41 It has recently been shown that optical coherence tomography reflects the macrophage content of the fibrous cap.42 Noninvasive options include MRI with superparamagnetic iron oxide35,36 and gadolinium fluorine compounds.43–45

2. A Thin Cap With a Large Lipid Core
These plaques have a cap thickness of <100 µm and a lipid core accounting for >40% of the plaque’s total volume.8 Possible intravascular diagnostic techniques include optical coherence tomography (OCT),46,47 intravascular ultrasonography (IVUS),48 high-resolution IVUS,49 elastography (palpography),50,51 MRI,52 angioscopy,53 near infrared (NIR) spectroscopy,54–56 and radiofrequency IVUS analysis.57,58 The only noninvasive options are presently MRI and possibly CT.34,35,59–62

3. Endothelial Denudation with Superficial Platelet Aggregation
These plaques are characterized by superficial erosion and platelet aggregation or fibrin deposition.5 Possible intravascular diagnostic techniques include angioscopy with dye63 and matrix-targeted/fibrin-targeted immune scintigraphy and OCT.46,47 Noninvasive options include fibrin/matrix-targeted contrast enhanced MRI,64 platelet/fibrin-targeted single photon emission computed tomography,41 and MRI.52

4. Fissured/Injured Plaque
Plaques with a fissured cap (most of them involving a recent rupture) that did not result in occlusive thrombi may be prone to subsequent thrombosis, entailing occlusive thrombi or thromboemboli.5 Possible intravascular diagnostic techniques include OCT,46,47 IVUS, high-resolution IVUS,49 angioscopy, and MRI.34,35 A noninvasive option is fibrin-targeted CE-MRI.64,65

5. Severe Stenosis
On the surface of plaques with severe stenosis, shear stress imposes a significant risk of thrombosis and sudden occlusion. Therefore, a stenotic plaque may be a vulnerable plaque regardless of ischemia. Moreover, a stenotic plaque may indicate the presence of many nonstenotic or less stenotic plaques that can be vulnerable to rupture and thrombosis24,66 (Figure 5). The current standard technique is invasive x-ray angiography.32 Noninvasive options include multislice CT,67,68 magnetic resonance angiography with or without a contrast agent, and electron-beam tomography angiography.59,69–71



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  		Figure 5. Plaques with nearly similar morphology in terms of lipid core and fibrous cap (middle panel) may look similar with diagnostic imaging aimed at morphology only (bottom panel). However, they might look very different using diagnostic methods capable of detecting activity and physiology of the plaques. The top left plaque is hot (as evidenced in a thermography image), whereas the top right plaque is inactive and detected relatively as a cool plaque.

Minor Criteria
For techniques that focus on the plaque level, minor criteria include the following features.

1. Superficial Calcified Nodules
These plaques have a calcified nodule within, or very close to, their cap, and this structure protrudes through and can rupture the cap. This event may or may not be associated with severe coronary calcification and a high calcium score.5 Possible intravascular diagnostic techniques include OCT,46,47 IVUS and elastography (palpography).48 Noninvasive options include electron-beam CT,72 multisection spiral CT,73 and MRI.34,35

2. Yellow Color (on Angioscopy)
Yellow plaques, particularly glistening ones, may indicate a large lipid core and thin fibrous cap, suggesting a high risk of rupture. However, because plaques in different stages can be yellow and because not all lipid-laden plaques are destined to rupture or undergo thrombosis, this criterion may lack sufficient specificity.53,74 Possible intravascular diagnostic techniques include angioscopy73 and transcatheter colorimetry.75 No diagnostic method has yet been developed for noninvasive angioscopy.

3. Intraplaque Hemorrhage
Extravasation of red blood cells, or iron accumulation in plaque, may represent plaque instability.76 Possible intravascular diagnostic techniques include NIR spectroscopy,54,55 tissue Doppler methods,77 and intravascular MRI. A noninvasive option is MRI.34,35,61

4. Endothelial Dysfunction
Impaired endothelial vasodilator function occurs in a variety of acute and chronic disease states. Patients with cardiovascular risk factors have endothelial dysfunction. Endothelial dysfunction predicts CHD and stroke.89,95 Vulnerable plaques have sites of active inflammation and oxidative stress and are likely to be associated with impaired endothelial function. Possible diagnostic techniques are endothelium-dependent coronary artery dilatation (intravascular)78 and measurement of flow-mediated dilatation by brachial artery ultrasonography and other emerging techniques (noninvasive).79

5. Expansive (Positive) Remodeling
Many of the nonstenotic lesions undergo "expansive," "positive," or "outward" remodeling, namely compensatory enlargement before impinging significantly on the vascular lumen. This phenomenon was considered as positive remodeling because the luminal area was not affected and stenosis was the only measure of risk. However, with the emphasis on plaque rupture in nonstenotic lesions, the so-called positive remodeling may not be truly positive and beneficial. Several studies have suggested that such remodeling is a potential surrogate marker of plaque vulnerability.80,81 In these studies, intravascular ultrasound was used to evaluate remodeling in coronary arteries. A recent study by Kim et al82 introduced a noninvasive method for detection of expansive remodeling in coronary arteries by MRI. CT might also provide a noninvasive method for studying arterial remodeling.

Few of the above techniques have been tested in clinical trials showing ability to predict events. MRI and CT-based approaches are being developed. These technologies and strategies must also be evaluated with regard to their cost effectiveness.

Functional Versus Structural Assessment
A growing body of evidence indicates that different types of vulnerable plaque with various histopathology and biology exist. To evaluate plaque vulnerability, it is evident that a combined approach capable of evaluating structural characteristics (morphology) as well as functional properties (activity) of plaque may be more informative and may provide higher predictive value than a single approach. For instance, a combination of IVUS or OCT with thermography80,83 may provide more diagnostic value than each of these techniques alone. Such an arrangement can be useful for both intravascular as well as noninvasive diagnostic methods (Figure 6). Autopsy84 and IVUS studies85 have shown that atherosclerotic lesions are frequently found in young and asymptomatic individuals. It is unclear what percentage of these lesions present morphologies of rupture-prone vulnerable plaques. Moreover, chronic inflammation86 and macrophage/foam cell formation are an intrinsic part of the natural history of atherosclerosis. These data suggest that screening only based on plaque morphology and/ or chronic markers of inflammation may not provide satisfactory predictive value for detection of vulnerable patients.



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  		Figure 6. Correlation between frequency of plaques, degree of stenosis, and risk of complication per plaque as a function of plaque progression. Although the average absolute risk of severely stenotic plaques may be higher than the average absolute risk of mildly stenotic plaques, there are more plaques with mild stenoses than plaques with severe stenoses.

Pan-Arterial Approach
Diagnostic and therapeutic methods may focus on the total burden of coronary artery disease.27 The coronary Calcium Score is a good example of using CT for this purpose.72 The total burden of calcified atherosclerotic plaques in all coronary arteries is identified by ultrafast CT. Extensive efforts are underway to improve image quality, signal processing, and interpretation of detailed components of coronary arteries that lend hope of a new calcium scoring and risk stratification technique based on CT information.87 Like systemic indexes of inflammation (eg, high sensitive CRP), endothelial dysfunction as measured by impaired flow-mediated vasodilation in the brachial artery can aid in the detection of pan-arterial vulnerability and may serve as a screening tool.88,89

Another emerging technique is the measurement of the transcoronary gradient (difference in concentration between coronary ostium and coronary sinus or between proximal and distal segments of each coronary segment) of various factors, including cytokines,90 adhesion molecules,91 temperature, etc.

It will be important in the future to identify plaques that are on a trajectory of evolution toward a vulnerable state, to find out how long they will stay vulnerable, and to be able to target interventions to those plaques most likely to develop thrombosis. Similarly, factors that protect plaques from becoming vulnerable also need to be identified. It is likely that local hemodynamic factors and 3-dimensional morphology may provide insight regarding the temporal course of an evolving plaque.

New studies are unraveling the role of the adventitia and periadventitial connective and adipose tissue in vulnerability of atherosclerotic plaques.92 Further studies are needed to define the importance of these findings in the detection and treatment of vulnerable plaques.

Acknowledgments

We are indebted to Valentin Fuster, MD, and Salim Yusuf, MD, for their insightful reviews and thoughtful comments.

Footnotes

Many of the authors of this work, in addition to their research activities, have served as consultants to and/or employees of pharmaceutical, medical equipment, and other related companies.

Guest editor for this article was Eugene Braunwald, MD, Brigham and Women’s Hospital.

This article is Part I of a 2-part article. Part II will appear in the October 14, 2003 issue of Circulation.

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