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J Am Coll Cardiol, 2001; 38:99-104 © 2001 by the American College of Cardiology Foundation |
a Department of Internal Medicine, Nippon Medical School, Chiba Hokusoh Hospital, Chiba, Japan
Manuscript received November 20, 2000; revised manuscript received March 13, 2001, accepted March 26, 2001.
Reprint requests and correspondence: Dr. Kyoichi Mizuno, Department of Internal Medicine, Nippon Medical School, Chiba Hokusoh Hospital, 1715 Kamakari, Imba, Imba, Chiba, Japan
mizunok{at}nms.ac.jp
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Abstract |
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Mechanical and structural characteristics of vulnerable plaques were evaluated using coronary angioscopy and intravascular ultrasound.
BACKGROUND
Mechanical stress and composition of plaques play an important role in plaque disruption.
METHODS
Thirty-eight lesions in 38 patients were examined pre-interventionally. The plaques were classified as either yellow or white using coronary angioscopy. Intravascular ultrasound imaging was performed simultaneously with electrocardiographic and intracoronary pressure recordings to calculate distensibility index and stiffness ß. Moreover, the type of remodeling was classified.
RESULTS
We identified 27 patients with yellow plaques and 11 patients with white plaques. Patients with yellow plaques presented acute coronary syndromes more frequently than stable angina (85% vs. 36%, p < 0.01). The distensibility index in yellow plaques was significantly greater than it was in white plaques (2.7 ± 0.8 mm Hg–1 vs. 0.7 ± 0.8 mm Hg–1, p < 0.0001), while stiffness ß for white plaques was significantly greater than it was for yellow plaques (34.9 ± 16.3 vs. 8.7 ± 2.7, p < 0.0001). Compensatory enlargement occurred more frequently with yellow plaques than with white plaques (56% vs. 9%, p < 0.01), while paradoxical shrinkage occurred more frequently with white plaques than it did with yellow plaques (64% vs. 4%, p < 0.001).
CONCLUSIONS
Yellow plaques with an increased distensibility and a compensatory enlargement may be mechanically and structurally weak. As a result, mechanical "fatigue," caused by repetitive stretching, may lead to plaque disruption. Plaques with a high distensibility and a compensatory enlargement may be vulnerable.
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Disruption of plaques, which is considered to be the cause of acute coronary syndromes, occurs through the action of extrinsic mechanical stresses, such as shear forces, acute changes in coronary blood pressure or bending and twisting of an artery during each cardiac contraction (6). However, the relationship between mechanical properties, such as coronary artery distensibility, and plaque color is unknown. Intravascular ultrasound (IVUS) allows for the evaluation of plaque morphology, vascular remodeling and vessel wall distensibility (7–10).
Arterial remodeling of the coronary artery was originally described in a necropsy study by Glagov et al. (11) and later confirmed in vivo using IVUS (12). Compensatory enlargement is defined as an increase in local vessel size in response to increased plaque burden (13). Paradoxical shrinkage is defined as the local shrinkage of vessel caliber and has been implicated in the development of native atherosclerosis and restenosis after angioplasty (14,15). Recently, Schoenhagen et al. (8) reported that compensatory enlargement commonly occurs in patients with acute coronary syndromes, whereas paradoxical shrinkage commonly occurs in those with stable angina. However, the relationship between plaque color and the presence of arterial remodeling has not been determined.
We hypothesized that yellow plaques are softer and more distensible and are associated with compensatory enlargement more frequently than white plaques. In this study, we determined whether yellow plaques have high distensibility and are associated with compensatory enlargement using coronary angioscopy and IVUS.
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Methods |
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20 mm, n = 1) and distal lesions of 50% stenosis (n = 2) were also excluded from this study. The cases of disagreement between observers concerning plaque color (n = 2) were also excluded. Finally, five cases with poor angioscopic or IVUS image quality were excluded. Therefore, 38 patients (32 men and 6 women) were included in the study. Clinical data and information about the clinical presentation were collected from patients’ charts. Written informed consent approved by our institutional review board was obtained from all patients. Clinical presentation. Unstable angina was defined using American Heart Association criteria (16). Acute myocardial infarction was defined by the presence of typical chest pain, ST-segment elevation and an increase in the serum creatine kinase-MB isoenzyme activity more than two times the upper limit of normal range. Stable angina was defined as the presence of exertional chest pain (unchanged over the previous two months) or the presence of an abnormal stress test.
Coronary angioscopic procedure. After routine coronary angiography, an additional 100 U/kg heparin was administered, and an 8F guiding catheter was used to engage the coronary artery. Coronary angioscopy was performed with an image catheter (Vecmova, Clinical Supply Co., Gifu, Japan). Before use, the white balance was adjusted for color correction. The light power was adjusted to avoid refraction and to determine the color of the plaque.
IVUS procedure. A 30 MHz, 3.2F monorail IVUS catheter (Ultra Cross, Boston Scientific Scimed, Inc., Boston, Massachusetts) was used. The IVUS catheter was advanced at least 10 mm distal to the culprit lesion over a guide wire. The IVUS catheter was then withdrawn automatically at a rate of 0.5 mm/s using a motorized transducer pullback device. The IVUS catheter was then fixed just proximal to the culprit lesion for measurements of coronary artery distensibility. The culprit lesion was defined as the site with the smallest luminal diameter. The proximal and distal reference sites were chosen 10 mm proximal and distal to the culprit lesion. All IVUS studies were performed immediately after the intracoronary administration of 400 µg nitroglycerin to prevent coronary spasm.
Qualitative angioscopic analysis. Angioscopic images were analyzed independently by two observers. Both observers had no knowledge of the IVUS findings or patient history. The plaques were classified as either yellow or white (Fig. 1A and E). Intra-observer agreement was measured by having an observer repeat the assessment of 20 images (presented in random order) after one week. The inter-observer agreement was measured by comparing the assessment of 20 images by the two observers. Intra- and inter-observer agreements were both 95%. If there was no consensus concerning plaque color, the data were excluded from the study.
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For the measurement of coronary artery distensibility, simultaneous electrocardiogram and intracoronary pressure were recorded on a strip chart at 200 mm/s during IVUS imaging. Electrocardiographic gating was used to determine the largest lumen CSA (lumen CSA in systole) and the smallest lumen CSA (lumen CSA in diastole) (Fig. 2).
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lumen CSA/(lumen CSA in diastole x P) x 103 (mm Hg–1), where lumen CSA is the difference between lumen CSA in systole and lumen CSA in diastole and P is the difference between systolic intracoronary pressure (SBP) and diastolic intracoronary pressure (DBP). We also calculated stiffness ß, which is a pressure-independent vascular stiffness index (17).
Stiffness ß = [ln (SBP/DBP)]/(D/diastolic mean luminal diameter), where D is the difference between the systolic and diastolic mean luminal diameters. The systolic and diastolic mean luminal diameters were calculated from the CSA, assuming that the cross-section was circular [diameter = 2 (EEM CSA/
)1/2]. Distensibility index and stiffness ß were measured at the proximal site near to the culprit lesion. All measurements represent the average values for three cardiac cycles.
IVUS definitions of remodeling. The remodeling ratio (RR) was defined as follows: RR = EEM CSA at the culprit lesion/mean reference EEM CSA, where mean reference EEM CSA is the average of the proximal and the distal reference site EEM CSA. Three remodeling categories were defined as follows: compensatory enlargement, an RR > 1.10; no remodeling, an RR between 0.90 and 1.10; paradoxical shrinkage, an RR < 0.90 (Fig. 1., B to D and F to H).
Qualitative IVUS analysis. The observer classified plaque morphology visually according to commonly used criteria (18). Echo-lucent plaques were defined as lesions with an echo density less than that of the adventitia for >75% of the plaque area. Echo-dense plaques were defined as lesions with an echo density equivalent to or greater than the adventitia for >75% of the plaque area without acoustic shadowing. All other lesions were defined as mixed plaques.
Statistical analysis. Data are presented as the mean ± SD. If the data were normally distributed, an unpaired Student t test was used to compare two groups. Otherwise, a Mann-Whitney U test was used. Categorical variables were analyzed using Fisher exact probability test. The correlation between two parameters was evaluated by linear regression analysis. Analysis of residual variance was used to detect significant differences in a dependent variable (e.g., stiffness ß) between the categories of a factor (e.g., plaque color). Statistical analyses were performed using SAS for Windows, version 6.12. A value of p < 0.05 was considered statistically significant.
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Results |
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lumen CSA and distensibility index were greater in the yellow plaques than they were in the white plaques (p < 0.0001). Stiffness ß was greater in the white plaques than it was in the yellow plaques (p < 0.0001). However, the percentage of plaque area was not different between the two groups (Table 2). Figure 3 shows the relationship between percentage of plaque area and stiffness ß in the yellow plaques and white plaques. In both groups, the percentage of plaque area correlated with stiffness ß. Stiffness ß in the white plaques was significantly greater than it was in the yellow plaques (p < 0.005). The frequency of compensatory enlargement was significantly higher in the yellow plaque group than it was in the white plaque group (p < 0.01). In contrast, the frequency of paradoxical shrinkage was significantly higher in the white plaques than it was in the yellow plaques (p < 0.001). Moreover, the mean RR was significantly greater in the yellow plaques than it was in the white plaques (p < 0.0001, Table 2).
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Discussion |
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Plaque disruption is triggered by intrinsic plaque changes, such as increased pressure in atheroma and thinning of fibrous caps caused by inflammatory cells and by extrinsic stress. Prior investigators have suggested that several kinds of stress, including shear stress (6) and circumferential and localized wall stress (19,21), play an important role in plaque disruption. However, the mechanical responses of vulnerable plaques to local stress are unknown. Coronary artery distensibility is one parameter of vessel wall stiffness (9). Previous investigators have reported that a few factors, including thickness of the intima–media complex (9) and plaque distribution (10), influence distensibility. However, the difference in the distensibility of vulnerable and stable plaques is not known.
The difference of distensibility. The distensibility index of yellow plaques was higher than it was in white plaques. There are two possible reasons for this difference. One of the reasons may be that the composition of the plaque differs between yellow and white plaques. The yellow plaques are closely related to atheromas, which have thin fibrous caps and large lipid pools (3), while the white plaques have thick fibrous caps or are entirely fibrous by histomorphologic analysis (3,4). The results of this study demonstrate that coronary artery distensibility may be regulated by plaque composition. Another reason may be that vascular remodeling also influences distensibility. Paradoxical shrinkage results from adventitial fibrosis, which compresses the vessel (22). As a result, the white plaques mainly cause paradoxical shrinkage and may be less distensible and stiffer than the yellow plaques that present with compensatory enlargement.
Regional distensibility may play an important role in plaque disruption. The border of normal intima and plaque, the so-called "shoulder lesion" must be exposed to mechanical stress caused by difference of regional distensibility. However, circumferential plaques were frequently observed, and the border of the normal intima and the plaque was not clear at the distensibility measurement site. Therefore, whole vessel distensibility was calculated in this study.
The difference of plaque morphology. A recent IVUS study demonstrated that echo-lucent plaques are more frequently found in the setting of acute coronary syndromes than in the setting of stable angina (8). In our study, there was no difference in the frequency of echo-lucent plaques between yellow and white plaques. However, yellow plaques detected in patients with acute coronary syndromes are frequently accompanied by thrombus, and these thrombi are seen as echo-dense masses by IVUS (23). Therefore, the frequency of echo-lucent plaques in the yellow plaques may be underestimated.
The difference of remodeling. Collagen is one of the components of plaque, and its content regulates plaque growth and mechanical stability (24,25). Recent studies have shown that inadequate collagen content leads to plaque weakness and vulnerability, whereas excessive collagen accumulation leads to arterial stenosis (24–26). Moreover, collagen density correlates with the type of arterial remodeling. Specifically, the collagen density is higher in arteries with constrictive remodeling than it is in those with dilatory remodeling (26). Other histologic studies have shown that the numbers of collagen fibers in yellow plaques is much lower than they are in white plaques (3). These findings help to explain the fact that compensatory enlargement was more common with yellow plaques, while paradoxical shrinkage was more common in white plaques. Theoretically, the yellow plaques have greater plaque area than the white plaques. However, there was no difference in plaque area between the yellow and the white plaques. The reason for this result may be explained by the fact that there were a small number of patients in this study.
A prospective IVUS study showed that large plaque area contributed to the acute coronary events (27). The results of this study showed that the increasing of the percentage of plaque area correlated with decreasing distensibility of the plaque. Percentage of plaque area does not always correlate with absolute plaque area. The plaques having large absolute area and a low percentage of plaque area may be most vulnerable. Vulnerable plaque may be observed as mild to moderate stenosis by angiogram and as compensatory enlargement by IVUS.
Yellow plaques with increased distensibility and inadequate collagen content may be mechanically and structurally weak. As a result, mechanical "fatigue," caused by repetitive stretching, may lead to plaque disruption. We concluded that plaques with high distensibility and compensatory enlargement may be vulnerable to rupture.
Study limitations. Coronary artery distensibility is influenced by intracoronary pressure. In this study, measurements of intracoronary pressure were obtained through the tip of the guiding catheter, which may not accurately reflect intracoronary pressure. Therefore, in this study, the lesions that have >50% stenosis in proximal sites were excluded because the intracoronary pressure in this type of lesion differs significantly from the pressure at the tip of the guiding catheter. The pressure measurements at sites of severe stenosis and at sites distal to severe stenosis were not exact, because the coronary lumen was narrowed or obstructed by the IVUS catheter. Therefore, measurements of distensibility were performed at sites proximal to the culprit lesion. However, stiffness ß is a pressure-independent index of vascular stiffness. Based on both pressure-dependent and pressure-independent measurements, yellow plaques were more distensible than white plaques. The detection of differences between thrombus and plaque by IVUS is often difficult because thrombus appears similar to intima or plaque (23). Therefore, the plaque CSA and percentage of plaque area at the culprit lesion, especially in patients with acute coronary syndromes, may be overestimated. Moreover, the presence of thrombus may influence the distensibility and the morphologic classification of the plaque.
Conclusions. This study demonstrated that yellow plaques were associated with the presence of compensatory enlargement, while white plaques were associated with paradoxical shrinkage. Furthermore, yellow plaques are more distensible than white plaques. We concluded that yellow plaques with a high distensibility and a compensatory enlargement are more vulnerable to disruption.
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