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Clustering of several cardiovascular risk factors affects tissue characteristics of the carotid artery
Atherosclerosis 198 (2008) 208–213
Clustering of several cardiovascular risk factors affects tissue characteristics of the carotid artery Naoto Katakami a,b,∗ , Hideaki Kaneto a , Munehide Matsuhisa a , Yutaka Umayahara b , Keisuke Kosugi b , Yoshimitsu Yamasaki a a
Department of Internal Medicine and Therapeutics, Osaka University Graduate School of Medicine, Japan b Department of Internal Medicine, Osaka Police Hospital, Japan Received 1 June 2007; received in revised form 13 August 2007; accepted 16 August 2007 Available online 23 October 2007
Abstract Integrated backscatter (IBS) signal obtained by extracranial ultrasound examination of the carotid artery can be used to evaluate the tissue characteristics of arterial plaque. To examine whether clustering of several cardiovascular risk factors, metabolic syndrome (MetS), affects the tissue characteristics of the carotid artery, we measured Calibrated-IBS values of the intima–media complex in the carotid artery for 142 patients (61.6 ± 9.5 years old) with at least one component of MetS. Calibrated-IBS values were inversely correlated with the number of MetS components (r = −0.224, p = 0.0073). Furthermore, the number of MetS components was an independent risk factor for a low Calibrated-IBS. The subjects with coronary heart disease (CHD) (n = 20) showed significantly lower Calibrated-IBS than those without it (n = 122) (−17.8 ± 5.1 vs. −21.7 ± 5.5 dB, p = 0.0020), and the logistic regression analysis showed that the presence of CHD was associated with Calibrated-IBS (RR = 0.898, 95%CI: 0.811–0.994, p = 0.0374). These results suggest that clustering of the cardiovascular risk factors affects the Calibrated-IBS, an acoustic parameter of tissue characteristics of the intima–media complex in the carotid artery. © 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Ultrasonography; Metabolic syndrome; Integrated backscatter; Intima–media thickness
1. Introduction Metabolic syndrome (MetS) is a condition in which several cardiovascular risk factors including impaired glucose tolerance, hypertension, dyslipidemia, and obesity accumulate in an individual and synergistically accelerate the progression of atherosclerosis. Although the definitions of MetS given by WHO, the National Cholesterol Education Program (NCEP) Adult Treatment Panel III, and international diabetes federation (IDF) have slightly different tests and/or cut-offs [1–3], there is no doubt that this syndrome markedly increases the risk of cardiovascular diseases [4–6]. ∗ Corresponding author at: Department of Internal Medicine and Therapeutics (A8), Osaka University Graduate School of Medicine, 2-2, Yamadaoka, Suita, Osaka 565-0871, Japan. Tel.: +81 6 6879 3633; fax: +81 6 6879 3639. E-mail address: [email protected] (N. Katakami).
0021-9150/$ – see front matter © 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.atherosclerosis.2007.08.022
Disruption of an atherosclerotic plaque has been shown to play a crucial role in the pathogenesis of cardiovascular events [7]. As the plaque disruption is dependent on the content of lipid in the atheroma and the thickness of the fibrous cap [7,8], tissue characterization of a plaque may be useful for determining its fragility. It has not been fully evaluated, however, whether MetS affects the tissue characteristics of the carotid artery. Recently, several studies have shown that integrated backscatter (IBS) signal obtained by extracranial ultrasound examination of the carotid artery can be used to noninvasively distinguish among the tissue characteristics of arterial plaque [9–11]. Furthermore, our recent study revealed that Calibrated-IBS values in the carotid arteries were significantly lower in type 2 diabetic patients with a recent history of cardiovascular events and were correlated with serum HDL-cholesterol and triglyceride levels compared to those without it [12]. It remains to be evaluated, however,
N. Katakami et al. / Atherosclerosis 198 (2008) 208–213
whether clustering of several cardiovascular risk factors is related to the tissue characteristics of the carotid artery. The aim of this study was to examine a possible association between clustering of several cardiovascular risk factors and the tissue characteristics of the carotid artery evaluated with the IBS analysis.
2. Materials and methods 2.1. Subjects Study subjects were selected from outpatients at Osaka Police Hospital who fulfilled the following criteria: (I) more than 30 years old, (II) without malignancy, connective tissue disease, and severe liver or renal dysfunction, and (III) with at least one following four conditions. (1) Obesity defined as BMI 25.0 kg/m2 . (2) Elevated blood pressure defined as systolic blood pressure (SBP) 130 mmHg or diastolic blood pressure (DBP) 85 mmHg or having been treated for hypertension. (3) Dyslipidemia defined as serum total cholesterol 220 mg/dl or serum triglyceride (TG) 150 mg/dl or HDL-cholesterol <40 mg/dl or having been treated for dyslipidemia. (4) Elevated plasma glucose defined as fasting glucose 110 mg/dl or having been diagnosed as diabetes mellitus (DM). The hospital ethics committee approved the study protocol, and informed consent was obtained from each patient after a full explanation of the study. After all, a total of 142 patients (91 men and 51 women, aged 61.6 ± 9.5 years (mean ± S.D.) were enrolled. 2.2. Assessment of cardiovascular and metabolic risk factors The laboratory data and blood pressure measurements for the prior 6 months were collected and averaged. Blood pressure was measured at rest with a mercury sphygmomanometer. Fasting blood was withdrawn for analyses of serum total cholesterol, serum HDL-cholesterol, serum triglycerides, plasma glucose and HbA1c levels by standard laboratory techniques. A structured questionnaire was used to determine medical history, current medication use and smoking status. 2.3. Integrated backscatter (IBS) signal analysis IBS analysis was performed with a software package “Acoustic Densitometry” with the Hewlett-Packard SONOS 5500. In this system, the return echoes that impinge on the individual elements of the transducer are amplified, mixed to the intermediate frequency signal, and sent to either a standard ultrasound video processing chain for the B-mode image or a special IBS processor. In the IBS image, the gray level is displayed in proportion to the integrated backscattered power, which is calibrated in dB and has a dynamic range of 64 dB. Thus, IBS is calculated as the average power of the ultra-
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sonic backscattered signal from a region of interest (ROI) and represents its tissue structure [10,11]. A series of ultrasonographic scanning of the carotid artery was performed with an electrical linear transducer (midfrequency of 7.5 MHz). The detection limit of this system was about 0.1 mm. Initially, the conventional B-mode imaging of the extracranial common carotid artery, the carotid bulb, and the internal carotid artery in the neck was performed bilaterally, and the carotid intima–media thickness (IMT) was measured, as reported in previous studies [13]. The site of the greatest IMT including the plaque lesion was sought along the arterial wall and measured for each projection. We defined the greatest value among all projections as Max-IMT. IBS data in the intima–media complex and those in the adventitia were sampled at a location close to where the MaxIMT was measured. We placed the rectangular-shaped ROIs (1.0 mm × 0.1 mm) in a line from the leading edge of the first echogenic line to the leading edge of the second echogenic line of the thickest site and measured the IBS values of these ROIs. We defined the average of these IBS values as the IBS value in intima–media complex (Fig. 1A). Similarly, we measured the IBS value in the adventitia. According to a previous report, the relative IBS values of the adventitia of pathologically different samples were almost the same, and thus atherosclerotic change has been considered to occur mainly in the intima–media complex of the carotid artery wall [10]. Therefore, the IBS values in the intima–media complex were calibrated by subtracting the IBS values in the adventitia as follows: Calibrated-IBS = IBS values in intima–media complex − IBS values in the adventitia In this way, the calibrated-IBS has been considered to be a parameter that likely reflects the tissue characteristics of the intima–media complex. Fig. 1B shows backscatter imaging of normal (left panel) and increased IMT (right panel). Two observers who were unaware of the clinical characteristics of the subjects conducted all scans. The intraobserver and interobserver variabilities of the IBS values (±S.E.) were 3.2 ± 0.4% and 3.5 ± 0.4%, respectively. 2.4. Statistical analysis Data are given as mean ± S.D. Data between two groups were compared by the two-tailed unpaired Student’s t test and data among more than three groups by the one-way ANOVA followed by Scheffe’s test. Single linear univariate correlations and forward and backward stepwise multivariate regression analyses were performed to evaluate the relationship between Calibrated-IBS and the following variables: gender (female = 0, male = 1), age, BMI, SBP, DBP, total cholesterol, HDL-cholesterol, TG, fasting plasma glucose, smoking status (no = 0, yes = 1) and number of MetS components. Variables were considered for the multivariate models when their univariable p-value was less than 0.05. For the
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N. Katakami et al. / Atherosclerosis 198 (2008) 208–213
Fig. 1. (A) Scheme of sampling the region of interest (ROI). IBS data in the intima–media complex (IMC) and the adventitia were sampled at a location close to where the Max-IMT was measured. We placed the setup rectangular-shaped ROIs (1.0 mm × 0.1 mm) one after another, in a line from the leading edge of the first echogenic line to the leading edge of the second echogenic line, and measured the IBS values of these ROIs. After that, we calculated the average of these IBS values as the representative IBS value in intima–media complex. (B) Backscatter imaging of normal (left) and increased IMT (right).
forward and backward stepwise multivariate regression analyses, the F value for the inclusion and exclusion of variables was set at 2.0. Multiple logistic regression method was used to select variables significantly associated with an increase in the risk of coronary heart disease. Variables were considered for the multivariable models when their univariable p-value was less than 0.05. These statistical analyses were performed using Stat-View statistical software (Version 5.0 for Windows; Abacus Concepts, Berkeley, CA) on a personal computer. The threshold of statistical significance was defined as p < 0.05.
demia; 97 (68%) subjects with raised fasting plasma glucose or DM. The study group was divided into four subgroups: (1) subjects with one component of the MetS (n = 18, 13%); (2) subjects with two components (n = 53, 37%); (3) subjects with three components (n = 51, 36%); (4) subjects with four components (n = 20, 14%). One hundred and twenty-two subjects had no history of coronary heart disease. The remaining 20 subjects had cardiovascular diseases; 11 chronic stable angina cases and 9 acute coronary syndrome cases. These diagnoses were made by skilled cardiologists according to established guidelines.
3. Results
3.2. Clustering of cardiovascular risk factors and acoustic parameters
3.1. Characteristics of the study subjects Patients’ characteristics are shown in Table 1. There were 43 (30%) subjects with obesity; 103 (73%) subjects with raised SBP and/or DBP; 114 (80%) subjects with dyslipi-
Max-IMT in the subjects with one MetS component, two MetS components, three MetS components and four MetS components was 1.10 ± 0.28 mm, 1.06 ± 0.25 mm, 1.25 ± 0.52 mm, and 1.43 ± 0.80 mm, respectively. There
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Table 1 Patient characteristics (overall) Parameters
Total (n = 142)
Men (n = 91)
Women (n = 51)
Age (years old) Body mass index (kg/m2 ) Systolic blood pressure (mmHg) Diastolic blood pressure (mmHg) Total-cholesterol (mg/dl) HDL-cholesterol (mg/dl) Triglyceride (mg/dl) Fasting plasma glucose (mg/dl) Smoking status (pack-years) Obesity (yes/no) Elevated blood pressure (yes/no) Dyslipidemia (yes/no) Elevated plasma glucose (yes/no) Number of MetS components (1/2/3/4) Coronary heart disease (yes/no)
62 ± 10 23.5 ± 2.8 134 ± 14 75 ± 9 212 ± 36 54 ± 15 155 ± 118 133 ± 48 16 ± 27 43/99 103/39 114/28 97/45 18/53/51/20 20/122
61 ± 10 24.0 ± 2.8 133 ± 14 75 ± 10 207 ± 33 52 ± 14 162 ± 96 134 ± 44 22 ± 30 33/58 66/25 75/16 67/24 9/30/36/16 18/73
62 ± 9 22.6 ± 2.5 135 ± 12 75 ± 8 221 ± 40 59 ± 15 141 ± 148 133 ± 55 3 ± 12 10/41 37/14 39/12 30/21 9/23/15/4 2/49
Data are shown as mean ± S.D. or number.
was a graded relationship between the number of MetS components and Max-IMT (p for trend = 0.0132). Pearson’s linear regression analysis revealed that Max-IMT was correlated with the number of MetS components (r = 0.245, p = 0.0031). Also, a stepwise multivariate regression analysis showed that the number of MetS was an independent risk factor for MaxIMT (F = 8.3). Calibrated-IBS in the subjects with one MetS component, two MetS components, three MetS components and four MetS components was −15.3 ± 4.2 dB, −18.4 ± 5.0 dB, −18.8 ± 5.8 dB, and −20.2 ± 4.7 dB, respectively. There was a graded relationship between the number of MetS components and Calibrated-IBS (p for trend = 0.0308). Calibrated-IBS in the subjects with four MetS components was significantly lower than that in the subjects with one MetS component (p = 0.0422) (Fig. 2). Furthermore, Pearson’s lin-
Fig. 2. Calibrated-IBS in the subjects with 1–4 MetS components. Data are given as mean ± S.D. Data between the groups were compared by the one-way ANOVA followed by Scheffe’s test. The results showed a graded relationship between the number of MetS components and Calibrated-IBS (p for trend = 0.0308). Calibrated-IBS in the subjects with four MetS components was significantly lower than that in the subjects with one MetS component (p = 0.0422).
ear regression analysis revealed that Calibrated-IBS values were inversely correlated with the number of MetS components (r = −0.224, p = 0.0073). In addition, Calibrated-IBS values were positively correlated with serum HDL-C levels (r = 0.286, p = 0.0006), but there was no statistically significant association between Calibrated-IBS and the other variables. There was also no significant correlation between Max-IMT and Calibrated-IBS (r = −0.042, p = 0.6203). To examine whether the number of MetS components is a determinant of the low Calibrated-IBS values independent of conventional risk factors, we performed a stepwise multivariate regression analysis with HDL-C levels and the number of MetS components as independent variables and calibratedIBS as objective variables. This analysis showed that the number of MetS was an independent risk factor for a low calibrated-IBS (F = 3.4), suggesting that clustering of cardiovascular risk factors is closely associated with the tissue characteristics of the carotid artery independently of conventional risk factors. Gender separated analysis was also performed. In the male subjects with one MetS component (n = 9), two MetS components (n = 30), three MetS components (n = 36) and four MetS components (n = 16), Max-IMT was 1.13 ± 0.28 mm, 1.01 ± 0.19 mm, 1.25 ± 0.58 mm, and 1.48 ± 0.89 mm, respectively. In the female subjects with one MetS component (n = 9), two MetS components (n = 23), three MetS components (n = 15) and four MetS components (n = 4), Max-IMT was 1.07 ± 0.30 mm, 1.12 ± 0.31 mm, 1.24 ± 0.33 mm, and 1.22 ± 0.21 mm, respectively. There was a graded relationship between the number of MetS components and Max-IMT (p for trend = 0.0414) and MaxIMT was significantly correlated with the number of MetS components (r = 0.261, p = 0.0123) in the male subjects, whereas such statistically significant relationship was not observed in the female subjects. In the male subjects with one MetS component, two MetS components, three MetS components and four MetS components, Calibrated-IBS
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was −16.4 ± 4.3 dB, −18.9 ± 5.2 dB, −18.9 ± 6.2 dB, and −19.9 ± 4.6 dB, respectively. In the female subjects with one MetS component, two MetS components, three MetS components and four MetS components, Calibrated-IBS was −14.4 ± 4.1 dB, −17.6 ± 4.8 dB, −18.5 ± 5.0 dB, and −21.2 ± 5.6 dB, respectively. There was a trend of graded relationship between the number of MetS components and Calibrated-IBS (p for trend = 0.0732), and Calibrated-IBS value was significantly correlated with the number of MetS components (r = −0.349, p = 0.0116) in the female subjects, whereas such statistically significant relationship was not observed in the male subjects. Analyses on subjects without apparent history of CHD (n = 122) were also performed. Max-IMT in the subjects with one MetS component (n = 15), two MetS components (n = 47), three MetS components (n = 46) and four MetS components (n = 14) was 1.07 ± 0.29 mm, 1.06 ± 0.26 mm, 1.26 ± 0.54 mm, and 1.24 ± 0.38 mm, respectively. There was a trend of graded relationship between the number of MetS components and Max-IMT (p for trend = 0.0805) and Max-IMT value was significantly correlated with the number of MetS components (r = 0.196, p = 0.0348), although there was no statistically significant difference among the groups. Calibrated-IBS in the subjects with one MetS component, two MetS components, three MetS components and four MetS components was −14.5 ± 3.9 dB, −17.9 ± 4.8 dB, −18.8 ± 5.8 dB, and −18.3 ± 3.1 dB, respectively. There was a graded relationship between the number of MetS components and Calibrated-IBS (p for trend = 0.0403) and Calibrated-IBS was significantly correlated with the number of MetS components (r = −0.191, p = 0.0348), although there was no statistically significant difference among the groups. The results were similar to those led by the analyses on subjects including patients with apparent history of CHD. 3.3. Calibrated-IBS and cardiovascular diseases As compared to the subjects without coronary heart disease (n = 122), the subjects with coronary heart disease (n = 20) showed significantly lower Calibrated-IBS (−17.8 ± 5.1 vs. −21.7 ± 5.5 dB, p = 0.0020). Also, the subjects with coronary heart disease showed relatively greater Max-IMT than the subjects without it (1.16 ± 0.41 vs. 1.35 ± 0.79 mm, p = 0.1070). Pearson’s test revealed that the presence of coronary heart disease (“no” = 0 and “yes” = 1) was associated with gender (r = 0.219, p = 0.0088), HDLC (r = −0.270, p = 0.0011) and Calibrated-IBS (r = −0.257, p = 0.0019). Furthermore, the logistic regression analysis demonstrated that the presence of coronary heart disease was associated with gender (RR = 4.76, 95%CI: 1.02–22.28, p = 0.0474), HDL-C (RR = 0.950, 95%CI: 0.908–0.994, p = 0.0273) and Calibrated-IBS (RR = 0.898, 95%CI: 0.811–0.994, p = 0.0374) (Table 2). These results suggest that Calibrated-IBS values are associated with the development of coronary heart disease.
Table 2 Multivariate logistic regression analysis to identify independent determinant(s) for coronary heart disease Parameters
Odds ratio (95%CI)
p-Value
Gender HDL-cholesterol Calibrated-IBS
4.76 (1.02–22.28) 0.95 (0.91–0.99) 0.90 (0.81–0.99)
0.0474 0.0273 0.0374
Multivariate logistic regression analysis was done for 142 subjects with at least one component of metabolic syndrome to select variables significantly associated with an increase in the risk of coronary heart disease. Variables were considered for the multivariable models when their univariable p-value was less than 0.05. Abbreviation: CI, confidential interval.
4. Discussion It is well known that clustering of several cardiovascular risk factors, so called “metabolic syndrome”, markedly increases the risk of cardiovascular diseases [4–6]. Although disruption of an atherosclerotic plaque has been shown to play a crucial role in the pathogenesis of cardiovascular events [7], it has not been fully evaluated whether MetS affects the tissue characteristics of the arterial wall. In the present study, to assess the tissue characteristics of the carotid arterial wall, we performed the IBS analysis, one of the clinically useful methods for evaluating the acoustic characteristics of tissue structure. Interestingly, previous studies indicated that a low Calibrated-IBS value in the carotid artery corresponded to lipid-rich plaque or intraplaque hemorrhage [9–11,14]. Furthermore, it has been reported that Calibrated-IBS values in the carotid artery were significantly correlated with serum HDL-cholesterol and triglyceride levels in type 2 diabetic patients [12]. It remains to be evaluated, however, whether clustering of several cardiovascular risk factors is related to the tissue characteristics of the carotid artery. The present study has shown that the Calibrated-IBS value in the intima–media complex of the carotid was significantly lower in subjects with MetS. Furthermore, a stepwise multivariate regression analysis revealed that the number of the MetS components is an independent risk factor for the low Calibrated-IBS. Although we cannot infer the causality from the cross-sectional survey, a low Calibrated-IBS value observed in subjects with MetS may reflect the lipid accumulation in the arterial wall that would be accelerated by clustering of the cardiovascular risk factors. Thus, the clustering of the cardiovascular risk factors would affect the tissue characteristics and the size of an arterial plaque both of which could be involved in plaque’s vulnerability leading to cardiovascular events. Gender separated analysis revealed that relationship between the number of MetS components and Calibrated-IBS was observed in the female subjects but not in male subjects. One possible explanation for this discrepancy is that the sample size was too small to show a statistical significance when gender separated analyses were performed. Another possible explanation is that this discrepancy is truly derived from gender difference, although its mechanism remains unclear.
N. Katakami et al. / Atherosclerosis 198 (2008) 208–213
Further study with a larger scale would be necessary to clarify this point. The present study also demonstrated that the subjects with coronary heart disease showed significantly lower Calibrated-IBS (−21.7 ± 5.5 vs. −17.8 ± 5.1 dB, p = 0.0020) and that a low Calibrated-IBS value was independently associated with the coronary heart disease. These results suggest that the lower Calibrated-IBS value in the carotid may reflect the increase of the risk of the coronary heart disease. However, since the number of the subjects is quite small (especially in CHD positive group), future prospective longitudinal studies using larger populations would be necessary to conclude that the Calibrated-IBS can be used to predict future cardiovascular events. In conclusion, clustering of the cardiovascular risk factors affects the Calibrated-IBS value, an acoustic parameter of tissue characteristics of the intima–media complex in the carotid artery.
References [1] Alberti KG, Zimmet PZ. Definition, diagnosis and classification of diabetes mellitus and its complications. Part 1. Diagnosis and classification of diabetes mellitus provisional report of a WHO consultation. Diabetes Med 1998;15:539–53. [2] Executive Summary of the Third Report of The National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III). JAMA 2001;285:2486–2497. [3] Alberti KG, Zimmet PZ, Shaw J, IDF Epidemiology Task Force Consensus Group. The metabolic syndrome—a new worldwide definition. Lancet 2005;366:1059–62.
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[4] Lakka HM, Laaksonen DE, Lakka TA, et al. The metabolic syndrome and total and cardiovascular disease mortality in middle-aged men. JAMA 2002;288:2709–16. [5] Isomaa B, Almgren P, Tuomi T, et al. Cardiovascular morbidity and mortality associated with the metabolic syndrome. Diabetes Care 2001;24:683–9. [6] McNeill AM, Rosamond WD, Girman CJ, et al. Prevalence of coronary heart disease and carotid arterial thickening in patients with the metabolic syndrome (The ARIC Study). Am J Cardiol 2004;94:1249–54. [7] Fuster V, Badimon L, Badimon JJ, Chesebro JH. The pathogenesis of coronary artery disease and the acute coronary syndromes. N Engl J Med 1992;326:242–50. [8] Lee RT, Grodzinsky AJ, Frank EH, Kamm RD, Schoen FJ. Structuredependent dynamic mechanical behavior of fibrous caps from human atherosclerotic plaques. Circulation 1991;83:1764–70. [9] Urbani MP, Picano E, Parenti G, et al. In vivo radiofrequency-based ultrasonic tissue characterization of the atherosclerotic plaque. Stroke 1993;24:1507–12. [10] Takiuchi S, Rakugi H, Honda K, et al. Quantitative ultrasonic tissue characterization can identify high-risk atherosclerotic alteration in human carotid arteries. Circulation 2000;102:766–70. [11] Kawasaki M, Takatsu H, Noda T, et al. Noninvasive quantitative tissue characterization and two-dimensional color-coded map of human atherosclerotic lesions using ultrasound integrated backscatter: comparison between histology and integrated backscatter images. J Am Coll Cardiol 2001;38:486–92. [12] Katakami N, Yamasaki Y, Kosugi K, et al. Tissue characterization identifies subjects with high risk of cardiovascular diseases. Diabetes Res Clin Pract 2004;63:93–102. [13] Yamasaki Y, Kawamori R, Matsushima H, et al. Atherosclerosis in carotid artery of young IDDM patients monitored by ultrasound highresolution B-mode imaging. Diabetes 1994;43:634–9. [14] Waki H, Masuyama T, Mori H, et al. Ultrasonic tissue characterization of the atherosclerotic carotid artery: histological correlates or carotid integrated backscatter. Circ J 2003;67:1013–6.
Clustering of several cardiovascular risk factors affects tissue characteristics of the carotid artery Naoto Katakami a,b,∗ , Hideaki Kaneto a , Munehide Matsuhisa a , Yutaka Umayahara b , Keisuke Kosugi b , Yoshimitsu Yamasaki a a
Department of Internal Medicine and Therapeutics, Osaka University Graduate School of Medicine, Japan b Department of Internal Medicine, Osaka Police Hospital, Japan Received 1 June 2007; received in revised form 13 August 2007; accepted 16 August 2007 Available online 23 October 2007
Abstract Integrated backscatter (IBS) signal obtained by extracranial ultrasound examination of the carotid artery can be used to evaluate the tissue characteristics of arterial plaque. To examine whether clustering of several cardiovascular risk factors, metabolic syndrome (MetS), affects the tissue characteristics of the carotid artery, we measured Calibrated-IBS values of the intima–media complex in the carotid artery for 142 patients (61.6 ± 9.5 years old) with at least one component of MetS. Calibrated-IBS values were inversely correlated with the number of MetS components (r = −0.224, p = 0.0073). Furthermore, the number of MetS components was an independent risk factor for a low Calibrated-IBS. The subjects with coronary heart disease (CHD) (n = 20) showed significantly lower Calibrated-IBS than those without it (n = 122) (−17.8 ± 5.1 vs. −21.7 ± 5.5 dB, p = 0.0020), and the logistic regression analysis showed that the presence of CHD was associated with Calibrated-IBS (RR = 0.898, 95%CI: 0.811–0.994, p = 0.0374). These results suggest that clustering of the cardiovascular risk factors affects the Calibrated-IBS, an acoustic parameter of tissue characteristics of the intima–media complex in the carotid artery. © 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Ultrasonography; Metabolic syndrome; Integrated backscatter; Intima–media thickness
1. Introduction Metabolic syndrome (MetS) is a condition in which several cardiovascular risk factors including impaired glucose tolerance, hypertension, dyslipidemia, and obesity accumulate in an individual and synergistically accelerate the progression of atherosclerosis. Although the definitions of MetS given by WHO, the National Cholesterol Education Program (NCEP) Adult Treatment Panel III, and international diabetes federation (IDF) have slightly different tests and/or cut-offs [1–3], there is no doubt that this syndrome markedly increases the risk of cardiovascular diseases [4–6]. ∗ Corresponding author at: Department of Internal Medicine and Therapeutics (A8), Osaka University Graduate School of Medicine, 2-2, Yamadaoka, Suita, Osaka 565-0871, Japan. Tel.: +81 6 6879 3633; fax: +81 6 6879 3639. E-mail address: [email protected] (N. Katakami).
0021-9150/$ – see front matter © 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.atherosclerosis.2007.08.022
Disruption of an atherosclerotic plaque has been shown to play a crucial role in the pathogenesis of cardiovascular events [7]. As the plaque disruption is dependent on the content of lipid in the atheroma and the thickness of the fibrous cap [7,8], tissue characterization of a plaque may be useful for determining its fragility. It has not been fully evaluated, however, whether MetS affects the tissue characteristics of the carotid artery. Recently, several studies have shown that integrated backscatter (IBS) signal obtained by extracranial ultrasound examination of the carotid artery can be used to noninvasively distinguish among the tissue characteristics of arterial plaque [9–11]. Furthermore, our recent study revealed that Calibrated-IBS values in the carotid arteries were significantly lower in type 2 diabetic patients with a recent history of cardiovascular events and were correlated with serum HDL-cholesterol and triglyceride levels compared to those without it [12]. It remains to be evaluated, however,
N. Katakami et al. / Atherosclerosis 198 (2008) 208–213
whether clustering of several cardiovascular risk factors is related to the tissue characteristics of the carotid artery. The aim of this study was to examine a possible association between clustering of several cardiovascular risk factors and the tissue characteristics of the carotid artery evaluated with the IBS analysis.
2. Materials and methods 2.1. Subjects Study subjects were selected from outpatients at Osaka Police Hospital who fulfilled the following criteria: (I) more than 30 years old, (II) without malignancy, connective tissue disease, and severe liver or renal dysfunction, and (III) with at least one following four conditions. (1) Obesity defined as BMI 25.0 kg/m2 . (2) Elevated blood pressure defined as systolic blood pressure (SBP) 130 mmHg or diastolic blood pressure (DBP) 85 mmHg or having been treated for hypertension. (3) Dyslipidemia defined as serum total cholesterol 220 mg/dl or serum triglyceride (TG) 150 mg/dl or HDL-cholesterol <40 mg/dl or having been treated for dyslipidemia. (4) Elevated plasma glucose defined as fasting glucose 110 mg/dl or having been diagnosed as diabetes mellitus (DM). The hospital ethics committee approved the study protocol, and informed consent was obtained from each patient after a full explanation of the study. After all, a total of 142 patients (91 men and 51 women, aged 61.6 ± 9.5 years (mean ± S.D.) were enrolled. 2.2. Assessment of cardiovascular and metabolic risk factors The laboratory data and blood pressure measurements for the prior 6 months were collected and averaged. Blood pressure was measured at rest with a mercury sphygmomanometer. Fasting blood was withdrawn for analyses of serum total cholesterol, serum HDL-cholesterol, serum triglycerides, plasma glucose and HbA1c levels by standard laboratory techniques. A structured questionnaire was used to determine medical history, current medication use and smoking status. 2.3. Integrated backscatter (IBS) signal analysis IBS analysis was performed with a software package “Acoustic Densitometry” with the Hewlett-Packard SONOS 5500. In this system, the return echoes that impinge on the individual elements of the transducer are amplified, mixed to the intermediate frequency signal, and sent to either a standard ultrasound video processing chain for the B-mode image or a special IBS processor. In the IBS image, the gray level is displayed in proportion to the integrated backscattered power, which is calibrated in dB and has a dynamic range of 64 dB. Thus, IBS is calculated as the average power of the ultra-
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sonic backscattered signal from a region of interest (ROI) and represents its tissue structure [10,11]. A series of ultrasonographic scanning of the carotid artery was performed with an electrical linear transducer (midfrequency of 7.5 MHz). The detection limit of this system was about 0.1 mm. Initially, the conventional B-mode imaging of the extracranial common carotid artery, the carotid bulb, and the internal carotid artery in the neck was performed bilaterally, and the carotid intima–media thickness (IMT) was measured, as reported in previous studies [13]. The site of the greatest IMT including the plaque lesion was sought along the arterial wall and measured for each projection. We defined the greatest value among all projections as Max-IMT. IBS data in the intima–media complex and those in the adventitia were sampled at a location close to where the MaxIMT was measured. We placed the rectangular-shaped ROIs (1.0 mm × 0.1 mm) in a line from the leading edge of the first echogenic line to the leading edge of the second echogenic line of the thickest site and measured the IBS values of these ROIs. We defined the average of these IBS values as the IBS value in intima–media complex (Fig. 1A). Similarly, we measured the IBS value in the adventitia. According to a previous report, the relative IBS values of the adventitia of pathologically different samples were almost the same, and thus atherosclerotic change has been considered to occur mainly in the intima–media complex of the carotid artery wall [10]. Therefore, the IBS values in the intima–media complex were calibrated by subtracting the IBS values in the adventitia as follows: Calibrated-IBS = IBS values in intima–media complex − IBS values in the adventitia In this way, the calibrated-IBS has been considered to be a parameter that likely reflects the tissue characteristics of the intima–media complex. Fig. 1B shows backscatter imaging of normal (left panel) and increased IMT (right panel). Two observers who were unaware of the clinical characteristics of the subjects conducted all scans. The intraobserver and interobserver variabilities of the IBS values (±S.E.) were 3.2 ± 0.4% and 3.5 ± 0.4%, respectively. 2.4. Statistical analysis Data are given as mean ± S.D. Data between two groups were compared by the two-tailed unpaired Student’s t test and data among more than three groups by the one-way ANOVA followed by Scheffe’s test. Single linear univariate correlations and forward and backward stepwise multivariate regression analyses were performed to evaluate the relationship between Calibrated-IBS and the following variables: gender (female = 0, male = 1), age, BMI, SBP, DBP, total cholesterol, HDL-cholesterol, TG, fasting plasma glucose, smoking status (no = 0, yes = 1) and number of MetS components. Variables were considered for the multivariate models when their univariable p-value was less than 0.05. For the
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Fig. 1. (A) Scheme of sampling the region of interest (ROI). IBS data in the intima–media complex (IMC) and the adventitia were sampled at a location close to where the Max-IMT was measured. We placed the setup rectangular-shaped ROIs (1.0 mm × 0.1 mm) one after another, in a line from the leading edge of the first echogenic line to the leading edge of the second echogenic line, and measured the IBS values of these ROIs. After that, we calculated the average of these IBS values as the representative IBS value in intima–media complex. (B) Backscatter imaging of normal (left) and increased IMT (right).
forward and backward stepwise multivariate regression analyses, the F value for the inclusion and exclusion of variables was set at 2.0. Multiple logistic regression method was used to select variables significantly associated with an increase in the risk of coronary heart disease. Variables were considered for the multivariable models when their univariable p-value was less than 0.05. These statistical analyses were performed using Stat-View statistical software (Version 5.0 for Windows; Abacus Concepts, Berkeley, CA) on a personal computer. The threshold of statistical significance was defined as p < 0.05.
demia; 97 (68%) subjects with raised fasting plasma glucose or DM. The study group was divided into four subgroups: (1) subjects with one component of the MetS (n = 18, 13%); (2) subjects with two components (n = 53, 37%); (3) subjects with three components (n = 51, 36%); (4) subjects with four components (n = 20, 14%). One hundred and twenty-two subjects had no history of coronary heart disease. The remaining 20 subjects had cardiovascular diseases; 11 chronic stable angina cases and 9 acute coronary syndrome cases. These diagnoses were made by skilled cardiologists according to established guidelines.
3. Results
3.2. Clustering of cardiovascular risk factors and acoustic parameters
3.1. Characteristics of the study subjects Patients’ characteristics are shown in Table 1. There were 43 (30%) subjects with obesity; 103 (73%) subjects with raised SBP and/or DBP; 114 (80%) subjects with dyslipi-
Max-IMT in the subjects with one MetS component, two MetS components, three MetS components and four MetS components was 1.10 ± 0.28 mm, 1.06 ± 0.25 mm, 1.25 ± 0.52 mm, and 1.43 ± 0.80 mm, respectively. There
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Table 1 Patient characteristics (overall) Parameters
Total (n = 142)
Men (n = 91)
Women (n = 51)
Age (years old) Body mass index (kg/m2 ) Systolic blood pressure (mmHg) Diastolic blood pressure (mmHg) Total-cholesterol (mg/dl) HDL-cholesterol (mg/dl) Triglyceride (mg/dl) Fasting plasma glucose (mg/dl) Smoking status (pack-years) Obesity (yes/no) Elevated blood pressure (yes/no) Dyslipidemia (yes/no) Elevated plasma glucose (yes/no) Number of MetS components (1/2/3/4) Coronary heart disease (yes/no)
62 ± 10 23.5 ± 2.8 134 ± 14 75 ± 9 212 ± 36 54 ± 15 155 ± 118 133 ± 48 16 ± 27 43/99 103/39 114/28 97/45 18/53/51/20 20/122
61 ± 10 24.0 ± 2.8 133 ± 14 75 ± 10 207 ± 33 52 ± 14 162 ± 96 134 ± 44 22 ± 30 33/58 66/25 75/16 67/24 9/30/36/16 18/73
62 ± 9 22.6 ± 2.5 135 ± 12 75 ± 8 221 ± 40 59 ± 15 141 ± 148 133 ± 55 3 ± 12 10/41 37/14 39/12 30/21 9/23/15/4 2/49
Data are shown as mean ± S.D. or number.
was a graded relationship between the number of MetS components and Max-IMT (p for trend = 0.0132). Pearson’s linear regression analysis revealed that Max-IMT was correlated with the number of MetS components (r = 0.245, p = 0.0031). Also, a stepwise multivariate regression analysis showed that the number of MetS was an independent risk factor for MaxIMT (F = 8.3). Calibrated-IBS in the subjects with one MetS component, two MetS components, three MetS components and four MetS components was −15.3 ± 4.2 dB, −18.4 ± 5.0 dB, −18.8 ± 5.8 dB, and −20.2 ± 4.7 dB, respectively. There was a graded relationship between the number of MetS components and Calibrated-IBS (p for trend = 0.0308). Calibrated-IBS in the subjects with four MetS components was significantly lower than that in the subjects with one MetS component (p = 0.0422) (Fig. 2). Furthermore, Pearson’s lin-
Fig. 2. Calibrated-IBS in the subjects with 1–4 MetS components. Data are given as mean ± S.D. Data between the groups were compared by the one-way ANOVA followed by Scheffe’s test. The results showed a graded relationship between the number of MetS components and Calibrated-IBS (p for trend = 0.0308). Calibrated-IBS in the subjects with four MetS components was significantly lower than that in the subjects with one MetS component (p = 0.0422).
ear regression analysis revealed that Calibrated-IBS values were inversely correlated with the number of MetS components (r = −0.224, p = 0.0073). In addition, Calibrated-IBS values were positively correlated with serum HDL-C levels (r = 0.286, p = 0.0006), but there was no statistically significant association between Calibrated-IBS and the other variables. There was also no significant correlation between Max-IMT and Calibrated-IBS (r = −0.042, p = 0.6203). To examine whether the number of MetS components is a determinant of the low Calibrated-IBS values independent of conventional risk factors, we performed a stepwise multivariate regression analysis with HDL-C levels and the number of MetS components as independent variables and calibratedIBS as objective variables. This analysis showed that the number of MetS was an independent risk factor for a low calibrated-IBS (F = 3.4), suggesting that clustering of cardiovascular risk factors is closely associated with the tissue characteristics of the carotid artery independently of conventional risk factors. Gender separated analysis was also performed. In the male subjects with one MetS component (n = 9), two MetS components (n = 30), three MetS components (n = 36) and four MetS components (n = 16), Max-IMT was 1.13 ± 0.28 mm, 1.01 ± 0.19 mm, 1.25 ± 0.58 mm, and 1.48 ± 0.89 mm, respectively. In the female subjects with one MetS component (n = 9), two MetS components (n = 23), three MetS components (n = 15) and four MetS components (n = 4), Max-IMT was 1.07 ± 0.30 mm, 1.12 ± 0.31 mm, 1.24 ± 0.33 mm, and 1.22 ± 0.21 mm, respectively. There was a graded relationship between the number of MetS components and Max-IMT (p for trend = 0.0414) and MaxIMT was significantly correlated with the number of MetS components (r = 0.261, p = 0.0123) in the male subjects, whereas such statistically significant relationship was not observed in the female subjects. In the male subjects with one MetS component, two MetS components, three MetS components and four MetS components, Calibrated-IBS
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was −16.4 ± 4.3 dB, −18.9 ± 5.2 dB, −18.9 ± 6.2 dB, and −19.9 ± 4.6 dB, respectively. In the female subjects with one MetS component, two MetS components, three MetS components and four MetS components, Calibrated-IBS was −14.4 ± 4.1 dB, −17.6 ± 4.8 dB, −18.5 ± 5.0 dB, and −21.2 ± 5.6 dB, respectively. There was a trend of graded relationship between the number of MetS components and Calibrated-IBS (p for trend = 0.0732), and Calibrated-IBS value was significantly correlated with the number of MetS components (r = −0.349, p = 0.0116) in the female subjects, whereas such statistically significant relationship was not observed in the male subjects. Analyses on subjects without apparent history of CHD (n = 122) were also performed. Max-IMT in the subjects with one MetS component (n = 15), two MetS components (n = 47), three MetS components (n = 46) and four MetS components (n = 14) was 1.07 ± 0.29 mm, 1.06 ± 0.26 mm, 1.26 ± 0.54 mm, and 1.24 ± 0.38 mm, respectively. There was a trend of graded relationship between the number of MetS components and Max-IMT (p for trend = 0.0805) and Max-IMT value was significantly correlated with the number of MetS components (r = 0.196, p = 0.0348), although there was no statistically significant difference among the groups. Calibrated-IBS in the subjects with one MetS component, two MetS components, three MetS components and four MetS components was −14.5 ± 3.9 dB, −17.9 ± 4.8 dB, −18.8 ± 5.8 dB, and −18.3 ± 3.1 dB, respectively. There was a graded relationship between the number of MetS components and Calibrated-IBS (p for trend = 0.0403) and Calibrated-IBS was significantly correlated with the number of MetS components (r = −0.191, p = 0.0348), although there was no statistically significant difference among the groups. The results were similar to those led by the analyses on subjects including patients with apparent history of CHD. 3.3. Calibrated-IBS and cardiovascular diseases As compared to the subjects without coronary heart disease (n = 122), the subjects with coronary heart disease (n = 20) showed significantly lower Calibrated-IBS (−17.8 ± 5.1 vs. −21.7 ± 5.5 dB, p = 0.0020). Also, the subjects with coronary heart disease showed relatively greater Max-IMT than the subjects without it (1.16 ± 0.41 vs. 1.35 ± 0.79 mm, p = 0.1070). Pearson’s test revealed that the presence of coronary heart disease (“no” = 0 and “yes” = 1) was associated with gender (r = 0.219, p = 0.0088), HDLC (r = −0.270, p = 0.0011) and Calibrated-IBS (r = −0.257, p = 0.0019). Furthermore, the logistic regression analysis demonstrated that the presence of coronary heart disease was associated with gender (RR = 4.76, 95%CI: 1.02–22.28, p = 0.0474), HDL-C (RR = 0.950, 95%CI: 0.908–0.994, p = 0.0273) and Calibrated-IBS (RR = 0.898, 95%CI: 0.811–0.994, p = 0.0374) (Table 2). These results suggest that Calibrated-IBS values are associated with the development of coronary heart disease.
Table 2 Multivariate logistic regression analysis to identify independent determinant(s) for coronary heart disease Parameters
Odds ratio (95%CI)
p-Value
Gender HDL-cholesterol Calibrated-IBS
4.76 (1.02–22.28) 0.95 (0.91–0.99) 0.90 (0.81–0.99)
0.0474 0.0273 0.0374
Multivariate logistic regression analysis was done for 142 subjects with at least one component of metabolic syndrome to select variables significantly associated with an increase in the risk of coronary heart disease. Variables were considered for the multivariable models when their univariable p-value was less than 0.05. Abbreviation: CI, confidential interval.
4. Discussion It is well known that clustering of several cardiovascular risk factors, so called “metabolic syndrome”, markedly increases the risk of cardiovascular diseases [4–6]. Although disruption of an atherosclerotic plaque has been shown to play a crucial role in the pathogenesis of cardiovascular events [7], it has not been fully evaluated whether MetS affects the tissue characteristics of the arterial wall. In the present study, to assess the tissue characteristics of the carotid arterial wall, we performed the IBS analysis, one of the clinically useful methods for evaluating the acoustic characteristics of tissue structure. Interestingly, previous studies indicated that a low Calibrated-IBS value in the carotid artery corresponded to lipid-rich plaque or intraplaque hemorrhage [9–11,14]. Furthermore, it has been reported that Calibrated-IBS values in the carotid artery were significantly correlated with serum HDL-cholesterol and triglyceride levels in type 2 diabetic patients [12]. It remains to be evaluated, however, whether clustering of several cardiovascular risk factors is related to the tissue characteristics of the carotid artery. The present study has shown that the Calibrated-IBS value in the intima–media complex of the carotid was significantly lower in subjects with MetS. Furthermore, a stepwise multivariate regression analysis revealed that the number of the MetS components is an independent risk factor for the low Calibrated-IBS. Although we cannot infer the causality from the cross-sectional survey, a low Calibrated-IBS value observed in subjects with MetS may reflect the lipid accumulation in the arterial wall that would be accelerated by clustering of the cardiovascular risk factors. Thus, the clustering of the cardiovascular risk factors would affect the tissue characteristics and the size of an arterial plaque both of which could be involved in plaque’s vulnerability leading to cardiovascular events. Gender separated analysis revealed that relationship between the number of MetS components and Calibrated-IBS was observed in the female subjects but not in male subjects. One possible explanation for this discrepancy is that the sample size was too small to show a statistical significance when gender separated analyses were performed. Another possible explanation is that this discrepancy is truly derived from gender difference, although its mechanism remains unclear.
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Further study with a larger scale would be necessary to clarify this point. The present study also demonstrated that the subjects with coronary heart disease showed significantly lower Calibrated-IBS (−21.7 ± 5.5 vs. −17.8 ± 5.1 dB, p = 0.0020) and that a low Calibrated-IBS value was independently associated with the coronary heart disease. These results suggest that the lower Calibrated-IBS value in the carotid may reflect the increase of the risk of the coronary heart disease. However, since the number of the subjects is quite small (especially in CHD positive group), future prospective longitudinal studies using larger populations would be necessary to conclude that the Calibrated-IBS can be used to predict future cardiovascular events. In conclusion, clustering of the cardiovascular risk factors affects the Calibrated-IBS value, an acoustic parameter of tissue characteristics of the intima–media complex in the carotid artery.
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