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the Division of Clinical Epidemiology (M.W., S.M.H., M.P.S.), Department of Medicine, University of Texas Health Science Center at San Antonio, San Antonio; the Centro de Estudios en Diabetes (C.G.), American British Cowdray Hospital, and Endocrinology and Metabolism Service, Division of Internal Medicine, Specialty Hospital of The National Medical Center, Mexican Social Security Institute, Mexico City; and the Department of Radiology (D.H.O'L.), New England Medical Center, Tufts University School of Medicine, Boston, Mass.
Correspondence to Ming Wei, MD, Division of Clinical Epidemiology, Department of Medicine, University of Texas Health Science Center at San Antonio, San Antonio, TX 78284-7873.
| Abstract |
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Key Words: artery wall thickness Mexican American Mexico risk factors
| Introduction |
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Increased carotid artery IMT at one site is only modestly correlated with thickened artery walls at other carotid sites.31 However, most studies report the CCA IMT only or the average of the CCA and ICA IMTs. Information on the relation of risk factors to arterial IMT at different carotid sites may increase our understanding of atherosclerosis. We therefore assessed city differences and associations with risk factors separately for the CCA and ICA.
| Methods |
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The San Antonio sample consisted of Mexican Americans who participated in the IRAS. The IRAS sampling design called for one third of subjects to be diabetic. San Antonio IRAS participants were derived from the San Antonio Heart Study cohort, a random sample of the San Antonio population. Diabetic and nondiabetic subjects were recalled separately in random order. The present analyses are based on the 202 San Antonio subjects (55 diabetics) who were Mexican Americans and between 35 and 64 years of age. Again, the cardiovascular risk factor profile was quite similar in the US sample and the original San Antonio Heart Study population (data not shown). All survey and examination procedures were approved by the Institutional Review Boards of the University of Texas Health Science Center at San Antonio, and all subjects gave informed consent.
Diabetes was defined according to the plasma glucose criteria of the World Health Organization, ie, fasting plasma glucose
140 mg/dL and/or 2-hour postload glucose level
200 mg/dL.32 Subjects who did not meet these criteria but who gave a history of diabetes and reported current therapy with either oral antidiabetic agents or insulin were also considered to have diabetes. Plasma cholesterol, TG, and HDL-C levels were measured by using enzymatic procedures.33
Measurement of Carotid Artery Wall Thickness
Carotid artery wall thickness was measured from US recordings made with a Toshiba SSA-270A imaging unit. The imaging protocol involved obtaining a single longitudinal lateral view of the distal 10 mm of the right and left CCAs and three longitudinal views (anterior, lateral, and posterior obliques) in different imaging planes of each ICA. Carotid ultrasonography was performed by certified sonographers who attended a week-long training course in San Antonio and received training in the IRAS ultrasonography protocol. This protocol is essentially identical to the protocol used in the Cardiovascular Health Study.9 All US readings were read by a single reader from the Ultrasound Reading Center at the Geisinger Medical Center, Danville, Pa, which developed standardized protocols for both scanning and interpretation of carotid ultrasonographic images. The images were digitized, and a single reader drew lines at the lumen-intimal, medial-adventitial, and adventitial-periadventitial interfaces on the near and far walls. Maximum IMT for each segment was determined by using a specially designed computer program. To quantify the degree of IMT of the carotid artery wall, the maximum wall thickness of the CCA was defined as the mean of the maximum wall thicknesses of the far wall on the left and right sides; the maximum wall thickness of the ICA was defined in the same way, with the results of the three views averaged for the right and left sides separately.7 9 34 35 36 37 The use of a single reader and the reading of both studies during the same approximate time frame eliminated concerns about interreader variability and temporal drift.
Statistical Methods
Based on a two-sided test with a .05 significance level and 80% power, the magnitude of the crude differences that could be detected between study subjects in the two cities was 0.044 mm for CCA IMT and 0.067 mm for ICA IMT. Variation in crude IMT and covariate-adjusted means were assessed by using ANCOVA. Since both US samples were stratified by diabetic status, although in slightly different ways, analyses are presented either separately for diabetic and nondiabetic subjects or adjusted for diabetes by using ANCOVA. Multivariate analyses were initially performed by using general linear regression models. Analyses were repeated by using log-transformation to normalize the distribution of TGs and IMTs. All probability values were calculated by using two-tailed tests. All statistical analyses were performed by using SAS software.
| Results |
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The age- and diabetes-adjusted and full risk factoradjusted CCA and ICA IMTs according to city and sex are presented in Table 4
. Even though subjects in Mexico City had higher levels of TGs and lower levels of HDL-C (Table 2
), they had thinner CCA IMTs than subjects in San Antonio (0.76 versus 0.81 mm in men and 0.71 versus 0.77 mm in women; P<.0001 for city difference). For ICA IMT, an interaction between city and sex was detected: in San Antonio men had thicker (0.88 versus 0.83 mm) and women had thinner (0.73 versus 0.77 mm) ICA IMTs (P<.05 for the interaction between sex and city). CCA and ICA IMTs in men were thicker than in women, both in Mexico City and San Antonio (all P<.01). Table 5
shows the association of risk factors with carotid wall thickness. Since tests for interactions between sex and risk factors in their associations with carotid artery wall thickness were not significant, we pooled the sexes to increase statistical power. Older age, male sex, diabetes, high SBP, and low HDL-C were significantly associated with both CCA and ICA IMTs. However, a significant association between current cigarette smoking and carotid artery wall thickness was demonstrated only for ICA IMT, and a significant relationship between TC level and carotid artery wall thickness was seen only in CCA IMT in multivariate analysis. Overall, the combination of age, sex, and other risk factors explained 22% of the variance in CCA IMT and 12% of that in ICA IMT.
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| Discussion |
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In our study, San Antonio men had thicker but women had thinner ICA IMTs than their counterparts in Mexico City. These differences were not explained by differences in age, levels of TC, HDL-C, or TGs, or prevalence of smoking or diabetes between the two groups. One limitation in our study was that reproducibility between sonographers was not tested, so this source of measurement error could not be evaluated. However, both sonographers were extensively trained and certified in a common protocol.
Current population-based studies have documented that cardiovascular risk factors, including cigarette smoking, elevated SBP, high TC, and low HDL-C, are consistently associated with carotid artery wall thickness in the general population11 16 17 18 21 23 ; however, these studies contain few Hispanic individuals. Racial differences in carotid artery wall thickness have been found. For example, Tell et al22 report that whites have more carotid artery plaques than blacks. For white subjects, age, sex, diabetes, hypertension, and smoking history were significantly associated with CCA plaque thickness, but in black subjects only age and smoking were associated.22 Few studies based on populations of Mexican origin have thus far been reported. Our results corroborate findings of an association between carotid wall thickness and atherosclerosis risk factors after multivariate statistical adjustment. The effects of these risk factors on carotid artery wall thickness are similar to those reported for the general population of the U.S.17 21
The ability to accurately predict wall thickness at one site given the wall thickness at another site is modest in our study, but is consistent with previous studies.31 Risk factors for carotid plaques may vary somewhat by arterial segment.12 22 Most previous carotid wall thicknessrisk factor studies have used single CCA measurements or have combined a few sites into a single summary index. Comparisons of ICA and CCA IMTs are infrequent. In the present study, age, male gender, diabetes mellitus, SBP, TC, and HDL-C were all associated with both ICA and CCA IMT; smoking was significantly associated with ICA but not CCA IMT (Table 5
). The association between smoking and carotid artery wall thickness seems to be stronger for the ICA than the CCA wall.9 It is also important to note that Mexican American smokers smoke, on average, only 9 cigarettes per day, and smokers in Mexico City smoke, on average, only 7 cigarettes per day (Table 2
). The lack of a smoking effect on CCA IMT may be due to the low rate of cigarette consumption among Mexican smokers. Based on the patchy distribution of atherosclerosis in human arteries, the thickness at a given location of an artery should affect blood flow changes in the artery, and a greatly thickened arterial wall at one site may have a larger effect on local blood flow than modestly increased thickness at two sites. Therefore, we favor using both ICA IMT and CCA IMT separately as indicators of atherosclerosis, rather than the average of the thickness at both sites.
The reports of associations between diabetes and carotid artery wall thickness are controversial. Although a history of diabetes has not been independently associated with carotid wall thickness in some hospital-based studies,19 20 population-based studies have demonstrated that diabetes is a risk factor for maximal intimal-medial carotid thickening.11 23 Our data support such a relationship in populations of Mexican origin (Table 3
).
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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Received August 7, 1995;
revision received April 10, 1996;
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