(Stroke. 1996;27:2012-2015.)
© 1996 American Heart Association, Inc.
Articles |

the Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, Mass (J.F.P.); Cardiovascular Health Study Coordinating Center, University of Washington (Seattle) (R.A.K., N.O.B.); Department of Medicine, University of California, Irvine (Orange) (J.M.G.); Department of Public Health and Primary Health Care, University of Bergen (Norway) (G.S.T.); Department of Radiology, New England Medical Center, Boston, Mass (D.H.O'L.); Epidemiology and Biometry Program, Division of Epidemiology and Clinical Applications, National Heart, Lung, and Blood Institute, Bethesda, Md (P.J.S.); and Memphis Veterans Administration Medical Center, Memphis, Tenn (G.H.R.).
| Abstract |
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Methods We performed a cross-sectional, community-based study of adults 65 years of age and older, measuring inner and outer diameter of the CCA in vivo with carotid sonography. Findings were correlated against risk factors for atherosclerosis, CCA intima-media thickness (IMT), and echocardiographically determined left ventricular (LV) mass.
Results Independent variables showing strong positive associations with outer and inner CCA diameter included age, male sex, height, weight, and systolic blood pressure. As an independent variable, LV mass (r=.40 and r=.37, respectively; P<.00001) had a strong positive relation to inner and outer CCA diameters. The relationship between diameter and IMT was different. In a model that controlled for age, sex, and estimated LV mass, an increase of 1 mm in CCA IMT corresponded to a 1.9-mm increase in the outer diameter of the artery (P<.00001) but was not significantly related to the inner diameter (slope=+0.07 mm; P=.26).
Conclusions Increase in the outer diameter of the CCA is associated with subject size, sex, age, echocardiographically estimated LV mass, and CCA IMT. Increases in internal diameter of the CCA have similar relationships but are not related to IMT. This supports the hypothesis that the human CCA dilates as the thickness of the artery wall increases.
Key Words: carotid arteries risk factors ultrasonics
| Introduction |
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Artery size is also affected by blood pressure: the inner diameter of the aorta increases in the presence of hypertension.8 It therefore seems possible that CCA diameter might be related to LV mass, since prolonged exposure to elevated blood pressure levels is related to increased LV mass.9
The goal of this work was to further study the phenomenon of compensatory arterial enlargement of the CCA. Diameter measurements made in the distal CCA were related to artery IMT, blood pressure, and estimated LV mass by echocardiography to determine whether CCA outer diameter was related to CCA IMT.
| Subjects and Methods |
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Data Acquisition
Seated blood pressures were measured in the right arm after a 5-minute rest. The average of two measures obtained from appropriately sized cuffs and a Hawksley random-zero sphygmomanometer (model 7076, Hawksley and Sons Limited) was used. All data were collected according to standard protocols by personnel trained and certified by the CHS Coordinating Center. Procedures have previously been described and consist of determination of the presence of prevalent disease by self-report or, whenever possible, by supporting examinations: electrocardiography, prescription medication, hospital discharge diagnoses, and medical reports obtained from treating physicians.10 Body height and weight were measured. Blood samples were drawn for measurements of lipids in the central laboratory. Transthoracic echocardiography was performed with 2.5- and 3.5-MHz transducers and included the acquisition of M-mode, two-dimensional, and Doppler velocity data.12 M-mode measurements were performed using the leading-edge method and used to estimate LV mass according to a necropsy-validated regression equation.13 The carotid arteries were evaluated with a high-resolution linear-array transducer capable of variable-depth focusing and Doppler velocity measurements (model SSA-270A, Toshiba Corp). Longitudinal images of the right and left CCA were acquired from the anterior neck and centered 1 cm proximal to the carotid bulb. The arterial wall boundaries defined in the NHLBI-sponsored Atherosclerosis Risk in Communities study were identified.14 Gross and microscopic techniques have shown that the distance between the lumen-intima interface and the media-adventitia interface corresponds to the combined thickness of the intima and media.3 4 In each individual, CCA IMT values were the mean of the maximal IMT measured at the near and far walls of the CCAs on both sides of the neck.15 CCA diameter measurements were made in the portion of the CCA where the wall interfaces are parallel. The outer diameter of the artery was the mean of the maximal distance between the adventitia-to-periadventitia interface of the near wall and the periadventitia-to-adventitia interface on the far wall of the right and left CCAs. The inner diameter was the mean of the minimal distance between the near-wall intima-lumen interface to the far-wall lumen-intima interface of the right and left CCAs. Focal lesions were also imaged and evaluated in the internal carotid arteries of all participants but were not used in the present analyses. All sonographers and readers underwent training sessions and were certified after review of their performance.15
Data Analysis
The two-sample t test was used to compare CCA diameters for variables with two categories (Table 1
). Only values of P<.01 were considered significant. The F test was used when comparisons were made of the age-adjusted measurements (Table 2
).
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Correlation coefficients for the variables shown in Table 3
were computed as descriptive measures of the respective relationships with inner and outer diameters as the dependent variables.
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Two regression equations16 were generated with the following variables used as independent variables: CCA IMT, sex, age, and estimated LV mass. The inner and outer diameters of the CCA were selected as the dependent variables.
All analyses were conducted using a standard statistical package (SPSS/PC, version 4.0; SPSS Inc). The regression analyses included only participants with complete data for all variables.
| Results |
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Outer diameter of the CCA was significantly greater in men than in women (Table 1
). The size of the CCA was also on average larger in subjects with a history of hypertension (9.41±0.98 versus 9.14±0.99 mm; P<.0001). A similar pattern was seen for measurements of the inner diameter, which were on average approximately 3 mm less than those of the outer diameter. A similar pattern was seen when controlling for sex and adjusting for age (Table 2
).
The correlation coefficients between the CCA outer diameter and key variables are shown in Table 3
. The strongest relationship was seen with CCA IMT (r=.51; P<.0001), followed by the M-mode estimate of LV mass (r=.40; P<.0001), then height (r=.33; P<.0001) and weight (r=.30; P<.0001). CCA outer diameter increased with age (r=.23; P<.0001) and systolic blood pressure (r=.24; P<.0001) but increased less with diastolic pressure. The relationship of CCA inner diameter (Table 3
) with CCA IMT (r=.14) is not as strong as what was seen with the CCA outer diameter (r=.51). Relationships with systolic blood pressure (r=.17; P<.001) and echocardiographic LV mass estimates (r=.37; P<.0001) were also weaker. The strength of these relationships was weaker after adjustment for age and sex (Table 3
).
The following regression equations were obtained for predicting the diameter of the CCA: Outer Diameter (mm)=1.91 CCA IMT (mm)+0.43 Sex (Male=1)+0.016 Age(years)+0.0042 Echo LV Mass (g)+5.3 mm (r=.63; all coefficients significant to P<.0001), and Inner Diameter(mm)=0.07 CCA IMT (mm)+0.40 Sex (Male=1)+0.013 Age (years)+0.0040 Echo LV Mass (g)+4.1 mm (r=.45; all coefficients significant to P<.0001 except for wall thickness with P=.26).
| Discussion |
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Our diameter and IMT measurements are made, by design, during diastole. This minimizes the likelihood of image blurring due to arterial wall motion during mechanical systole. Despite this, diastolic pressure was not as strongly associated with inner or outer CCA diameter as was systolic pressure. This may simply be due to the known plateau and even decrease in diastolic pressures with age, whereas systolic pressures continue to increase with age.18 Variables such as male sex and increases in height and weight are also strongly positively correlated to CCA diameters. These morphometric relationships are consistent with other studies6 and appear intuitively correct: the larger the individual, the larger the CCA lumen. The arterial diameters of hypertensive subjects when compared with normal subjects also correlate with blood pressure and parameters such as age, height, and weight: Safar et al19 noted a relationship between brachial artery internal diameter and arterial pressure (r=.60), whereas Isnard et al8 showed that internal diameter of the aorta is related to body surface area (r=.66) and arterial pressure (r=.54). Measurement techniques may explain the failure to see this relationship in certain studies.20
We confirm that CCA outer diameter increases with increases in the CCA IMT. Without such a mechanism, the arterial wall, as it thickens, would cause the lumen of the artery to narrow, thereby subjecting the endothelium to an increase in shear stress caused by flowing blood.21 The artery appears to compensate for the presence of thickened walls by dilating, thereby stabilizing the shear stress experienced at the interface between blood and endothelium.22 This is seen for coronary artery lesions that compromise less than 40% of the lumen area.2 Without an increase in artery diameter, resistance to blood flow would increase, as would cardiac work. For example, a change of 0.3 mm in wall thickness in a 6.0-mm-diameter CCA translates to a 10% (0.6 mm) decrease in diameter. Because resistance to blood flow is a function of the fourth power of diameter, a decrease in CCA lumen diameter would increase resistance to blood flow by 34%. The increase in outer diameter of the CCA prevents this from happening. Other studies have established a possible relationship between vessel diameter and the extent of wall thickening.23 Aortic diameter has been shown to progressively increase with age; this increase is more pronounced as the severity of atherosclerotic disease increases.24 Although thickening of the CCA wall can be considered a subclinical manifestation of atherosclerosis in the CCA,5 25 26 its role as a predictor of incident cardiovascular disease (ie, stroke or transient ischemic attacks) has yet to be proven.
Few large cross-sectional studies have investigated what factors relate to increases in the size of the CCA. Riley et al27 have noted that CCA diameter (measured as the distance between adventitia-media interfaces) increases with age. The population studied is, however, younger than ours and limited to those less than 65 years of age. Roman et al28 looked at the relationship between LV mass and internal diameter of the CCA and also found correlations between CCA inner diameter, LV mass, and systolic blood pressure. They did not see any effect of age in multivariate analyses predicting inner diameter of the CCA, nor did they measure the outer diameter of the CCA. Crouse et al6 and Bonithon-Kopp et al7 report a relationship between CCA IMT and CCA diameter. In both these studies, the final diameter measurements of the artery are calculated with a term that uses the far-wall IMT as a substitute for the near-wall IMT. This might introduce collinear effects on subsequent regression models looking at the relationship between artery diameter and risk factors for atherosclerosis. We have avoided this problem by performing direct measurements of the artery diameters. This may explain why we do not see an "overcompensation" of CCA diameter to CCA IMT.7 Crouse et al and Bonithon-Kopp et al did not study the relationship between CCA diameter and LV mass. Measurements of CCA IMT and diameters have a major limitation: the magnitude of the observed changes is quite small. This is in part due to the fact that the changes in CCA IMT tend to be small. The detection of these changes requires a large number of observations to compensate for the physical limits of the measuring device.29
Echocardiographic estimates of LV mass were obtained less often than any other measure (Table
s 1 to 3). This seems linked to the age and state of health of the participant.30 Our findings linking CCA measures to LV mass apply as such to those participants for whom echocardiography was of sufficient quality to obtain appropriate M-mode measurements. LV mass is known to increase in response to increases in cardiac work such as prolonged elevations in blood pressures.9 The relationships between arterial diameter, wall thickness, and LV mass suggest a significant interaction between the arterial system and the heart. CCA IMT may increase in response to exposure to atherosclerosis risk factors and to increases in blood pressure.26 However, it is believed that this increase in IMT is distinct from the focal atherosclerotic process.5 The loss of elasticity known to occur as the artery wall thickens also increases the amount of cardiac work.31 This simple interaction may contribute to the observed relationship between LV mass and arterial diameter. This relation is stronger than the one with blood pressure, plausibly because LV mass reflects chronic exposure to blood pressure better than single measurements made in a clinic.9
We conclude that the inner and outer diameters of the CCA, measured with high-resolution sonography, are directly related to LV mass and to systolic blood pressure. The outer diameter is in addition related to CCA IMT, while the inner diameter of the CCA tends not to increase or decrease when IMT increases. The compensatory increase of the outer diameter (remodeling) seems to preserve the inner diameter of the CCA in the presence of wall thickening.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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| Footnotes |
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Received May 3, 1996; revision received July 15, 1996; accepted August 6, 1996.
| References |
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