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Circulation. 1995;91:734-740

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(Circulation. 1995;91:734-740.)
© 1995 American Heart Association, Inc.

Articles

Determinants of Echocardiographic Aortic Root Size

The Framingham Heart Study

Ramachandran S. Vasan, MD; Martin G. Larson, ScD; Daniel Levy, MD

From the Framingham Heart Study, Framingham, Mass (R.S.V., M.G.L., D.L.); the Divisions of Cardiology and Clinical Epidemiology, Beth Israel Hospital, Boston, Mass (D.L.); Boston (Mass) University School of Medicine (R.S.V., M.G.L., D.L.); and the National Heart, Lung, and Blood Institute, Bethesda, Md (D.L.).


*    Abstract
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 *Abstract
down arrowIntroduction
 down arrowMethods
down arrowResults
down arrowDiscussion
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Background Previous studies that evaluated the determinants of aortic root size have not yielded uniform results. We examined the relations of age, height, weight, body surface area, sex, and blood pressure to echocardiographically determined aortic root size in a population-based cohort.

Methods and Results The study sample consisted of 1849 men and 2152 women in the Framingham Heart Study and Framingham Offspring Study who were free of clinically apparent cardiac disease when echocardiography was performed. Aortic root measurements were made by M-mode echocardiography using a leading-edge-to-leading-edge technique. The relations of age, height, weight, body surface area, and blood pressure variables (contemporary and those obtained 8 years before) to aortic root dimension were examined by use of sex-specific correlations and linear regression analyses. Age, height, weight, and sex emerged as the principal determinants of aortic root dimensions in adults (cumulative R2=.2085 in men and .2327 in women). The additional effect of contemporary or previous blood pressure measures was small and revealed direct associations of aortic root dimension with mean arterial and diastolic blood pressures and inverse associations with pulse and systolic blood pressures. Previous blood pressure measurements did not contribute significantly to prediction of aortic root size once contemporary blood pressure variables entered the models. Results of regression analyses using a sex-pooled data set showed that on average, the aortic root measurement in women was 2.4 mm smaller than that of men of comparable age, height, and weight. Logistic regression was used to assess the likelihood of aortic root enlargement according to blood pressure levels. After adjustment for age, height, and weight, the odds ratio of aortic dilation for a 1-SD increment in systolic pressure was 0.70 (95% CI, 0.52 to 0.95) in men and 0.79 (95% CI, 0.60 to 1.04) in women; the odds ratio for a 1-SD increment in diastolic pressure was 1.22 (95% CI, 0.91 to 1.63) in men and 1.33 (95% CI, 1.01 to 1.73) in women.

Conclusions Age, height, weight, and sex emerged as the principal determinants of aortic root dimensions. The additional influences of blood pressure measurements were small; direct associations of aortic root dimensions with mean arterial and diastolic blood pressures and inverse associations with pulse and systolic blood pressures were observed. Additional prospective studies are needed to confirm these observations and to assess the impact of aortic root dimensions on the incidence of hypertension.


Key Words: aorta • echocardiography • blood pressure


*    Introduction
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 up arrowAbstract
 *Introduction
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down arrowResults
down arrowDiscussion
down arrowReferences
 
Age-related dilation of the aortic root is the most common cause of aortic regurgitation in developed countries.1 2 3 4 The prevalence of aortic regurgitation increases linearly with an increase in aortic root size.5 Aortic root size is also of prognostic importance in subjects with idiopathic annuloaortic ectasia and Marfan's syndrome.6 7 Therefore, the study of the normal biological determinants of aortic root size is important.

The biological variables recognized to influence aortic root size include age, sex, indexes of body size, systolic and diastolic blood pressures, and stroke volume.8 9 10 Studies that evaluated the determinants of aortic root size, however, have not yielded uniform results. These previous studies were not population-based, and the numbers of subjects were small. No prior study included a sufficient number of subjects beyond the seventh decade of life, at which time age-related changes in the structural and functional properties of the thoracic aorta become more pronounced.11

In the present study, we examined the relations of age, height, weight, body surface area (BSA), sex, contemporary and long-term blood pressures (systolic, diastolic, mean), and pulse pressure to echocardiographically determined aortic root size in subjects from the Framingham Heart Study.


*    Methods
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 up arrowAbstract
 up arrowIntroduction
 *Methods
down arrowResults
down arrowDiscussion
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Study Sample
The selection criteria and study designs of the Framingham Heart Study and the Framingham Offspring Study have been detailed extensively.12 13 Subjects of the Framingham Heart Study who participated in the 16th biennial examination (1979-1981) and subjects of the Framingham Offspring Study who participated in the second offspring examination (1979-1983) constituted the study sample used in this investigation. At these index examinations, M-mode echocardiograms were uniformly performed on participating subjects. Subjects were excluded if they fulfilled any of the following criteria, in hierarchical order: (1) clinically apparent aortic or mitral valvular disease identified by the presence of a diastolic or systolic murmur (grade 3/6 or more) on precordial auscultation (n=245); (2) clinically apparent coronary heart disease, congestive heart failure, or atrial fibrillation at or before the index examination (n=553); (3) use of cardiovascular medications at the index examination (n=1081); (4) age <20 or >=90 years (n=12); (5) morbid obesity defined as body mass index (BMI) >35 (n=119); (6) missing or unobtained blood pressure information (n=49); and (7) unavailable or technically inadequate echocardiograms at the index examination (n=154).

Methods of Measurement and Definitions
Blood pressure data were made up of two measurements of systolic and diastolic pressures determined by the first and fifth Korotkoff sounds, respectively, in the left arm of the seated subject obtained by a physician using a mercury column sphygmomanometer. The following definitions of blood pressure variables were used: (1) index systolic blood pressure was the mean of the two systolic pressure readings obtained at the index examination when the echocardiogram was obtained; (2) index diastolic blood pressure was the mean of two diastolic pressure readings obtained at the index examination; (3) index pulse pressure was the difference between the index systolic and diastolic blood pressures; (4) index mean arterial pressure was the sum of the index diastolic blood pressure plus one third of the index pulse pressure; and (5) previous blood pressures were the corresponding blood pressure variables obtained at a clinic examination 8 years before the index examination.

Body height and weight measurements obtained at the index examination were used to calculate BMI (weight in kilograms divided by the square of height in meters). BSA was calculated with the Du Bois14 formula.

All subjects underwent M-mode echocardiograms. More than 90% of the M-mode studies were obtained with two-dimensional (2D) echocardiographic guidance.15 All studies were performed with a 2.25 MHz, 1.25-cm diameter, unfocused transducer (K.B. Aerotech) and a Hoffrel 201 ultrasound receiver (Hoffrel Instruments) interfaced with a Honeywell 1856 strip chart recorder. Aortic root size was measured from the M-mode tracings in accordance with the American Society of Echocardiography (ASE) guidelines using a leading edge to leading edge measurement of the maximal distance between the anterior aortic root wall and the posterior aortic root wall at end diastole.16

Statistical Methods
All analyses were sex-specific. The relations of age, height, weight, BSA, stroke volume, and blood pressure (predictor variables) to aortic root dimension (dependent variable) were initially examined through calculation of simple and partial correlation coefficients.17 Multiple linear regression analyses were used to evaluate the relative influences of these predictor variables on aortic root dimensions.17 Logistic regression18 was used to assess the likelihood of aortic root enlargement (defined as a value exceeding the 95th percentile value) according to blood pressure variables, with adjustment for age and anthropometric measures. A two-sided significance level of .05 was used for each statistical test. All statistical analyses were performed with the STATISTICAL ANALYSIS SYSTEM (SAS)19 on a SUN sparc 2 workstation.


*    Results
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 up arrowIntroduction
 up arrowMethods
*Results
down arrowDiscussion
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Study Sample
Of the 2803 men and 3411 women who attended the index examinations, 1849 men (66%) and 2152 women (63%) ranging in age from 20 to 89 years were eligible for analysis after exclusions. Table 1Down lists the clinical and echocardiographic characteristics of the subjects. Mean aortic root size was larger in men than in women. Indexing for height or BMI resulted in a larger mean aortic root size in men; indexing for BSA or weight resulted in a higher value in women.


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  		Table 1. Clinical and Echocardiographic Characteristics of Subjects

Simple and Partial Correlation Analyses
Table 2Down gives the results of sex-specific correlations. These analyses revealed highly significant but modest correlations of each of the predictor variables with aortic root size, except for height in women. After adjustment for age, all anthropometric and blood pressure variables (other than systolic blood pressure) remained significantly related to aortic root size.


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  		Table 2. Correlations of Aortic Root Diameter With Age, Body Habitus, and Blood Pressure Variables

Stepwise Linear Regression
Effects of Anthropometric Measures on Aortic Root Dimensions
Linear regression models were initiated with the inclusion of age and anthropometric predictor variables. Models with (1) age, (2) height, and (3) weight, BMI, or BSA were studied. When squares of these variables were added to the models, only height squared in men was statistically significant, but its contribution to the model was negligible (the increment in R2 was .0017, P=.046). To create a simple, parsimonious model, squared terms of anthropometric measures were avoided. Table 3Down lists the regression coefficients and the standard errors of the basic regression model that included age, height, and weight.


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  		Table 3. Regression of Aortic Root Dimension on Age, Height, and Weight

Effects of Blood Pressure Variables
The blood pressure variables were then added to the basic model in a forward stepwise fashion. Both systolic and diastolic blood pressures were entered into the model, and both were highly significant (P<.0001). Addition of previous blood pressure terms (variables from 8 years before) indicated that previous diastolic blood pressures in men and women and previous systolic blood pressures in men yielded statistically significant coefficients, but increments in R2 were minimal. Age–blood pressure interactions were examined; an age-related interaction term with systolic blood pressure was significant but small in men (increment in R2=.0036, P=.003) but not in women (P>.40). There was no significant age–diastolic blood pressure interaction. Because in samples this large, statistical significance can be achieved with very small effects, predictor variables relating to previous blood pressure and interaction terms were eliminated to avoid overfitting.

Additional regression models were considered with age, height, weight, and (1) systolic blood pressure, (2) diastolic blood pressure, (3) systolic and diastolic blood pressures, (4) mean arterial pressure, and (5) pulse pressure. Table 4Down lists the regression coefficients, R2, and incremental R2 values for these models. Although blood pressure variables did contribute to the prediction of aortic root size, the changes in R2 from the basic model (age, height, weight) were small. For instance, the inclusion of both systolic and diastolic blood pressures increased the R2 of the model by only .0115 in men and .0042 in women. The coefficients for diastolic blood pressure were slightly higher than those for systolic blood pressure and were similar in magnitude in men and women. When considered together, the mathematical signs of regression coefficients for systolic and diastolic blood pressures were opposite, suggesting a "pulse pressure effect." Collinearity is an unlikely explanation for this observation because the correlation between systolic and diastolic blood pressures was only 0.49.


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  		Table 4. Regression Coefficients and R2 Values for Selected Linear Regression Models With Blood Pressure Variables Fitted for Aortic Root Size

Figs 1Down and 2Down show observed data and fitted regression lines for the equation



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  		Figure 1. Graph showing predicted aortic root size as a function of age and mean arterial pressure (MAP). Upper two curves represent plots of aortic root size for men in the lower and upper quartiles of MAP computed with the following regression equation: Aortic root (mm)=27.83+0.0612xage+0.0234xMAP. Lower two curves represent similar plots for women computed with the following regression equation: Aortic root (mm)=22.65+0.0755xage+0.0224xMAP. Note that men and women in the upper quartile of MAP have a larger aortic root size compared with those of the same sex in the lowest quartile of MAP. Note also that the values for the intercept and regression coefficients differ from those in Tables 3Up and 4Up because height and weight were not included in models used to generate these curves. Triangles and squares represent observed values of aortic root size in men and women, respectively.



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  		Figure 2. Graph showing predicted aortic root size as a function of age and pulse pressure. Upper two curves represent plots of aortic root size for men in the lower and upper quartiles of pulse pressure (PP) computed with the following regression equation: Aortic root (mm)=30.21+0.0798xage-0.0318xPP. Lower two curves represent similar plots for women computed with the following regression equation: Aortic root (mm)=24.44+0.0889xage-0.0094xPP. Note that men and women in the lower quartile of PP have a larger aortic root size compared with those of the same sex in the highest quartile of PP. Note also that the values for the intercept and regression coefficients differ from those in Tables 3Up and 4Up because height and weight were not included in models used to generate these curves. Triangles and squares represent observed values of aortic root size in men and women, respectively.


based on models 4 and 5 from Table 4Up. Fig 1Up indicates that men and women in the fourth quartile of mean arterial pressure have larger predicted aortic root measurements than those in the first quartile. Fig 2Up shows that men and women in the first quartile of pulse pressure have larger aortic root measurements than subjects of the same sex in the fourth quartile.

Sex-Related Differences
Although the primary analyses were sex-specific, sex-related interactions were explored in a sex-pooled data set. Results of these analyses indicated that men and women have different aortic root dimensions (holding all other variables equal and constant). On average, the aortic root measurement in women was 2.4 mm smaller than that of men of comparable age, height, and weight.

Aortic Root Dilatation
Multiple logistic regression was used to determine the contributions of blood pressure variables to the prevalence of aortic root enlargement. Aortic root dilatation was defined as a value exceeding the 95th percentile of the raw values of aortic root dimensions. Logistic models were fitted that incorporated the covariates of age, height, and weight. Blood pressure variables then were entered in the following fashion: (1) contemporary systolic and diastolic blood pressures, (2) contemporary mean arterial and pulse pressures, (3) previous systolic and diastolic blood pressures, and (4) contemporary and previous systolic and diastolic blood pressures. Table 5Down gives the results. In general, contemporary and previous systolic and diastolic blood pressures were significant predictors of an enlarged aortic root in both sexes. After contemporary blood pressure measurements were accounted for, previous measurements of blood pressure did not contribute significantly. The regression coefficients for systolic and diastolic blood pressures had opposite mathematical signs: systolic blood pressure was inversely and diastolic blood pressure was directly related to the probability of having an enlarged aortic root measurement, suggesting a pulse pressure effect. In both sexes, every 1-SD increment in pulse pressure was associated with 25% lower odds of aortic root dilatation. When entered together into the model, mean arterial pressure was directly related and pulse pressure inversely related to the probability of having an enlarged aortic root.


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  		Table 5. Regression Coefficients of Blood Pressure Variables and Odds Ratios for Aortic Root Dilation


*    Discussion
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
up arrowResults
*Discussion
down arrowReferences
 
This study provides a systematic analysis of selected biological determinants of aortic root dimensions in an adult population-based cohort. Age, height, weight, and sex emerged as the principal determinants of aortic root dimensions. The additional influences of blood pressure measurements, although highly significant statistically, were small and revealed direct relations of aortic root dimensions with diastolic and mean arterial pressures and inverse relations with systolic and pulse pressures.

Effect of Anthropometric Variables
The relations of anthropometric variables to aortic dimensions have been recognized for a long time20 21 22 and were confirmed in more recent echocardiographic studies of the aortic root.9 10 23 24 It has been suggested that height is the most important determinant of aortic root size compared with other measurements such as BSA or weight.10 24 In the present study, height and weight (and their derivatives, BSA and BMI) predicted aortic root dimensions. Results of linear regression analyses revealed that a 10-cm increment in height was associated with 0.24- (men) and 0.38-mm (women) increments in aortic root size. Corresponding increments in aortic root dimensions for a 10-kg increment in weight were 0.87 and 0.68 mm. The correlation of aortic root dimension with anthropometric measures yielded smaller correlation coefficients in our study compared with those reported in the literature.9 10 24 Failure to account for sex differences in the previous studies may be an important reason for this difference because sex-pooled analyses can inflate correlations if the distributions of the variables differ in men and women.25 Pooling of children and adults in one study10 is another reason for the inflation of previously reported correlation coefficients because much of the variation of aortic root size among children is related to body growth.

Effect of Age
Age-related dilation of the aortic root has been consistently reported in autopsy series26 27 28 29 30 31 and in clinical studies where chest roentgenograms, computed tomography, and magnetic resonance imaging22 32 33 34 were used. In contrast, clinical echocardiographic studies have yielded disparate results. An age-related increase was evident in some studies9 23 35 but not in others.10 36 Some of these studies included a predominantly pediatric population10 ; none included a sufficient number of subjects beyond the seventh decade, when the age-related structural and functional changes in the aorta become more apparent. In the present study, aortic root dimensions increased gradually and smoothly with age in subjects of both sexes. Results of the linear regression models (Table 3Up) revealed 0.8 (men) and 0.9 mm (women) increases in aortic root dimension for each advancing decade of age, after adjustment for height and weight. Such an age-related change in vessel dimensions is seen in other vascular territories37 38 39 40 and is related to thinning and fracturing of the elastic laminae,11 41 42 43 44 presumably related to the effects of cyclic stress.45 46 47 It has been postulated that because aortic distensibility declines with age, the aorta diameter increases, so that the volume buffering capacity is constant.39 48

Effect of Sex
While it is generally accepted that aortic root dimensions are smaller in women than in men,8 9 the basis for this difference is not clear. Indexing for BSA has resulted in various results; similar,9 larger,8 and smaller29 values of indexed aortic root dimensions have been reported in men compared with women. In the present study, indexing the aortic root dimension produced variable results, depending on the male/female ratio of the divisor selected for indexation. Regression models that used a sex-pooled database indicated that women on average have a 2.4 mm smaller aortic root dimension than men, even after adjustment for anthropometric measures and age. Such sex-related differences have also been observed in other vascular territories.32 39 40

Effect of Blood Pressure Variables
Investigations of the effects of systolic and diastolic pressures on aortic root dimensions have produced inconsistent results. Direct correlations with systolic24 and diastolic49 pressures and no significant correlations with either systolic or diastolic pressure have been reported.9 Studies that compared aortic root dimensions in hypertensive subjects with normotensive age-matched control subjects have not found any consistent difference.50 51 52

In the present study, direct relations of aortic root size to diastolic and mean arterial pressures were observed. The magnitude of effect was very small after adjustment for the influence of age and anthropometric measures. Interestingly, inverse relations of aortic root diameter to systolic and pulse pressures were observed in the present study. Results of linear regression models (Table 4Up) revealed that a 10 mm Hg increment in pulse pressure was associated with a 0.25 (men) and 0.09 mm (women) decrease in aortic root size after adjustment for age and anthropometric measures. Although the direction of these pressure relations may appear counterintuitive, such an inverse relation between pulse pressure and the arterial lumen is predicted by the equation for estimation of elastic modulus of the aorta:


where R is the radius of the aorta, {Delta}P is the pulse pressure, and {Delta}R is the change in radius of the artery.53 A growing body of evidence suggests that in contrast to mean arterial pressure, pulse pressure may have an important effect on alteration of vascular structure in hypertensive subjects.54 55 56 Similar inverse relations of pulse pressure and arterial diameter have been observed in other vascular territories57 and may be related to pulse pressure–mediated intimomedial hypertrophy58 59 or to reflex smooth muscle activation in the vessel wall, causing a decrease in vessel diameter.60 61 In the present study, aortic intimomedial thickness was not measured; we cannot exclude medial hypertrophy as an explanation for the inverse relations of pulse pressure and arterial diameter that we observed.

Although the findings of a direct relation of aortic root dimension to mean arterial pressure and an inverse relation to pulse pressure may seem paradoxical, they actually are consonant with normal physiology. This is because the hemodynamic determinants of mean arterial pressure and pulse pressure are different62 and these blood pressure variables have been shown to have differential effects on other physiological variables such as echocardiographic left ventricular mass.55 Further prospective studies are needed to confirm these observations.

Limitations
It has been stated that 2D echocardiography offers advantages over M-mode studies for the measurement of aortic root dimensions.9 In the present study, M-mode measurements of the aortic root were guided by 2D scanning, which increases the quantitative reliability of the M-mode technique. However, only standard left parasternal views were used to measure the aortic root diameter. Ideally, high left parasternal, right parasternal, and other modified views should be combined with the standard views for obtaining the maximal aortic root diameter. Because these views are not routinely performed in our laboratory, aortic root diameter may have been underestimated in some patients. Interobserver and intraobserver variabilities in the measurement of aortic root diameter were not obtained in the present study; however, prior studies have demonstrated the excellent reproducibility of M-mode echocardiographic aortic root measurements.63

Because the ASE recommendations for measurement of aortic root diameter include the anterior wall of the aorta in the measure, the present study overestimates the true aortic root lumen. We have also assumed that blood pressure variables measured at the brachial artery are not substantially different from those at the aortic root. Whereas such an assumption is valid for mean arterial pressure (which remains the same throughout the vascular tree), pulse pressure is amplified at the level of the brachial artery because of earlier wave reflection,64 principally in younger subjects. We do not view this as a serious limitation of our study because other studies have demonstrated that aortic pulse pressure can be approximated to brachial pulse pressure in studies of central aortic distensibility.65

Conclusions
Aortic root dimension is highly dependent on age, sex, and body size measurements. The effect of blood pressure on aortic root dimensions over and above these variables is small, with direct associations with diastolic and mean blood pressures and inverse associations with systolic and pulse pressures.


*    Acknowledgments
 
Dr Vasan's research fellowship was made possible by a grant from Merck & Co Inc. Special thanks go to Sonya Vaziri, MD, for her assistance in compilation of the data set used in this manuscript and to Emelia Benjamin, MD, for her valuable comments on the manuscript.


*    Footnotes
 
Reprint requests to Daniel Levy, MD, Framingham Heart Study, 5 Thurber St, Framingham, MA 01701.

Received May 31, 1994; accepted September 23, 1994.


*    References
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 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
up arrowResults
up arrowDiscussion
*References
 

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