Screening and detection of asymptomatic aortic aneurysms is based largely on uniform cut-point diameters; therefore, establishing a normal reference value for aortic diameters is important. Despite the advances in aortic imaging, there are still fewer studies covering this area of research worldwide. So, we aimed in this study to establish a normal reference value for aortic diameters among a chosen sample of the Egyptian population using MDCT. Different diameters in all thoracic aorta segments were measured. This study also shows the effect of age, gender, weight and BSA and other cardiovascular risk factors on those diameters.
Aortic diameters
In our study, thoracic aortic diameters at its different levels (hinge to hinge measurements) were taken during systolic (40%) and diastolic (75%) phases of the cardiac cycle of contrast enhanced MSCT study. The aortic root was measured at the levels of the annulus, the sinuses of Valsalva and the STJ. The aortic diameters at the mid-level of ascending thoracic aorta and the descending thoracic aorta and at the level of pulmonary bifurcation and the diaphragm were measured. The arithmetic mean diameters (taken from systolic and diastolic diameters) and BSA-indexed diameters were calculated.
In our study, aortic root measurements were as follows: annulus mean diameter was 23.09 ± 2.55 mm, and its BSA-indexed diameter was 11.70 ± 1.39, sinus mean diameter was 33.75 ± 3.93 mm and its BSA-indexed diameter was 17.10 ± 2.1, STJ mean diameter was 26.13 ± 3.05 mm, and its BSA-indexed diameter was 13.25 ± 1.65. Ascending aorta mean diameter was 30.97 ± 4.16 mm and its BSA-indexed diameter was 15.71 ± 2.28. Descending aorta mean dimeter at the level of pulmonary bifurcation was 24.17 ± 2.92 mm, and its BSA-indexed diameter was 12.24 ± 1.52. Descending aorta mean dimeter at the diaphragmatic level was 21.94 ± 2.73 mm, and BSA-indexed diameter was 11.11 ± 1.44.
In Sang Hawn Lee et al. study, aortic diameters were: 29.9 ± 5.7 cm at the ascending aorta (at the middle level of the right main pulmonary artery), 23.6 ± 3.5 cm at the proximal DTA (at the middle level of the left main pulmonary artery), 21.7 ± 0.38 cm at the distal DTA (at the top of the diaphragmatic level) [6]. Ascending and descending aortic diameters were smaller by ~ 1 mm in this study than in ours. Although they measured the aorta at 9 levels, they didn’t measure any of the aortic root components and they used data from non-gated helical CT scans which could affect the accuracy of the measurements. Different ethnicity also may explain the difference.
In Michael H.C. Pham et al. study, seven anatomical segments were measured “inner edge to inner edge” using contrast-enhanced ECG-gated cardiac CTA during phase 75%. The aortic diameters were; sinus of Valsalva 33 ± 3 mm in men, 29 ± 2.5 mm in women, STJ 31 ± 3 mm in men, 27 ± 2.7 mm in women, ascending aorta at pulmonary trunk level (33 ± 4 mm in men, 30 ± 3.5 mm in women, descending aorta at pulmonary trunk level 25 ± 2 in men, 22 ± 2 mm in women, and aorta at diaphragm 23 ± 2.5 mm in men, 21 ± 2 mm in women [7]. The sinus diameters were larger in our study by ~ 2 mm, while the rest of the diameters were larger in Danish population more so at the STJ level ≥ 3 mm difference. This study is limited by including only Caucasians older than 40, and their results may, therefore, not apply in younger and non-Caucasian individuals. Furthermore, CTA images acquired at 75% of R-R interval in thoracic measures where we took our measurement both at 75% and 40% phases of the cardiac cycle.
Barbara L. McComb et al. study analyzed ungated, low-dose non-contrast CT scans. Measurements were taken outer wall to outer wall at five aortic levels; STJ, mid-ascending aorta, aortic arch, mid-descending aorta (at same level as the STJ measurements) and distal descending aorta at the diaphragmatic hiatus. Aortic diameters were: STJ 3.28 ± 0.38 cm, ascending aorta 3.38 ± 0.38 cm, mid-descending aorta 2.59 ± 0.29 cm, diaphragmatic hiatus 2.53 ± 0.28 cm [8]. Diameters were larger here than in our study, more noticeably STJ diameter ≥ 5 mm difference. This study did not include younger participants (participants were older than 55 y) and did not measure sinus and annulus diameters so we couldn’t compare those to our measurements. They also used data from non-gated CT, which results in motion artefacts and could affect the accuracy of the measurements.
In Fay Y. Lin et al. study, aortic measurements were taken during end systolic and end diastolic phases. Short-axis aortic root measurements were made at the sinuses of Valsalva in end diastolic phase. Short-axis anteroposterior and lateral end diastolic diameters of the ascending and descending thoracic aorta were measured at the level of the main pulmonary artery bifurcation. Ascending and descending thoracic aortic measurements were made at end systolic phase in a subset of 80 patients. Axial measurements were made at end systolic phase of anteroposterior and lateral axis of the ascending and descending thoracic aorta at the level of the pulmonary artery bifurcation in a subset of 36 patients. Aortic diameters were: aortic root (sinus) 3.1 ± 0.3 cm, ascending aorta (short axis, end diastolic) 2.8 ± 0.4 cm, (short axis, end systolic) 3 ± 0.3 cm and (axial, end systolic) 3 ± 0.3 cm, descending aorta (short axis, end diastolic) 2.1 ± 0.2 cm, (short axis, end systolic) 2.2 ± 0.2 cm and (axial, end systolic) 2.3 ± 0.2 cm in overall population [9]. Diameters in this study were smaller than our study, and sinus diameter was about 3 mm smaller here. This difference could be explained because in our study we measured the mean arithmetic dimeter at systolic and diastolic phases in all patient. The rest of aortic root components and the descending aorta at diaphragmatic level were not measured here so we couldn’t completely compare with our study.
In Alfred Hager et al. study, aortic diameters were measured at seven intrathoracic levels: aortic valve sinus, ascending aorta at its maximum size, aorta just proximal the right innominate artery, proximal transverse aortic arch, distal transverse aortic arch, aortic isthmus, and aorta at the level of the diaphragmatic wall of the left ventricle. Sinus diameter 29.8 ± 4.6 mm, ascending diameter 30.9 ± 0.4.1 mm and diameter at diaphragm 24.3 ± 3.5 mm [10]. Sinus diameter was smaller in this study than ours, while descending diameter at diaphragmatic level was larger here. The rest of aortic root components and descending diameter at pulmonary bifurcation level were not measured here so we couldn’t compare.
In Ian S. Rogers et al. study, measurements of the diameters of the ascending and descending thoracic aorta were acquired at the level of the right pulmonary artery. They were traced manually from outside wall to outside wall in the anteroposterior and transverse planes. For all men, the average diameters were 34.1 ± 3.9 mm for the ascending aorta, 25.8 ± 3.0 mm for the descending thoracic aorta. For all women, the average diameters were 31.9 ± 3.5 mm for the ascending aorta and 23.1 ± 2.6 mm for the descending thoracic aorta [11]. Ascending diameters in this study were larger than in our study in both men and women; however, there was no significant difference in the descending aortic diameters. One limitation of this study is the small number of participants compared to ours. Also, the measurements here were taken during one phase only (early diastole) of the cardiac cycle.
Arik Wolak et al. study performed non-contrast gated CT, and ascending and descending thoracic aortic diameters were measured at the level of pulmonary artery bifurcation. The mean aortic diameters for the ascending and descending aorta, respectively, were 33 ± 4 mm and 24 ± 3 mm for ascending and descending aorta, respectively [12]. Ascending diameter was larger here than in our study; however, they only measured the aorta at 2 levels, so more detailed comparison to our study could not be made.
Determinants of aortic diameters
Previous studies have shown that BSA, age and gender were major determinants of aortic diameters. Sang Hawn Lee et al. study showed that age and gender were major determinants of ascending aortic diameters in asymptomatic Korean adults. They found that men had slightly larger aortic diameters than women (P < 0.05). Women had slightly larger BSA-adjusted aortic diameters than men (P < 0.05). Women’s aortic diameters were bigger than men’s in terms of the ascending aorta, while the opposite was true for the aorta between the proximal descending thoracic aorta (P < 0.01), when adjusted by age, hypertension, height and weight. All aortic diameters increased with height (P < 0.05), and all aortic diameters increased with weight (P < 0.05). There was a significant increase in aortic diameter at all levels throughout adult life (P < 0.01). All diameters increased with hypertension when adjusted by sex, age, height and weight (P < 0.01). This study is limited by the use of data from non-gated helical CT scan, because ECG-gated MDCT provides high resolution images in near isotropic conditions [6]. BMI was not calculated here.
In Michael H. C. Pham et al. study, diameters were body height-adjusted. This study also showed that gender, age and body surface area were significantly associated with increasing aortic diameters at all aortic segments (P < 0.001). All diameters were found to be larger in men than in women, but when diameters were adjusted for body height to the power of 2.7, all aortic regions except the sinus of Valsalva were found to be marginally larger in women than in men (P < 0.001) [7].
Fay Y. Lin et al. study showed that aortic root diameter was greater in men than in women and it was associated strongly with body size and less strongly with SBP and DBP. Age and BSA were independent determinants of aortic root diameters, whereas gender was not. Age and BSA were also significantly related to end-diastolic ascending and descending thoracic aortic diameter [9].
In Brbara L. McComb et al. study, it was found that age, gender, BSA, and hypertension were significant predictors of aortic diameter. It also showed that the aortic diameter for the diaphragmatic hiatus might be larger in current smokers [8]. On the other hand, Hager et al. study revealed no influence of weight, height, or body surface area, but it did reveal influences of gender and age. Age being the significant influencer, as there was a significant increase of the aortic diameters at all intrathoracic levels throughout adult life [10].
Rogers et al., found that gender, age, BSA and diastolic blood pressure were significant determinants of all thoracic aortic diameters. This study was limited by the inclusion of only Caucasian participants. Also, it was limited by the lack of use of intravenous contrast [11].
Arik Wolak et al., found that for both the ascending and descending aorta, age, BSA, diabetes, hypertension and an interaction between age and male gender (such that older men have, on average, larger aorta than women of a similar age) were significant predictors of aortic diameter. Smoking, however, was found to be independent predictor of descending aortic diameter [12].
Our study showed that gender, age, BSA, BMI and hypertension were the major determinants of aortic diameters. Diabetes has no effect on aortic diameters at its different levels.
Gender was the most important determinant of aortic root diameters (R = 0.53, adjusted R square= 0.28, P < 0.001 with the highest standardized coefficients beta of 0.36 to predict the annulus diameter, R = 0.56, adjusted R square= 0.32, P = 0.001 with the highest standardized coefficients beta of 0.48 to predict the aortic sinus diameter, R = 0.45, adjusted R square= 0.2, P < 0.001 with highest standardized coefficients beta of 0.30 to predict the sino-tubular junction diameter). The effect of gender on the descending aorta is less pronounced. Mean aortic diameters were significantly larger in males, with the greatest difference at the aortic root (up to 4 mm) except BSA-indexed tubular aortic diameter that was larger in females.
Age was a major determinant of thoracic aortic diameters at all levels except the annulus. It should be noted that all diameters increased with age except the annulus diameter which appeared to be smaller in the older age groups (23.55 ± 2.54 mm, 23.20 ± 2.62 mm, 22.61 ± 2.34 mm, P = 0.025) at age group < 40 years, 40–60 years and > 60 years, respectively), that could be explained by our method of measurement of the oval shaped annulus which was done in a single plane and without perimeter or area derived measurements. Calcifications of the annulus at the older age group led to smaller measurements. The effect of age was most pronounced on the descending aorta at the diaphragmatic level (multiple regression analysis to predict descending aortic diameter at the diaphragmatic level; R = 0.59, adjusted R square= 0.35, P < 0.001 with the highest standardized coefficients beta that was 0.48).
BSA was a major determinant of thoracic aortic diameters at both the ascending and descending parts. BMI was the least important determinant of aortic diameters. It had little contribution to aortic annulus diameter and descending aorta diameter at the pulmonary bifurcation level.
Regarding other cardiovascular risk factors, smoking was associated with larger aortic root and descending aortic diameters (24.17 ± 2.42 mm, 22.58 ± 2.45 mm for smokers and non-smokers, respectively, with a P < 0.001 at the annulus, 35.31 ± 3.57 mm, 33.01 ± 3.88 mm for smokers and non-smokers, respectively, with a P < 0.001 at the aortic sinus, 27.09 ± 3.16 mm, 25.68 ± 2.89 mm for smokers and non-smokers, respectively, with a P < 0.001 at the aortic sino-tubular junction, 22.47 ± 2.85 mm, 21.68 ± 2.63 mm for smokers and non-smokers, respectively, with a P = 0.002 at the descending aorta at the diaphragm, 24.09 ± 2.89 mm, 23.88 ± 2.88 mm with a P < 0.001 at the level of pulmonary bifurcation). Aortic tubular BSA- indexed diameter was slightly smaller in smokers (15.18 ± 2.03 mm, 15.96 ± 2.36 mm for smokers and non-smokers, respectively, with a P = 0.001).
Dyslipidemia was associated with smaller aortic annulus and sinus BSA-indexed diameters (11.59 ± 1.37 mm, 11.87 ± 1.39 mm, P = 0.035 at the aortic annulus and 16.93 ± 2.03 mm, 17.36 ± 2.17 mm, P = 0.026 at the aortic sinus for patients with dyslipidemia and those with no dyslipidemia, respectively).
HTN was an important determinant of tubular aortic diameter (R = 0.38, adjusted R square= 0.14, P < 0.001 with a standardized coefficients beta of 0.11). Aortic tubular diameters were larger in hypertensives (31.69 ± 4.17 mm, 29.94 ± 3.94 mm, P < 0.001 for hypertensive and normotensive patients, respectively, and the significance is maintained when the diameters were indexed to the BSA). Descending aortic diameters were larger in hypertensives (24.47 ± 2.68 mm, 23.37 ± 3.02. P = 0.005 for hypertensive and normotensive patients and 22.24 ± 2.61 mm, 21.49 ± 2.83 mm, P = 0.005 for hypertensive and normotensive patients at the level of pulmonary artery bifurcation and the diaphragm, respectively). However, BSA-indexed annulus diameter was smaller in hypertensive group (11.49 ± 1.32 mm, 12.00 ± 1.42 mm, P < 0.001 for hypertensive and normotensive patients. The mean annulus diameter is smaller in hypertensive group, but statistically non-significant.