Arteries experience the highest blood pressures because they are the first vessels through which the heart pumps blood into the body, so they have thick, muscular walls to withstand high pressure and maintain their shape[1]. At the same time arterial walls are very elastic, which allows them to expand and accommodate the spike in blood volume that occurs with every heartbeat. After expanding, the arteries quickly contract back to their original size, and in the process help to push the blood along its way.

From the centre to the periphery, the vascular tree shows three main modifications. The arteries increase in number by sending out branches in both, the systemic and the pulmonary circulation[2]. The walls of arteries decrease in thickness towards the periphery, although this is not as substantial as the reduction in vessel diameter. Consequently, in the smallest arteries (arterioles), the thickness of the wall represents about half the outer radius of the vessel, whereas in a large vessel it represents between one-fifteenth and one-fifth, e.g. in the thoracic aorta the radius is ~17 mm and the wall thickness ~1.1 mm. For example, the aorta, which carries blood from the heart to the systemic circulation, gives rise to about 4 × 106 arterioles and four times as many capillaries[3-4]. The arteries also decrease in diameter, although not to the same extent as their increase in number. At its emergence from the heart, the aorta of an adult man has an outer diameter of 30 mm (sectional area of nearly 7 cm2). The diameter decreases along the arterial tree until it is as little as 10 μm in arterioles (each with a sectional area of 80 μm2).

The aortic arch and supra-aortic branches are important anatomical structures for both surgeons and interventionalists[5-6]. Aneurysms or dissections of the aortic arch need to be treated with complex surgical procedures such as deep hypothermic circulatory arrest and selective antegrade cerebral perfusion. These procedures evolved to enable replacement of the aortic arch and reconstruction of its continuity to the aorta and supra-aortic arteries with less risk of ischemic and/or embolic cerebral damage.

General and neurological morbidity, however, are still significant especially in elderly patients and in patients already burdened with significant co-morbidities. This is the motivation to find less invasive treatment options such as aneurysm exclusion with endovascular stent grafts[7].

Study was conducted in department of Anatomy, department of Radiology of M. L. N. Medical College, Allahabad, Indu Scans and Kriti scanning center, Allahabad covering the peripheral population of Allahabad. Populations from Allahabad region were included in our study and they were informed about the study.

The selection of participants depends on following inclusion and exclusion criteria, these are as follows:

Inclusion Criteria:

1. Age from 1year to 88years,
2. Both male and females were taken,
3. Without any medical, surgical complication,
4. subject was either urban or rural resident of Allahabad or North Indian ancestry,

Exclusion Criteria:

1. Poor visualization of the aorta.
2. Subjects with any aortic arch variations.
3. Long standing hypertension.
4. Previous history of thoracic aortic aneurysm.
5. High total cholesterol level.
6. Diabetes mellitus.
7.  
8. History of prolonged substance abuse, such as alcohol, smoking or tobacco.
9. Medical, surgical or obstetric vascular complications.

Participants were interviewed to assess their name, demographic data like age, sex, habitat –urban (subjects belonging to proper Allahabad city) or rural (subjects belonging to villages in the periphery of Allahabad), and any underlying disease, and factors that might cause abnormal dilation or growth of thoracic aorta and brachiocephalic trunk, left common carotid artery, left subclavian artery . The nutritional status has not been taken into this account.

The basic characteristics viz. age and sex distribution of studied subjects are summarized. All the subjects were ranged from 1 to 88 years with mean age (± SD) 50.14 ± 19.61 years and median 54 years. Further, among subjects, 33 were 41-60 years aged (39.3%), 24 were ≥ 61 years aged (28.6%), 20 were 21-40 years aged (23.8%) and 7 were ≤ 20 years aged (8.3%). Furthermore, among subjects, 32 were females (38.1%) and 52 were males (61.9%). 35 (41.7 %) subjects were from rural area and 49 (58.3%) subjects were from urban area.

TABLE 2: Summary of observed diameters of branches of arch aorta and thoracic aorta of studied subjects.

Among, branches of arch aorta, the mean diameter of brachiocephalic trunk was the highest and left common carotid artery the least (BCT > LSA > LCCA). Similarly, the mean diameters of thoracic aorta at aortic valve, ascending aorta, proximal to brachiocephalic trunk, proximal transverse arch, distal transverse arch, distal to left subclavian artery and aorta at diaphragm was found to be in descending order decrease from aortic valve sinus to diaphragm i.e. highest at aortic valve and least at aorta at diaphragm (at aortic valve > ascending aorta > proximal to brachiocephalic trunk > proximal transverse arch > distal transverse arch > distal to left subclavian artery > aorta at diaphragm).

TABLE 3: Effect of age on diameters

The age wise distributions of diameters (diameters of arch aorta and diameters of thoracic aorta) are summarized in Table 6. Table 6 showed that age increases mean diameters also increases except left common carotid artery. Evaluating the effect of age on diameters, ANOVA revealed significant (p<0.05 or p<0.01 or p<0.001) effect of age on all diameters except left common carotid artery.

TABLE 4: Comparison (p value) of mean diameters with different age groups by Tukey test

Further, Tukey test, mean diameter of brachiocephalic trunk of ≥61 yrs aged subjects was significantly (p<0.01) different and higher as compared to both ≤ 20 yrs and 21-40 yrs aged subjects. Similarly, mean diameter of LSA of both 41-60 yrs and ≥61 yrs aged subjects were also found significantly (p<0.05) different and higher as compared to ≤ 20 yrs aged subjects.

In contrast, the mean diameters of thoracic aorta (at aortic valve, ascending aorta, proximal to brachiocephalic trunk, proximal transverse arch, distal transverse arch, distal to left subclavian artery and aorta at diaphragm) of 21-40 yrs, 41-60 yrs and ≥61 yrs aged subjects were significantly (p<0.001) different and higher as compared to ≤ 20 yrs aged subjects. Further, the mean diameters of ascending aorta, proximal to brachiocephalic trunk, proximal TA, distal transverse arch and distal to LSA of both 41-60 yrs and ≥61 yrs aged subjects were also found to be significantly (p<0.05 or p<0.01) different and higher as compared to 21-40 yrs aged subjects. However, the mean diameters of all thoracic aorta not differed (p>0.05) between 41-60 yrs and ≥61 yrs aged subjects i.e. found to be statistically the same

The embryological origin of the descending aorta is different from that of the ascending aorta, i.e. dorsal aorta versus truncus arteriosus. Therefore, it is of interest to know whether the alterations in vessel calibre, such as those reported here for these parts of the aortic arch, are accompanied by comparable histological changes in the vascular wall[8]. The thoracic aorta provides compliance with elastic recoil to maintain blood pressure and antegrade blood flow throughout diastole. The more distal abdominal aorta functions mainly as a conduit. The varying functions are reflected in the histologic structure of the aorta. The elastin/collagen ratio is highest in the thoracic part and decreases distally. With age, the aortic wall structure changes. Elastin fragmentation, fibrosis, and media necrosis occur in the aorta as signs of aging[9-10]. Furthermore, various diseases alter aortic structure and function and may cause obstruction or dilatation of the aorta. Both obstruction and dilatation may be circumscript, segmental, or spread throughout the entire aorta.

In the present study we have taken demographic factors like age, sex as they can affect the diameters. The patients in this study group ranged from 01 year to 88 years and their mean age was (± SD) 50.14 ± 19.61 years and median 54 years. Further, among subjects, 33 were 41-60 years aged (39.3%), 24 were ≥ 61 years aged (28.6%), 20 were 21-40 years aged (23.8%) and 7 were ≤ 20 years aged (8.3%)[11].

In our study we have found that measured diameter of thoracic aorta at 7 predefined levels are found to be in descending order, decrease from proximal to distal i.e. highest at aortic valve and least at aorta at diaphragm (at aortic valve > ascending aorta > proximal to brachiocephalic trunk > proximal transverse arch > distal transverse arch > distal to left subclavian artery > aorta at diaphragm), and Hager et al (2002) also observed highest diameter at ascending aorta. All the mean diameter of thoracic aorta found in this study was slightly lower than diameter found by Hager et al[12-14].

On performing ―t test on both the study we found no significant difference in diameters at aortic valve, ascending aorta, proximal to BCT and significant (P< 0.005) difference found at proximal transverse arch, distal transverse arch, distal to left subclavian artery and aorta at diaphragm

Among, branches of arch aorta, the mean diameter of brachiocephalic trunk was the highest and left common carotid artery the least (BCT > LSA > LCCA). The mean diameter of thoracic aorta at aortic valve, ascending aorta, proximal to brachiocephalic trunk, proximal transverse arch, distal transverse arch, distal to left subclavian artery and aorta at diaphragm was found to be in descending order, decrease from proximal to distal i.e. highest at aortic valve and least at aorta at diaphragm. There is significantly high correlation between brachiocephalic trunk and left common carotid artery (r=0.62, p<0.001), brachiocephalic trunk and left subcavian atrtery (r=0.59, p<0.001) and left common carotid artery and left subclavian artery (r=0.59, p<0.001).

1. Cleemann L, Mortensen KH, Holm K, Smedegaard H, Skouby SO, Wieslander SB, Leffers AM, Leth-Espensen P, Pedersen EM, Gravholt CH. (2010) ―Aortic dimensions in girls and young women with turner syndrome: a magnetic resonance imaging study.‖ Pediatr Cardiol. Vol.31(4):P.497-504.
2. Craiem D, Casciaro ME, Graf S, Chironi G, Simon A, Armentano RL. (2012) ― Effects of aging on thoracic aorta size and shape: a non-contrast CT study‖ Conf Proc IEEE Eng Med Biol Soc. Vol.2012:P. 4986-9.
3. Davis AE, Lewandowski AJ, Holloway CJ, Ntusi NAB, Banerjee R, Nethononda R, Pitcher A, Francis JM, Myerson SG, Leeson P, Donovan T, Neubauer S and Rider OJ (2014) ―Observational study of regional aortic size referenced to body size: production of a cardiovascular magnetic resonance nomogram‖ Journal of Cardiovascular Magnetic Resonance. Vol.16: P.9.
4. Davies RR, Gallo A, Coady MA, et al. (2006) ―Novel measurement of relative aortic size predicts rupture of thoracic aortic aneurysms‖ Ann Thorac Surg Vol. 81:P.169–177.
5. Cleemann L, Mortensen KH, Holm K, Smedegaard H, Skouby SO, Wieslander SB, Leffers AM, Leth-Espensen P, Pedersen EM, Gravholt
6. (2010) ―Aortic dimensions in girls and young women with turner syndrome: a magnetic resonance imaging study.‖ Pediatr Cardiol. Vol.31(4):P.497-504.
7. Craiem D, Casciaro ME, Graf S, Chironi G, Simon A, Armentano RL. (2012) ― Effects of aging on thoracic aorta size and shape: a non-contrast CT study‖ Conf Proc IEEE Eng Med Biol Soc. Vol.2012:P. 4986-9.
8. Davis AE, Lewandowski AJ, Holloway CJ, Ntusi NAB, Banerjee R, Nethononda R, Pitcher A, Francis JM, Myerson SG, Leeson P, Donovan T, Neubauer S and Rider OJ (2014) ―Observational study of regional aortic size referenced to body size: production of a cardiovascular magnetic resonance nomogram‖ Journal of Cardiovascular Magnetic Resonance. Vol.16: P.9.
9. Davies RR, Gallo A, Coady MA, et al. (2006) ―Novel measurement of relative aortic size predicts rupture of thoracic aortic aneurysms‖ Ann Thorac Surg Vol. 81:P.169–177.
10. Van Meurs-Van Woezik H, Krediet P (1982), ―Measurements of the descending aorta in infants and children: comparison with other aortic dimensions‖ Journal of anatomy, Vol. 135(2): P. 273-9.
11. Shakeri A,  Pourisa  M,  Deldar,  Goldust    ―  Anatomic  Variations  of Aortic Arch Branches and Relationship with Diameter of Aortic Arch by 64-row CT Angiography‖ Pakistan Journal of Biological Sciences Vol. 16(10): P.496-500.
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13. Rogers IS, Massaro JM, Truong QA, Mahabadi AA, Kriegel MF, Fox CS, Thanassoulis G, Isselbacher EM, Hoffmann U, O'Donnell CJ. (2013) Distribution, determinants, and normal reference values of thoracic and abdominal aortic diameters by computed tomography (from the Framingham Heart Study).‖ Am J Cardiol. Vol.111(10):P.1510-6.
14. Voges I, Michael JH, Hedderich J, Pardun E,Hart C, Gabbert DD, Hansen JH, Petko C, Rickers HHKC (2012) ―Normal values of aortic dimensions, distensibility, and pulse wave velocity in children and young adults: a cross-sectional study‖ Journal of Cardiovascular Magnetic Resonance, Vol. 14: P.77.
15. Hager A, Kaemmerer H, Ulrike RB, Blucher S, Rapp K, Bernhardt TM, Galanski M, Hess J. (2002) ―Diameters of the thoracic aorta throughout life as measured with helical computed tomography‖ The Journal of Thoracic and Cardiovascular Surgery Vol. June :P. 1060-1066.