Title: Possible Sites of Rupture at the Aorta Ascendens and the Aortic Arch following Aortic Dissection
Abstract: To the Editor: Aortic dissection, a relatively common illness, is caused by the sudden development of a tear in the aortic intima, opening the way for a column of blood driven by the force of arterial pressure to enter the aortic wall, destroying the media and stripping the intima from the adventitia for variable distances along the length of the aorta. Finally, the dissecting column may rupture through the adventitia anywhere along the aorta.1.Slater E. Desanctis R. Diseases of the aorta.in: Braunwald E. Heart disease. W.B. Saunders, Philadelphia1980: 1606Google Scholar Considering the possibility of a rupture through the adventitia along the ascending aorta and aortic arch, I would like to emphasize that such ruptures should appear more frequently at the inside than at the outside surface of the aortic curvature. The above hypothesis is based on the following: To assess an equilibrium between transmural pressure and circumferential wall tension at any point on the outside of the aorta ascendens and the aortic arch where the surface is synclastic, the former should equal the latter times 1R1+1R2 where R1 represents the radius of the aorta and R2 the radius of the outer curvature of the arch. Instead, for the assessment of an equilibrium at any point on the inside of the aorta ascendens and the aortic arch where the surface is anticlastic, transmural pressure should equal circumferential wall tension times 1R1−1R2 where R1 represents the radius of the aorta and R2 the radius of the inner curvature of the arch (Fig 1). Since transmural pressure is similar everywhere along the aorta, the circumferential tension required in the wall on the inside perimeter of the curvature to hold the aortic pressure in equilibrium is much greater than that required on the outside perimeter of the curvature. Not surprisingly, the wall at the inner perimeter of the curvature is thicker than at the outer perimeter of the curvature. It has been calculated that the ratio of thickness at the inner, to thickness at the outer perimeter is given by (n – 1) (n +2)/(n +1) (n – 2), where n is the ratio of the radius of the arch (to the axis of the aorta) to the radius of the aorta.2.Burton A. Physical principles of circulatory phenomena: the physical equilibria of the heart and blood vessels.in: Hamilton WF Handbook of physiology. Waverly Press, Circulation I Baltimore1963: 88Google Scholar In the case of circumferential aortic dissections along the ascending aorta and aortic arch, the ratio of thickness at the bottom to thickness at the top would approximately equal 1 (the thickness of the adventitia). This would mean multiple circumferential wall tension at the inside compared to the outside curvature of the aorta and a greater possibility of rupture at the inner than at the outer surface of this vessel. Pathologically, it is not easy to demonstrate the external rupture when this has occurred into the superior mediastinum or pleural cavity, because the loose tissues outside the aorta are so readily infiltrated and distended with clotted blood that the opening, which is usually ragged and irregular, is obscured and may be readily overlooked.8 However, ruptures into the pericardium occur mostly in the recess between the aorta and pulmonary artery,3.Shennan T. Analysis of the anatomical and clinical features in 300 cases of dissecting aneurysms of the aorta and pulmonary artery, including the author’s 17 cases in dissecting aneurysms.in: Med Res Council (Gt Britain) Spec Rep Ser No 193. His Majesty’s Stationery Office, London1934: 69Google Scholar a site where this should be expected to occur if the hypothesis described is taken into account.