Aortic dimensions and the risk of dissection

Abstract

T he aorta represents a complex organ system which begins in the aortic ring adjacent to the aortic root with the origin of the two major coronary arteries, and ends at the iliac bifurcation. The subdivision into seven segments seems to be clinically important-the aortic root, the ascending aorta, the ascending aortic arch, the vessel bearing arch, the aortic isthmus, and the thoracic descending and abdominal aorta. The aorta as an organ can be regarded as a biological ''windkessel'', storing kinetic energy during systole which is delivered during diastole in order to maintain a relative constant mean aortic pressure. In particular, a high diastolic blood pressure is important for the coronary perfusion.The size of the aorta decreases with distance from the aortic valve in a tapering fashion. The normal diameter of the ascending aorta has been defined as ,2.1 cm/m 2 and of the descending aorta as ,1.6 cm/m 2 .1 The normal diameter of the abdominal aorta is regarded to be less than 3.0 cm. The normal range has to be corrected for age and sex, as well as daily workload.The aortic wall consists of three layers: intima, media, and adventitia. The intima is thin, the media contains the elastic fibres and smooth muscle cells forming a spiral layer of tissue providing the strength of the aortic wall, and the adventitia provides the nutrition with the arterial and venous vasa vasorum. An inner vasa vasorum from the aortic lumen also seems to be present.2 A wall thickness of , 4 mm is regarded as normal. AGEING cDuring life the size of the aorta increases. The normal expansion rate is about 1-2 mm/year. It involves all segments which, during childhood and in young adulthood, result in an increase of the luminal diameter of the entire aorta (figs 1 and 2). 3 In adulthood the aortic size is related to exercise and workload. The ageing of the aorta is accompanied by a loss of compliance, and an increase of wall stiffness caused by structural changes including an increase in the collagen content and formation of intimal atherosclerosis with calcium deposits.1 4 5 As a result pulse pressure is enhanced and pulse wave velocity raised, decreasing the resulting organ perfusion, particularly the predominant diastolic myocardial perfusion.Aortic sclerosis is subdivided into four stages: c intimal thickening c atheroma formation c protruding plaques c intimal flap and/or thrombus formation.Aortic sclerosis risk factors are hypertension, hyperlipidaemia, diabetes mellitus, and smoking. A thickness up to 7 mm is regarded as the upper normal tolerance limit. 1The incidence of ruptured thoracic aneurysms in individuals aged 60-69 years is about 100 cases per 10 000, in those aged 70-79 years it is about 300/10 000, and in those aged 80-89 years the incidence is 550/10 000. 6 DEFINITIONSAn enlargement of the aortic diameter exceeding at least 50% of the normal range represents an ectasia, which results in aneurysm formation when the ectasia exceeds tolerance limits. 5There are different forms of aneurysms: c true aneurysm means enlarge...

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