A thoracoabdominal aortic aneurysm is defined as a dilatation of the aorta involving the origins of the co-eliac, superior mesenteric or renal arteries. Crawford's classification system is universally accepted (Fig. 4.1.1).
Aneurysms arising proximal to the left subclavian or ending proximal to the level of the diaphragm should be classified as arch or descending thoracic aneurysms and are outside of the scope of this chapter. It is important to differentiate operations performed electively from those performed for acute dissection and free rupture, as mortality in these latter patients is higher.
Rupture of thoracoabdominal aortic aneurysms of less than 4 cm diameter is uncommon. In a follow-up study of 1600 patients the mean rate of aneurysm expansion was 1 mm per year and the risk of aneurysm rupture became significant when the aortic diameters exceeded 6 cm . The rate of expansion and size at which rupture occurs are consistent with past studies [7, 12, 18, 27]. Conservative management of thoracoabdominal aortic aneurysms of greater than 6 cm diameter has a poor prognosis: survival is as low as 40% at 1 year and 7% at 5 years [2, 8, 11].
Fig. 4.1.1 Safi's modification of Crawford classification of thoracoabdominal aortic aneurysms 
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