Aortic dissection occurs when an intimal tear allows blood to enter the media, creating a false lumen that propagates along the aortic wall. The Stanford classification divides dissections into Type A (involves ascending aorta regardless of origin — surgical emergency) and Type B (descending aorta only, distal to left subclavian — typically managed medically). The DeBakey classification further subdivides: I (ascending + descending), II (ascending only), III (descending only). The medial layer is weakened by cystic medial degeneration (loss of elastic fibers and smooth muscle cells), accelerated by hypertension, connective tissue disorders (Marfan, Ehlers-Danlos, Loeys-Dietz), and bicuspid aortic valve. Propagation is driven by shear stress (dP/dt) — the rate of pressure rise during systole. This is why both HR and BP must be reduced emergently. Complications include aortic rupture (most common cause of death), coronary malperfusion (MI), cerebral malperfusion (stroke), mesenteric ischemia, renal ischemia, and limb ischemia.