Porth's Essentials of Pathophysiology, 4e

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Disorders of Blood Flow and Blood Pressure

C h a p t e r 1 8

Marfan syndrome. It also may occur during pregnancy because of changes in the aorta that occur during this time. Other factors that predispose to dissection are congenital defects of the aortic valve (i.e., bicuspid or unicuspid valve structures) and aortic coarctation. Aortic dissections are commonly classified into two types, A and B, as determined by the level of dissec- tion. 1,25 Type A aneurysms, which involve the proximal aorta (ascending aorta only or both the ascending and the descending aorta), are the most common and poten- tially serious in terms of complications. Type B aneu- rysms usually begin distal to the subclavian artery and do not involve the ascending aorta. Dissections usually extend distally from the intimal tear. When the ascending aorta is involved, expansion of the wall of the aorta may impair closure of the aortic valve. There also is the risk of aortic rupture with blood moving into the pericardium and compressing the heart. Although the length of dis- section varies, it is possible for the abdominal aorta to be involved with progression into the renal, iliac, or femoral arteries. Partial or complete occlusion of the arteries that arise from the aortic arch or the intercostal or lumbar arteries may lead to stroke, ischemic peripheral neuropa- thy, or impaired blood flow to the spinal cord. A major symptom of a dissecting aneurysm is the abrupt presence of excruciating pain, described as tear- ing or ripping. 1,2,25 Pain associated with dissection of the ascending aorta frequently is located in the ante- rior chest, and pain associated with dissection of the descending aorta often is located in the back. In the early stages, blood pressure typically is moderately or markedly elevated. Later, the blood pressure and pulse rate become unobtainable in one or both arms as the dissection disrupts arterial flow to the arms. Syncope, hemiplegia, or paralysis of the lower extremities may occur because of occlusion of blood vessels that sup- ply the brain or spinal cord. Heart failure may develop when the aortic valve is involved. Diagnosis of aortic dissection is based on history and physical examination. Aortic angiography, transesoph- ageal echocardiography, CT scans, and MRI studies aid in the diagnosis. The treatment of dissecting aor- tic aneurysm may be medical or surgical. Aortic dis- section is a life-threatening emergency; persons with a probable diagnosis are stabilized medically even before the diagnosis is confirmed. Two important factors that participate in propagating the dissection are high blood pressure and the steepness of the pulse wave. Without intervention, these forces produce continued extension of the dissection. Medical treatment therefore focuses on control of hypertension and the use of drugs that lessen the force of systolic blood ejection from the heart. 25 Two commonly used drugs, often given in combination, are an intravenous β -adrenergic blocking drug and sodium nitroprusside (a vasodilator). Adequate pain control is necessary to alleviate anxiety and relieve sympathetic stimulation, which will raise blood pressure. Surgical treatment consists of resection of the involved segment of the aorta and replacement with a prosthetic graft. Despite recent advances in diagnosis and treatment, the mortality rate in acute aortic dissection remains high.

SUMMARY CONCEPTS

■■ Disorders of the arterial circulation produce ischemia due to narrowing and obstruction of blood vessels, thrombus formation associated with platelet adhesion, and weakening of the vessel wall with development of an aneurysm. ■■ Cholesterol and triglycerides are transported within lipoproteins, macromolecules made up of a hydrophobic lipid core surrounded by an apoprotein-containing outer shell.The high- density lipoproteins (HDLs), which are protective, remove cholesterol from the tissues and carry it back to the liver for disposal.The low-density lipoproteins (LDLs) carry cholesterol to the liver and extrahepatic tissues to be removed from the blood. Low-density lipoproteins that are not removed from the blood are taken up by phagocytic scavenger cells in the arterial wall, leading to an accumulation of cholesterol-laden macrophages and development of atherosclerosis. ■■ Atherosclerosis is a progressive arterial disease characterized by the formation of fibrofatty plaques in the inner lining of large and medium- sized arteries, including the aorta, coronary arteries, and cerebral vessels.The major risk factors for development of atherosclerosis are hypercholesterolemia and inflammation. ■■ The vasculitides are a group of vascular disorders characterized by inflammation and necrosis of the blood vessels in various tissues and organs of the body.The inflammatory process may be initiated by direct injury, infectious agents, or immune mechanisms. ■■ The peripheral arterial disorders, such as Raynaud phenomenon and Buerger disease, interrupt arterial flow of blood and interfere with the delivery of oxygen and nutrients to the tissues. Occlusion of flow can result from a thrombus or emboli, vessel compression, vasospasm, or structural changes in the vessel. ■■ Aneurysms are localized areas of vessel dilation caused by weakness of the arterial wall. A berry aneurysm is a small spherical dilation usually found in the circle ofWillis in the cerebral circulation. The most serious consequence of thoracic and abdominal aortic aneurysms is rupture. A dissecting aneurysm is an acute, life-threatening condition. It involves tearing (dissection) of the tunica intima, which allows formation of a blood-filled channel between the layers of the vessel and reduces blood flow through the vessel’s true lumen.