Porth's Essentials of Pathophysiology, 4e

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Disorders of Blood Flow and Blood Pressure

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phenomenon by excluding other disorders known to cause vasospasm. Treatment measures are directed toward eliminating factors that cause vasospasm and protecting the digits from trauma during an ischemic episode. Abstinence from smoking and protection from cold are priorities. The entire body must be protected from cold, not just the extremities. Avoidance of emotional stress is another important factor in controlling the disorder because anxiety and stress may precipitate a vascular spasm in predisposed persons. Vasoconstrictor medications, such as the decongestants contained in allergy and cold prep- arations, should be avoided. Treatment with vasodila- tor drugs may be indicated, particularly if episodes are frequent, because frequency encourages the potential for development of thrombosis and gangrene. A recent advancement in the treatment of Raynaud phenomenon therapy is phosphodiesterase inhibitors (e.g., sildenafil, tadalfil, vardenafil) that produce arterial vasodilation. Surgical interruption of sympathetic nerve pathways (sympathectomy) may be used for persons with severe symptoms. 21 Aneurysms and Dissection Aneurysm is a pathological outpouching or sac-like dilatation in the wall of a blood vessel usually caused by weakening of the vessel wall. Aneurysms can occur in arteries and veins, but they are most common in the arteries. There are two types of aneurysms. 1,23 A true aneurysm is bounded by a complete vessel wall. The blood in a true aneurysm remains within the vascular compartment. A false aneurysm represents a localized dissection or tear in the inner wall of the artery with for- mation of an extravascular hematoma that causes vessel enlargement (Fig. 18-10B). Unlike true aneurysms, false aneurysms are bounded only by the outer layers of the vessel wall or supporting tissues. Aneurysms can assume several forms and may be classified according to their cause, location, and ana- tomic features (Fig. 18-10). A berry aneurysm consists of a small, spherical dilation of the vessel at a bifur- cation (Fig. 18-10A). 1,2 This type of aneurysm usually is found in the circle of Willis in the cerebral circula- tion. A fusiform aneurysm involves the entire circumfer- ence of the vessel and is characterized by a gradual and progressive dilation of the vessel (Fig. 18-10C). These aneurysms, which vary in diameter (up to 20 cm) and length, may involve the entire ascending and transverse portions of the thoracic aorta or may extend over large segments of the abdominal aorta. A saccular aneurysm extends over part of the circumference of the vessel and appears saclike. A dissecting aneurysm is a false aneu- rysm resulting from a tear in the intimal layer of the ves- sel that allows blood to enter the vessel wall, dissecting its layers to create a blood-filled cavity. The weakness that leads to aneurysm formation may be caused by a number of factors, including congenital defects, trauma, infections, and atherosclerosis. Once initiated, the aneurysm grows larger as the tension in

Berry aneurysm

Aneurysm of abdominal aorta

A

Aortic dissection (longitudinal section)

C

B

FIGURE 18-10. Three forms of aneurysms: (A) berry aneurysm in the circle of Willis, (B) aortic dissection, and (C) fusiform-type aneurysm of the abdominal aorta.

the vessel increases. This is because the tension in the wall of a vessel is equal to the pressure multiplied by the radius (i.e., tension = pressure × radius; see Chapter 17). In this case, the pressure in the segment of the vessel affected by the aneurysm does not change but remains the same as that of adjacent portions of the vessel. As an aneurysm increases in diameter, the tension in the wall of the vessel increases in direct proportion to its increased size. If untreated, the aneurysm may rupture because of the increased tension. Even an unruptured aneurysm can cause damage by exerting pressure on adjacent structures and interrupting blood flow. Aortic Aneurysms Aortic aneurysms may involve any part of the aorta: the ascending aorta, aortic arch, descending aorta, thora- coabdominal aorta, or abdominal aorta. Multiple aneu- rysms may be present. The signs and symptoms of aortic aneurysms depend on the size and location. With both thoracic and abdominal aneurysms, the most dreaded complication is rupture. The likelihood of rupture corre- lates with increasing aneurysm size. The risk of rupture rises from less than 2% for small abdominal aneurysms