C h a p t e r 1 8
Disorders of Blood Flow and Blood Pressure
417
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
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
Aortic
dissection
(longitudinal
section)
Berry aneurysm
Aneurysm of
abdominal aorta
A
B
C
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.
