C h a p t e r 1 8
Disorders of Blood Flow and Blood Pressure
419
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.
