SPECIAL SITUATIONS AND EXTRAVASCULAR SOURCES OF PULMONARY EMBOLI
The risk of thrombosis among cancer patients is substantially higher than in the general population and may be even higher in those receiving chemotherapy. Patients with cancer and venous thromboembolism are more likely to develop recurrent thromboembolic complications and major bleeding during anticoagulant therapy than those without malignancy. Low- molecular-weight heparin (LMWH) may be more effective than warfarin in patients with cancer. LMWH should be considered for the first 3 to 6 months of therapy, and anticoagulant therapy should be continued indefinitely or until cure of the cancer.
PREGNANCY
Pulmonary embolism (PE) is an important potential
complication of pregnancy and is associated with substantial risk to the
mother. A complete discussion of the diagnosis and management of PE in
pregnancy is beyond the scope of this section.
FAT EMBOLISM
The most common cause of fat embolism is trauma to
bones, particularly the long bones of the legs. Fat embolism may also be
associated with air emboli in decompression sickness (caisson disease).
Microscopically, the fat emboli can be demonstrated with fat stains such as
Sudan III or IV or oil red O. With these stains, they appear as red-orange
droplets, several microns in diameter, filling the small arteries and alveolar
capillaries. With routine stains, they appear as optically clear spaces in the
vascular lumina. Clinically, fat embolism is often associated with acute
respiratory failure (adult respiratory distress syndrome). Cutaneous and
conjunctival petechial hemorrhages and embolism of retinal vessels are found in
about half the cases.
Bone marrow embolism is also a frequent complication of severe bone trauma
or fracture.
AMNIOTIC FLUID EMBOLISM
This relatively rare condition is caused by the
massive leakage of amniotic fluid into the uterine veins. The amniotic fluid
reaches the uterine venous circulation either as a result of vigorous uterine
contraction after rupture of the membranes or through tears or surgical
incisions in the myometrium or endocervix. Clinically, the condition is
characterized by sudden dyspnea, cyanosis, systemic hypotension, and death
during or immediately after delivery. The mechanism of death is not clear because
the emboli consist of a suspension of epithelial squamae, lanugo, and cellular
debris, usually occluding a few small blood vessels. Death has been attributed
to either anaphylactoid reaction to the amniotic fluid or disseminated
intravascular coagulation caused by activation of the clotting mechanism by
amniotic fluid thromboplastin.
AIR EMBOLISM
Air may be sucked into veins during attempts at
abortion, after chest injury as a result of a motor vehicle accident, during
the induction of artificial pneumothorax or pneumoperitoneum, placement of
central venous catheters, and in a number of other circumstances. The effects
of air embolism depend on the amount of air that reaches the circulation and
the rapidity of its entry. The volume of air necessary to cause death in humans
is usually more than 100 mL. In debilitated persons, a smaller volume of air
may be fatal. Death is caused by blockage by an air trap in the outflow tract of
the right ventricle, but small air bubbles can be seen in small pulmonary blood
vessels.
FOREIGN BODY EMBOLISM
A wide variety of organic or inorganic substances may
enter the venous circulation and reach the lungs. This type of embolism has become common
in certain groups addicted to narcotic drugs. The drugs are rarely chemically
pure and are frequently adulterated with vegetable seeds, talc, and other
substances.
As with the other kinds of emboli, the effect of
foreign bodies depends to a great extent on the rapidity and extent of
embolization. Such particles as fibers or talc are likely to produce an
inflammatory response in the wall of small pulmonary arteries, with formation of
foreign body granulomas composed of macrophages and multinucleated giant cells.
The granuloma may cause partial or total occlusion of the involved blood vessel.
If several blood vessels are involved, pulmonary vascular resistance may become
elevated and lead to pulmonary
hypertension.
