Medical Physiology: Integration Using Clinical Cases
689
drogen ions (pH), and hemoglobin. The fi ndings are shown
in
Table 19–2
.
Refl
ect and Review
#8
What is the cause of the change in arterial pH in
our patient (see Table 14–8)?
The results of these tests reveal that the patient has hypoxic hy-
poxia (hypoxemia), as indicated by the low arterial
P
O
2
, and has
an acute respiratory alkalosis, as indicated by the low arterial
P
CO
2
and bicarbonate, and high arterial pH. The normal hemo-
globin concentration indicates that the patient is not anemic.
Refl
ect and Review
#9
What are some possible causes of hypoxemia (see
Table 13–11)?
What are the two main types of alkalosis (see Table
14–8)?
How do we know the alkalosis in our patient was
acute (of recent, short-term origin) (see Table 14–8)?
The patient is given 100 percent oxygen to breathe through a
mask over his mouth and nose. This results in an increase in
arterial
P
O
2
to 205 mmHg, a small increase in arterial
P
CO
2
to 32 mmHg, and a small decrease in arterial pH to 7.48.
The normal response to breathing 100 percent oxygen is an
increase in arterial
P
O
2
to greater than 600 mmHg, and no
change in arterial
P
CO
2
or pH.
Refl
ect and Review #10
Explain why increasing arterial
P
O
2
with
supplemental oxygen caused the observed changes in
arterial
P
CO
2
and pH (see Figure 13–40).
Diagnosis
Because a heart attack has been ruled out, the physician sus-
pects that the patient has at least one
pulmonary embolism.
An
embolism
(plural, emboli) is a blockage of blood fl ow
through a blood vessel produced by an obstruction. It is of-
ten caused by a blood clot—or
thrombus
—in the pulmonary
arteries/arterioles. These clots usually arise from larger clots
in leg veins.
To confi rm his diagnosis, the physician orders a
ventilation-
perfusion scan.
In the ventilation scan, the patient inhales a
small amount of radioactive gas. Special imaging devices are
then used to detect the inhaled radiation and visualize which
parts of the lung are adequately ventilated. Poorly ventilated
areas of the lung will contain less radioactive gas. In the per-
fusion scan, a small amount of albumin, a naturally occurring
plasma protein, tagged with a radioactive tracer, is injected
into a vein. As the radioactive protein enters the pulmonary
circulation, its distribution can be monitored using the same
imaging device as just described. This procedure allows the
physician to determine if parts of the lungs are receiving less
than their normal share of blood fl ow because poorly perfused
areas of the lungs will contain less radioactive albumin. The
ventilation scan was normal, but the perfusion scan showed
abnormalities.
Figure 19–3
shows the results of the perfusion
scan, demonstrating dramatic decreases in perfusion in specif-
ic regions of the lung. These results supported the physician’s
diagnosis of several pulmonary emboli.
There are a variety of materials that can occlude pulmo-
nary arterial blood vessels, including air, fat, foreign bodies,
parasite eggs, and tumor cells. The most common embolus is
a thrombus that can theoretically come from any large vein,
but usually comes from the deep veins of the muscles in the
calves (
deep vein thrombosis
). The fact that our patient sat
on an 8-h fl ight without moving around greatly increased the
chances for the formation of a deep vein thrombosis in the leg.
This is because blood that is allowed to pool in the leg veins
has a tendency to form clots (see Figure 12–45). After the
abnormal lung perfusion scan, an ultrasound examination of
the legs was performed to confi rm whether clots were present
in the leg veins. The results showed a large clot in the femoral
and popliteal veins in the right leg.
Pulmonary embolism is a common and potentially fatal
result of deep vein thrombosis. In fact, pulmonary embolism
and deep vein thrombosis can be considered part of one syn-
drome. It may cause as many as 200,000 deaths each year in
the United States. Most cases are not diagnosed until after
death (on post-mortem examination) either because the symp-
toms are initially mild or because the syndrome is misdiag-
nosed. Most small clots that form in small veins in the calves
of the lower legs remain fi xed in place associated with the lin-
ing of the vein, and do not cause symptoms. However, if a clot
enlarges and migrates into larger veins such as the femoral and
popliteal veins, as in our patient, it can break off and migrate
up the vena cava, through the right atrium and right ventricle,
and into the pulmonary arterial circulation, where it can be-
come lodged (see Figure 12–2 for an overview of the circula-
tory system). When this happens, blood fl ow is reduced or cut
off to one or more large segments of the lung.
Refl
ect and Review
#11
Why will regional decreases in pulmonary blood
fl ow lead to hypoxemia (see Figure 13–24 and Table
13–11)?
Fortunately, these clots are too large to pass through the pul-
monary circulation into the systemic circulation. When clots do
form on the arterial side of the circulation, they can occlude
(a)
Normal
(b)
PE
Figure 19–3
Pulmonary embolism (PE) from a deep vein thrombosis shown on
a lung perfusion scan (posterior) with radiolabeled albumin. (a) A
normal perfusion scan. (b) Multiple perfusion defects are shown
(arrows).
Source: From Access Medicine On-Line–Current Medical Dx & Tx. McGraw-Hill, 2006.
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