Introduction to Acoustics

21.5.3 Agitated Saline
and Patent Foramen Ovale (PFO)
Agitated saline contrast bubbles are generated by forcing
fluid rapidly through a small orifice, such as an injection
needle. They have an important diagnostic application:
to determine whether there is a shunt from the right heart
to the left heart, bypassing the lungs. The most common
of these shunts is the patent foramen ovale (PFO). PFO
may be a cardiovascular defect of considerable medical
importance. If a person under the age of 50 has a stroke
due to occluded vessels in the brain (ischemic stroke),
the most likely cause is a PFO. One affected group is
divers, who return from great depths with large amounts
of gas dissolved in their body tissues, especially in fat.
Bubbles which form in the low-pressure venous system
as the gas is transported toward the lungs, might pass
through a PFO during a cough, releasing these large bubbles
into the arterial system where they cause occlusion
of arteries and ischemia of the tissues that were supplied
by the arteries. Ischemia of some tissues might be unnoticed.
Ischemia of other tissues might cause pain that is
ignored. Ischemia of some portions of the brain might
result in undetected changes in personality or memory.
Only ischemia of the motor and sensory cortices of the
brain causes loss of sensory and motor function that are
recognized as stroke symptoms. In divers, these symptoms
are called neurological symptoms of the bends, in
others they are called transient ischemic attacks (TIA)
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Medical Acoustics 21.5 Medical Contrast Agents 885
or stroke, depending on whether the symptoms last less
than 24 h or longer than 24 h, respectively.
The foramen ovale is a normal structure before birth,
along with the ductus arteriosus, allowing the blood to
circulate in parallel through the fetal body, lungs and
placenta. At birth, with the muscular constriction of
the ductus arteriosus to obliterate the lumen, pressure
shifts cause the flap valve of the foramen ovale to close
and over time it will seal in most people. However, in
some people the foramen ovale does not seal, opening
occasionally when the person coughs or strains. The
prevalence of PFO in the general population is 30% at
age 20 and 10% at age 70. The reason for the decline in
prevalence is either because in some people the foramen
seals after age 20 or because these people suffer repeated
embolic events from the venous to the arterial circulation,
which progressively damage body organs leading
to failure and death. In the absence of PFO, such venous
emboli pass to the lungs where they temporarily occlude
portions of the lung, but resolve over time.
The importance of the diagnosis of PFO is currently
disputed, but two factors may make testing for PFO
a part of every physical examination:
1. the cost of the contrast agent is only the 2 min that it
takes to handle off-the-shelf saline and
2. recently catheter methods of sealing a PFO have
been released for testing, avoiding the need for openchest
heart surgery to seal the defect. If 2/3 ofthe
V V
Fig. 21.66 Two-dimensional B-mode real-time echocardiogram of the left ventricle. Left: left ventricle without ultrasound
contrast agent. Right: left ventricle with intravenously infused Optison ultrasound contrast agent (courtesy Frances DeRook
and Keith Comess)
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Part F 21.5