C.
DISEASES OF AORTA
The aorta is the largest vessel which conducts the blood toward
the whole body. Its diameter is about 2 to 3 centimeters (just like
a ring made of thumb and index finger). The aorta starts from the
heart and is directed toward the head, then makes a U-turn below the
neck, where it gives off three branch arteries to the head and arms.
Then it runs downwards near the back toward the legs. As it passes
the diaphragm and enters the abdomen, it gives off several branch
arteries to the visceral organs, then divides into two arteries that
are directed to bilateral legs. Thus, the aorta is an important
route to deliver the blood loaded with oxygen and nutrition to the
entire body.
Diseases of the aorta, an important lifeline to every organ,
affect many organs at the same time. It is not simply the vascular
disease but can be a systemic disease. Main diseases include aortic
dissection and aortic aneurysm.
1) AORTIC DISSECTION
1) What is aortic dissection?
The wall of aorta consists of three layers: intima, tunica media,
and adventitia. The tunica media is most thick and tough. However, a
tear can appear at the intima and tunica media and dissect the
aortic wall to external and internal wall. This is aortic
dissection.
# Complication of aortic dissection
Thickness of the external wall becomes less than half of that of
the intact aorta. The external wall can burst to massive hemorrhage
and may be responsible for sudden death that occurs in 20 to 30% of
patients. The internal wall flaps and occasionally obstructs the
branch artery that arises from the aorta. As mentioned before, every
branch artery from the aorta supplies important organs. When such
artery is suddenly occluded, these organs are suddenly damaged. For
example, occlusion of coronary artery leads to myocardial
infarction, and that of carotid artery causes cerebral infarction.
In this way, aortic dissection can be complicated with either or
both of rupture and obstruction.
Among various types of aortic dissection, Stanford type A
dissection involves ascending aorta close to aortic valve and is
very dangerous. This type of dissection often leads to serious
complications of heart and/or brain and has so poor prognosis that
most of the patients die within three days.
# Treatment of aortic dissection
Treatment of aortic dissection is to replace the portion of aorta
with tear with vascular prosthesis. In this treatment, the blood
flow to the heart or brain needs to be temporarily stopped while the
aorta is manipulated. Therefore, this treatment can be complicated
with cerebral or myocardial infarction.
During this surgery, unexpected complications can occur in other
organs, such as ischemia of intestine or kidney. Although surgeon
expects that every organ is adequately perfused, dissection might
alter the blood flow and cause significant malperfusion. Thus, it is
important to monitor what is going on in the body. Otherwise,
complication can gum up successful surgery.
We have considered how we can detect such unexpected complications
that can occur behind the surgeon's view during cardiovascular
surgery. In our hospital, we have introduced intraoperative
ultrasonography such as transesophageal echocardiography in a
positive manner for 20 years, as intraoperative monitoring and
diagnostic imaging, and have minimized the blind zones during
surgery. Value of this modality depends on how one can utilize it to
detect essential findings or changes. Both surgeons and
anesthesiologists have been eager to learn and put it in practice,
as well as they have developed innovated way of utilizing it. We
have obtained a number of new clinical data and have presented these
results in the Scientific meetings and journals. Consequently,
surgery have become more safe and secure recently (mentioned in
detail later).
2) AORTIC ANEURYSM
Aortic aneurysm is dilatation of a portion of aortic wall like a
boss due to internal pressure of the aorta. Because the aneurysmal
wall is always exposed to high pressure inside, the aneurysm grows
and suddenly blasts.
Aortic
aneurysm is formed at various sites and is named, according to the
site, as ascending aortic aneurysm, arch aneurysm, descending aortic
aneurysm, thoracoabdominal aortic aneurysm, and abdominal aortic
aneurysm.
# Cause of aortic aneurysm
Causes of aortic aneurysm include atherosclerosis, aortic
dissection, and trauma.
The most popular cause is atherosclerosis. Most portion of the
aorta becomes rigid with calcification but some portion becomes
fragile. The latter gradually protrudes toward outside.
Atherosclerosis is related to metabolic syndrome.
Aortic dissection was described above. Although the aorta quits of
rupture, it may become aneurysmal due to its weakness (dissecting
aneurysm).
Traumatic injury is caused by traffic accident and other trauma.
The aortic wall is torn and forms aneurysm which is very close to
rupture.
# Symptoms of aortic aneurysm
It is problematic that there is no specific symptom of aortic
aneurysm. Because there is no symptom, the aneurysm grows without
being recognized and suddenly blasts. However, unspecific symptoms
appear in some patients.
a. Compression to the surrounding tissue
Aortic arch aneurysm may compress the recurrent nerve that courses
beside the aneurysm and reaches the vocal cord. Hoarseness,
dysphagia, or hemoptysis appears. The abdominal aorta aneurysm can
cause abdominal pain or back pain.
b. Rupture
As the aorta blasts, severe pain appears in the chest, back,
abdomen, or lumbar. Blood pressure drops and one loses
consciousness.
c. Ischemia due to dissection
In dissecting aneurysm, inadequate perfusion of the branch artery
can lead to syncope, chest pain, abdominal pain, coldness, palsy, or
pain of fingers or legs.
# Indication of surgery for aneurysm and outcomes
It is impossible to treat aortic aneurysm with drugs. To prevent a
death by rupture, surgical treatment or stent-graft therapy is
necessary. The aneurysm is larger, the risk of rupture is higher.
The saccular type is more likely to rupture than the spindle type.
Indication of surgery because of risk of rupture is as follows. ．
- Thoracic aneurysm
When the maximal diameter of aneurysm is larger than 55mm, surgery
is strongly recommended. In Marfan syndrome with higher risk of
rupture, aneurysm of larger than 50mm is indicated for surgery.
Traumatic aortic injury is indicated for emergency operation because
it easily blasts even if it is small. Dissecting aneurysm is
indicated for surgery when it is larger than 60mm.
- Thoracoabdominal aortic aneurysm
This type of aneurysm extends from thoracic to abdominal aorta.
Surgical treatment of this type is difficult because this region
often gives off an artery to the spinal cord. Indication of surgery
is similar to other aneurysms: larger than 55mm. In our hospital, we
do all kinds of things to perform this surgery without complication
such as paraplegia (details described in CONSIDERATIONS IN AORTIC
ANEURYSM SURGERY)
- Abdominal aorta aneurysm
Surgery is recommended when the aneurysm is larger than 50mm.
However, earlier surgery is recommended when the aneurysm is rapidly
growing or is protruding type of aneurysm.
# Surgical treatment of aortic aneurysm
Standard treatment of aortic aneurysm is to replace the diseased
portion of aorta with vascular prosthesis. The aorta is
cross-clamped above and below the aneurysm with a special clamp, the
aneurysm is cut open, and the graft (vascular prosthesis) is sutured
to the normal portion of aorta. The graft is a tube made of chemical
fabric. The graft and aorta is sutured with nylon-like string. After
the clamp is released, the blood flows through the graft.
In thoracic aneurysm, simple cross-clamping of aorta is difficult
because the aorta is close to the heart and perfusion to the lower
body or brain is stopped by cross-clamping. In this surgery, a
special extracorporeal circulation, is used for maintaining the
blood flow to these portion. By using artificial lung and blood
pump, the blood is passed to the system outside the body and sent to
the lower body and brain. It is one of the most complicated
operation.
CONSIDERATIONS IN AORTIC ANEURYSM SURGERY
1. Aortic arch aneurysm surgery with frozen
elephant trunk
A stent graft (vascular prosthesis with internal spring-like stent
at the tip) is placed into the aorta and opened at the normal-sized
aorta. The graft is fixed to the aortic wall with the expanding
force of the stent. The other side of the graft is sutured to the
aortic wall. In the conventional method, the left chest was widely
opened, the lung was compressed and the graft was sutured in the
deep surgical field. This new method has reduced the stress on the
lung and risk of bleeding at the deep anastomosis.
We have applied this method to about 40 patients. The longest
follow-up is longer than 9 years. The outcomes of this method are as
follows.
- If the graft is successfully implanted without leak between the
graft and aorta, the aneurysm portion is unloaded and shrinks
or occasionally disappears. Aortic dissection also disappears and
the aortic wall becomes normal.
- If the leak remains, the aneurysm is gradually dilated.
- Special attention should be paid for cerebral infarction and
spinal cord damage.
Although this method was introduced in many hospitals, a number of
complications occurred such as cerebral infarction, spinal cord
damage, rupture of aorta, massive leak, and so on. Most of them have
abandoned this method. We considered that such undesirable events
were caused by an insertion of stent graft into the descending aorta
in a blind way. We have used transesophageal echocardiography from
the beginning: 1) to visualize the blind portion of aorta
(descending aorta) to everybody while the surgeon manipulated the
catheter and graft; 2) to determine an appropriate graft size; and
3) to locate the catheter tip accurately. As the result, we have
experienced no event of aortic wall damage. Leakage due to an
inappropriate graft size occurred in one early case. Accurately
locating of the graft end which is essential for avoiding spinal
cord damage was achieved by meticulous guiding with transesophageal
echocardiography. Complete paraplegia occurred only in one early
case.
2. Stent graft implantation with catheter method
A stent graft is introduced to the aorta with a catheter through
the femoral artery and delivered to the thoracic aorta, and the
aortic aneurysm is covered with stent graft. The graft is fixed to
the aorta by the expanding force of the stent. Compared with
surgical treatment, stress on the body is extremely small. This
method is started in many hospitals for abdominal aorta aneurysm or
even thoracic aorta aneurysm. However, one should recognize not only
its merit but also its demerit.
Problems of this method is listed.
- In the patient with aortic aneurysm, the internal surface of
aorta is often covered with debris, called atheroma. By manipulating
catheters in the aorta, these debris can be detached and cause
embolism of visceral organs and legs.
- The graft is fixed to the aorta only by the expanding force of
stent. It is not certain how secure the stent holds the graft in the
high pressure blood flow. In some patients, the stent graft can
migrate.
- When the treatment is successful, the aneurysm shrinks. This
in turn can lead to migration or twisting of the stent graft.
- Some aneurysm gives off branch artery. After stent graft is
placed, blood flow can enter the aneurysmal cavity through this
artery and result in sustained pressure loading to the aneurysm.
In spite of these problems, this method is much less invasive and
suitable for the patients who cannot tolerate a major surgery. When
we select one treatment, we give first priority to "safety and
certainty". Thus, for the moment, we apply the catheter stent
grafting only to the aneurysm of thoracic descending aorta. As new
graft and catheter with better and more stable quality ideveloped,
we are to expand the indication of this method.
3. Prevention of stroke in thoracic aortic aneurysm
surgery
In surgery for thoracic aortic aneurysm, blood flow to the brain
is maintained with artificial heart-lung machine because the
surgical manipulation involves the carotid artery that delivers the
blood to the brain. If the blood flow is disrupted for any reason
during surgery, stroke ensues despite of successful aortic surgery.
It is essential to detect any event of inadequate brain perfusion as
soon as it occurs and to solve the problem early. In aortic
dissection, diseases often reach the carotid artery and the internal
membrane can occlude the carotid artery.
Conventional monitoring of brain perfusion is pressure monitoring
in the temporal artery. However, this demonstrates the blood
pressure outside the skull and does not necessarily reflect the
blood flow in the skull. We have explored other method that directly
indicates the blood flow in the skull and have established the
current method.
a) Brain perfusion monitoring with near-infrared
spectroscopy (NIRS)
A new monitor with NIRS was developed in 1990s. The figure shows
one model of such monitoring device. In this monitor, near-infrared
light is emitted from the sensor that is placed on the forehead. The
light reaches the brain through the skull. A portion of this light
is reflected and returns to the sensor through the skull. By
analyzing the returned near-infrared light, oxygen debt in the brain
tissue (oxygen saturation [%]) can be calculated. The data are
expressed as a trend graph on the screen.
When the brain perfusion becomes inadequate, oxygen saturation
drops. It is continuously monitored throughout surgery. Decrease in
oxygen saturation indicates that oxygen is insufficient in the brain
tissue. The cause needs to be explored and solved. Through clinical
study, we have clarified that sustained drop of oxygen saturation
below 60% is related to an increased incidence of brain
complications. Therefore, cause of oxygen insufficiency needs to be
removed as soon as possible. The most concern is reduced blood flow
in the brain. For assessing it, we use ORBITAL DOPPLER METHOD.
b) ORBITAL DOPPLER METHOD
The brain is surrounded by rigid skull and makes it difficult to
directly measure the blood flow in it. Standard method is
transcranial Doppler (TCD). However, it is difficult to obtain a
good Doppler signal with this method during surgery, because the
blood flow is usually low.
Thus, we have developed a novel method of assessing the blood flow
of brain tissue in the eye, which is called "the only site where one
can see the brain directly from the outside". As the eye is
visualized with ultrasonography, the artery in the optic papilla can
be clearly visualized with Doppler method through the lens and
vitreous body. Although this artery is only 1mm in diameter, it can
be clearly displayed because the eye ball itself transmits the
ultrasound freely. Changes in blood flow can be readily recognized.
Attention should be paid that ultrasound can damage the cornea if it
is exposed to the ultrasound energy for a long time. Therefore, we
usually finish the checkup within 10 seconds. No complication
related to this method is encountered so far.
Clinical investigation has demonstrated that incidence of brain
complication increases as the duration of time without detectable
blood flow in the eye is longer. This means that one should solve
the problem as soon as possible when the blood flow becomes
undetectable. However, orbital Doppler method does not provide a
clue to the cause of malperfusion. For explore the cause of
malperfusion, we use transesophageal echocardiography.
c) Transesophageal
echocardiography
Transesophageal echocardiography (TEE) is one of
echocardiography (a kind of ultrasonography). It visualize the
organs in the body with ultrasound as abdominal ultrasonography or
ordinary echocardiography. It is a safe examination without exposure
to radiation as fluoroscopy. It appears like an endoscope as one
uses for gastrointestinal fiberscopy. A transducer, that works as an
eye for visualization, is equipped near its tip. This probe is
inserted into the mouth and esophagus under anesthesia to visualize
the heart and vessels during surgery. Unlike ordinary
echocardiography, it does not interfere with surgical procedures and
provides very clear images. In our hospital, TEE was introduced 20
years ago. Not only it has been utilized in the operating room in a
positive way, a number of new clinical applications have been
developed to provide clinically useful information during surgery
and they have been reported in the scientific meetings and journals
in Japan and abroad.
By using TEE, the carotid artery is visualized to explore the
cause of decreased brain perfusion. The carotid artery had been
considered to be unvisualized zone for TEE, until we developed the
technique to visualize it and reported it to international journal
in 2000. Because this artery is situated beside the esophagus, it
can be more precisely visualized with TEE than with CT or MRI. What
is better, TEE can assess the blood flow without contrast media as
in CT or angiography.
We have established "3-stage monitoring" by combining TEE with
above two methods, orbital Doppler and NIRS. Recently this
monitoring is routinely used for assessing the brain perfusion
during surgery. This has often elucidated various events in the
vessels that cause malperfusion of the brain. Because these causes
can be specifically solved, incidence of neurological complications
has been reduced.
4. Prevention of spinal cord damage in
thoracoabdominal aorta surgery
Thoracoabdominal
aortic aneurysm, that is aneurysm extending from thoracic to
abdominal aorta, is treated with replacement of aneurysm with
vascular graft. However, spinal cord damage due to ischemia can be
complicated with this surgery. This is because an important artery
that supplies blood flow to the spinal cord can arise from this
replaced portion of aorta. The intercostal or lumbar artery
of the aneurysmal aorta is usually sutured. In many patients, an
important one arises around the diaphragm, called as Adamkiewicz
artery.
The blood flow for the spinal cord is supplied from three levels:
around the neck, diaphragm (Adamkiewicz artery), and lumbar.Because
these three arteries often has poor communications to each other,
occlusion of Adamkiewicz artery can lead to permanent damage of
spinalcord that presents with araplegia (palsy of lower body).One is
obliged to have life on the wheelchair for the rest of life.Sense of
urination and evacuation is also lost.Often decubitus develops on
the buttock with considerable infection.
Incidence of such tragedy is reported to be several to 20 %. If
the aneurysm is left untreated, it will blast soon. Emergency
operation for ruptured aneurysm is complicated with higher risk of
complications. Many scientists have searched for solution for this
problem in all over the world. This disease is not so simple that
can be treated in any hospital.
Current strategy for this disease is described.
a) Identification of Adamkiewicz
artery with CT
The Adamkiewicz artery is searched with preoperative CT and is
found in 80 to 90% of patients. Location of this artery is confirmed
before surgery so that it is identified during surgery. However,
this artery cannot be visualized with CT.
b) Motor-evoked potential
monitoring
We always have a concern that CT might not find every
artery to be identified. Thus, we use a special monitor, called as
motor-evoked potential (MEP) monitoring during surgery.
An electrode is inserted from the back into the vicinity of spinal
cord the day before surgery. During surgery, an electrode is placed
on the head and electric stimulation is given. Excitation is
conducted from the brain down to the spinal cord. This signal is
recorded with the electrode near the spinal cord.
If the Adamkiewicz artery is unintendedly injured during surgery
or blood flow of this artery with unusual course is blocked, the
spinal cord becomes ischemic and conduction of excitation is
blocked. When MEP is reduced, this indicates that something goes
wrong. We experienced several patients that escaped the spinal cord
damage with this monitoring.
c) cerebrospinal fluid drainage
Even if the Adamkiewicz artery is preserved, the spinal cord
suffers from a certain degree of hypoperfusion. The spinal cord can
be swollen for several days after surgery. Because the spinal cord
is surrounded by a duct (spinal canal) made of rigid bone, swollen
spinal cord is compressed by the surrounding tissue or fluid and
becomes ischemic. Within several days of surgery, palsy of lower
body can occur. To prevent this, a fine tube is placed into the
spinal canal through the back for letting the fluid out. This is
called as cerebrospinal fluid drainage. Several patients escaped
palsy with this treatment.
With all these efforts, the incidence of paraplegia in our
hospital is as low as 5% (much lower recently). However, it is not
perfect yet. Even a top level hospital in the world has 1 to 2% of
incidence. Investigation is still going for achieving a safer
surgery. While we treat the patients with best possible method and
technology, clinical data are being collected for analysis toward a
better certainty. In addition, we develop further methods with
animal studies. Some of new methods may be applied to the clinical
use in the near future. We believe that such attitude and efforts
are mandatory for those who treat the thoracoabdominal aortic
aneurysm. This is a typical disease that treatment should produce
something new for the future.
Even with novel techniques, emergency operation can be accompanied
with higher incidence of complication because preoperative
preparations are not sufficient. In this sense, what is important is
early detection and early treatment.
5. Strategy for bowel ischemia in aortic
dissection
This is another problem to be solved and some measures are needed.
When internal membrane occludes the artery that supplies the bowel
(superior and inferior mesenteric artery), bowel ischemia occurs. If
blood flow cannot be restored within several hours, the bowel
becomes necrotic. It leads to peritonitis, hepatic failure, or renal
failure, resulting to death within several days.
On the other hand, however, risk of aortic rupture necessitates
replacement of aorta early. One should determine which should be
treated first. However, diagnosis of bowel ischemia is not easy. The
CT images obtained in the previous hospital are not necessarily of
sufficient quality to make a diagnosis of ischemia. Usually the
condition of the patient is not stable enough to take another CT
examination. What is worse, bowel ischemia can newly occur in the
operating room often due to extracorporeal circulation (artificial
perfusion). It is impossible to have a CT examination in such a
situation.
We developed a novel method of making diagnosis of bowel ischemia
by using TEE. The abdominal aorta and visceral arteries were
considered to be out of the range of visualization with TEE.
However, we have found that some additional techniques have enabled
to visualize the superior mesenteric artery in the majority of
patients (over 90%) and reported to the scientific journal in 1999.
Observation is feasible during surgery without interrupting surgical
procedures and repeatedly or continuously without use of contrast
media.
Toward
the future
When we treat any disease, we do not just cure the patient but
always search for any better treatment. Every treatment should
produce any advancement in medicine: we believe this is important.
New ideas and findings are reported in the scientific meetings and
journals not only in Japan but in international base to confirm that
our output is not a complacence.
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