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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.
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.