An aneurysm is defined as a permanent restrictive dilatation of the arterial vessel wall exceeding 50% of the normal vessel diameter. Therefore, a precise definition of abdominal aortic aneurysm (AAA) would require calculation of the ratio of normal to dilated abdominal aorta in the same individual, with correction for influencing factors such as age, sex, race and body surface area. Normally, AAA can be diagnosed in abdominal aorta with a diameter of more than 3 cm.
1.Natural course of disease
The natural course of AAA is the gradual enlargement of the aneurysm and the formation of an appendage thrombus due to the continuous turbulent flow of blood in the aneurysmal cavity. Therefore, the most common complications of AAA are: rupture of the aneurysm, embolism of distal organs and compression of adjacent organs.
1.1 Natural course of AAA
Epidemiological data show that when AAA is less than 4 cm in diameter, the annual growth rate is around 1 mm to 4 mm; when the tumor is 4 cm to 5 cm in diameter, the annual growth rate is around 4 mm to 5 mm; when the tumor is more than 5 cm in diameter, the annual growth rate is greater than 5 mm, and the final rupture rate of the tumor reaches 20%; if the tumor is greater than 6 cm in diameter, the annual growth rate of the tumor is 7 mm to 8 mm The ultimate rupture rate also increases to 40%. The risk of ruptured AAA is extremely high, with a mortality rate of 90%. Therefore, it is now generally accepted that surgery is required when the AAA is greater than 5 cm in diameter. In women, due to the thin diameter of the abdominal aorta, surgical treatment should be considered if the aneurysm is greater than 4.5 cm. In addition, if the AAA aneurysm diameter grows too rapidly and is greater than the aforementioned average value, early surgical treatment should also be considered. factors associated with AAA rupture, in addition to aneurysm diameter, include hypertension, chronic obstructive pulmonary disease, long-term smoking, female and positive family history, all of which increase the risk of AAA rupture.
1.2 Natural course of common iliac artery aneurysm
Isolated common iliac artery aneurysms without concomitant AAA are rare, and therefore epidemiological data on this subject are scarce. Approximately 1/2 to 1/3 of common iliac aneurysms are bilateral, and most patients are asymptomatic at the time of diagnosis. Common iliac artery aneurysms larger than 5 cm in diameter are prone to rupture and require surgical treatment. There are few reports of rupture of common iliac artery aneurysms less than 3 cm in diameter. Therefore, it is generally believed that common iliac artery aneurysms less than 3 cm in diameter only require close monitoring and regular review.
1.3 Local compression or erosion of AAA aneurysm
When the AAA aneurysm is large, it may compress the duodenum and cause symptoms of upper gastrointestinal obstruction such as difficulty in eating, and in severe cases, it may invade the duodenum to form a duodenal fistula and cause gastrointestinal hemorrhage, which is one of the most fatal complications of AAA. In addition, AAA can also compress the inferior vena cava or renal vein, and even occur as abdominal aorta-inferior vena cava or abdominal aorta-renal vein fistula, leading to acute heart failure and death.
2.Conservative treatment
2.1 Close monitoring
For AAA found after screening, if the tumor is less than 4cm in diameter, color Doppler ultrasound examination is recommended every 2 to 3 years; if the tumor is larger than 4cm in diameter but less than 5cm, close monitoring is needed, and color Doppler ultrasound or CT angiography examination is recommended at least once a year. Once the tumor is found to be more than 5 cm, or the tumor grows too fast during the monitoring period, early surgery is needed.
2.2 Drug treatment
After the diagnosis of AAA is confirmed, smoking should be strictly abstained during the observation period, and attention should be paid to controlling blood pressure and heart rate. It has been found that oral beta-blockers can reduce the rate of expansion of AAA caused by atherosclerosis, effectively reducing the rupture rate and mortality due to perioperative adverse cardiac events, which is the only drug proven to be effective in the conservative treatment of AAA. The rationale may be that by slowing down the heart rate, the intra-aortic pressure is reduced, thus reducing the impact of blood flow on the aortic wall and slowing down the rate of aneurysm expansion.
3. AAA open surgery
The earliest AAA resection and artificial vascular grafting originated in the 1960s. After more than 40 years of development, it has evolved and matured, and has become one of the classic procedures. Although, in recent years, EVAR has developed rapidly, causing a great impact on the dominance of open surgery. However, for AAA patients with low risk factors who are in good general condition and can tolerate surgery, open surgery is still the standard procedure for treatment because of its definite immediate and long-term results.
3.1 Incision selection
The classical open surgical incision for AAA is chosen as a median abdominal incision to enter the abdominal cavity layer by layer and open the retroperitoneum to expose the AAA. a left-sided extraperitoneal incision approach has also been attempted and is considered suitable for patients who have had multiple abdominal surgeries and have heavy abdominal adhesions. However, there is no definitive evidence-based medical evidence to suggest a significant difference between the two approaches in terms of perioperative surgical complications and long-term treatment outcomes.
3.2 Preoperative evaluation
Patients with AAA are also at high risk for cardiovascular disease; therefore, preoperative cardiac evaluation is particularly important. Studies have demonstrated that perioperative mortality in open AAA surgery is significantly correlated with preoperative patient cardiac function, and mortality is significantly increased if patients have poor preoperative cardiac function. Therefore, a detailed preoperative cardiac evaluation with electrocardiogram and cardiac ultrasound and, if necessary, coronary angiography is required to adequately assess the degree of coronary stenosis. In addition to this, preoperative pulmonary function and liver and kidney function should also be carefully evaluated.
3.3 Perioperative outcomes
Comprehensive literature reports that mortality rates for elective open surgery for AAA range from 2% to 8%, with results varying among centers due to differences in experience. The mortality rate for ruptured AAA surgery is much higher, ranging from 40% to 70% at all centers. The higher the age of the patient, the higher the perioperative mortality; female patients had a significantly higher mortality rate than males. Preoperative patient cardiac function, pulmonary function, and renal function were all independent factors influencing perioperative mortality.
3.4 Long-term survival rate and complications
The 5-year survival rate for elective surgery for AAA ranges from 60% to 75%, and the 10-year survival rate ranges from 40% to 50%. The prognosis and long-term survival of AAA involving the renal artery is lower than that of common infrarenal AAA due to the need for renal artery grafting, with a 5-year survival rate of less than 50%. complications of AAA open surgery include: anastomotic bleeding, pseudoaneurysm; colonic ischemia; graft occlusion; graft infection, combined with duodenal fistula, etc. The incidence varies between 0.5% and 5%.
4.AAA Endovascular aneurysm repair (EVAR)
4.1 Introduction
Parodi et al. first used transfemoral AAA EVAR in an attempt to apply it to high-risk patients who were not suitable for open surgery. Over the next decade, interventional devices and related surgical techniques have rapidly evolved and improved and matured. Because EVAR avoids long abdominal incisions, it greatly reduces surgical trauma; it can be performed with regional block anesthesia or local anesthesia, and is particularly suitable for patients with severe combined cardiopulmonary insufficiency and other high-risk factors. Due to the minimally invasive nature of EVAR, its indications are rapidly expanding in some countries and medical centers, and it has even begun to replace traditional open surgery in AAA patients with low risk factors.
The stent grafts currently used in EVAR are made by suturing and fixing the artificial vessel inside a metal stent to prevent twisting and ectasia of the artificial vessel and to maintain stability. To accommodate aortic bifurcation structures and to increase the stability of the stented vessel, most current stent graft products use a patterned design in which the main body and one iliac branch are placed through one femoral artery and the other iliac branch is placed through the contralateral femoral artery and positioned for docking. An important prerequisite for the implementation of this procedure is the presence of a normal aorta of sufficient length beneath the renal artery that can serve as a proximal anchorage zone for the stent to prevent ectopic stent grafts to the distal side and to prevent the occurrence of postoperative endoleaks.
4.2 Preoperative evaluation
AAA EVAR has a low impact on the patient’s systemic status and is only equivalent to moderate to low surgical trauma, and its perioperative mortality and complication rates are significantly lower than those of conventional open surgery. However, preoperative assessment of cardiac function is still required to understand whether the patient has a previous history of acute heart attack or heart failure. Other organ function should also be assessed, with particular attention to renal function, to prevent the occurrence of postoperative contrast nephropathy.
4.3 Perioperative outcomes
Most of the data on comparing perioperative mortality between AAA open surgery and EVAR are non-randomized controlled studies, due to the fact that EVAR is chosen mostly in patients with high-risk procedures. Nevertheless, the perioperative mortality rate after EVAR is less than 3%, which is lower than that of open surgery. In addition, EVAR has a lower rate of fatal perioperative complications compared with open surgery, and patients recover quickly after surgery, with a much shorter ICU treatment time and overall length of stay.
4.4 Long-term survival rate and postoperative complications
The long-term survival rate of patients after AAA EVAR depends largely on the preoperative high-risk factors, and comprehensive literature reports that the 3-year survival rate after EVAR differs significantly between high-risk and general patients, 68% and 83%, respectively. post-EVAR complications mainly include endoleak, stent graft ectasia, torsion, graft occlusion, and infection. It has been shown that the larger the preoperative AAA tumor diameter, the higher the rate of postoperative endoleak, stent ectopic and other complications.
4.5 Problems with EVAR
With the continuous improvement of interventional devices and techniques, AAAEVAR has become increasingly mature, but there are still some problems with this procedure, which need to be further developed and improved.
Vascular anatomical limitations.
Compared with traditional open surgery, EVAR requires more vascular anatomical conditions. First, a normal aorta of at least 1.5 cm in length under the renal artery is required as the proximal anchorage area, i.e., the neck of the aneurysm must be at least 1.5 cm long; at the same time, the diameter of the neck of the aneurysm is required to be less than or equal to 28 mm, while it must not be severely angulated. It is also required that the external iliac artery and the femoral artery have sufficient diameter to ensure that the conveyer carrying the stent graft can pass through. Because of the thin external iliac artery in women, the percentage of women who forgo endoluminal treatment due to poor delivery routes is much higher than that of men, with approximately 17% of women compared to 2.1% of men reported in the literature
Endoleaks.
Endoleaks refer to persistent blood flow into the closed lumen after AAAEVAR and can be classified into the following four types. type I endoleaks refer to blood flow into the lumen due to failure of proximal or distal anchorage zone closure, which generally results in high pressure in the lumen and can easily lead to rupture of the tumor. Once detected, it needs to be corrected by adding extensions proximally or distally. Type II endoleaks refer to the return of blood into the tumor cavity through branch arteries (e.g., lumbar artery, inferior mesenteric artery, etc.) and occur in about 40% of cases. Most of them can close the lumen with prolonged thrombosis on their own, and some people perform selective branch artery embolization through catheterization.
However, current evidence suggests that type II endoleaks do not increase the incidence of proximal or distal rupture of the tumor. type III endoleaks are leaks at the interface due to breakage or distortion of the stent vessel and require immediate interventional or surgical correction once they occur. type IV endoleaks are those in which blood enters the tumor lumen due to high permeability of the stent vessel and usually occur within 30 days of stent vessel placement. In addition, some patients with persistent enlargement of the tumor lumen after AAAEVAR are not found to have significant endotension by CT scan, which is referred to as endotension. In conclusion, it is due to the presence of inexact factors such as endotension that patients after AAAEVAR need to be followed up regularly. The follow-up interval is usually 3, 6, and 12 months after surgery, and annually thereafter. If the imaging data reveal progressive enlargement of the tumor, further investigations are needed to clarify the cause.
Stent graft occlusion.
There is a high incidence of stent graft occlusion after early AAAEVAR. An important reason for occlusion is the twisting of the graft into an angle. It was later found that the use of a metal stent as an external support can reduce the twisting of the vascular graft, thus greatly reducing the incidence of graft thrombosis occlusion.
Tumor neck expansion.
After AAAEVAR, the aorta in the proximal anchorage area will dilate further over time, which can lead to ectasia of the stent graft distally. Currently, when EVAR is performed, the stent body diameter is generally selected to exceed the proximal aneurysm neck diameter by 10-20% to accommodate future aortic expansion, but even so, late ectasia of the stent graft is still not completely prevented.