The aortic valve is one of the four groups of valves in the heart, which is located at the end of the left ventricular outflow tract at the junction with the aorta. It acts like a one-way valve, allowing blood to be discharged smoothly from the systolic left ventricle into the lower pressure aorta during systole, and preventing the blood that has been discharged from the high pressure aorta from flowing back into the diastolic left ventricle during diastole, ensuring that blood flows with the contraction of the heart against the pressure The flow of blood in the reverse pressure step with the contraction of the heart. The total weight of the aortic leaflets is less than 1 gram, and the total leaflet area is about 1,000 mm2 with a thin leaflet. The orifice area is typically 4 cm2 when the valve leaflets are fully open, passing 5-10 liters of blood per minute without resistance in various physiological states, and ensuring no blood leakage at diastolic pressures of 100-60 mm Hg, opening and closing 60-100 times per minute, 24 hours per day, 365 days per year. 365 days a year, from before birth until the end of life (in total, approximately 2.5 to 3 billion times). The aortic valve should work properly for a long time, but of course there should be no problems with its quality in the first place. Some connective tissue diseases (e.g., Marfan’s syndrome) can cause poor “texture” of the aortic valve leaflets, which of course do not work properly for long periods of time. Second, the aortic valve should work in a good environment, and the blood flow through it should be very smooth and free of turbulence. When the aortic valve leaflet is not in optimal shape, the blood flow field through it will be abnormal, and this abnormal blood flow will cause continuous damage to the valve.
Figure 1 Normal and malformed aortic valve
The normal aortic valve is composed of three equal-sized leaflets, a pattern known as trivalvularization. This equal-sized trivalvularity is the optimal structure in terms of the environment in which the aortic valve is placed and its function. A few abnormalities of the human heart occur during embryonic development in which the aortic valve is not trivalved, such as univalved, bivalved, or tetravalved (Figure 1, Figure 2). Abnormalities in aortic valve structure result in two things: abnormalities in aortic valve function, either by not being able to fully open when blood flows out of the left ventricle (i.e., stenosis) or not being able to fully close when the left ventricle is diastolic (i.e., valvular insufficiency), or will be both (stenosis combined with insufficiency); and a poor blood flow field. Turbulence and eddy currents damage the aortic valve with premature leaflet prolapse, calcification, or infection.
Figure 2 Normal aortic valve leaflets versus diaphyseal aortic valve leaflets at work
The vast majority of congenital aortic valve malformations are bicuspid aortic valves (BAV). According to a 2002 U.S. article synthesizing 62 statistics on the incidence of congenital heart malformations since 1955, the incidence of BAV in live births was 13.6 per 1,000 live births, much higher than the total number of other congenital heart malformations requiring treatment (6 per 1,000). Males significantly outnumber females (3-4:1). Other congenital cardiac malformations of moderate or greater severity often require treatment in infancy or childhood. In contrast, most patients with BAV do not require treatment until the adult stage. In fact, most patients with BAV will require some form of treatment during their survival. Autopsy results show that only 20% of individuals with BAV maintain normal aortic valve function by age 70. 30% of patients will develop severe comorbidities. The lifetime incidence of infective endocarditis in patients is 30%, and the incidence of Stanford type A aortic coarctation is 10 times higher than in the normal population. In developed countries, approximately half of adults and three-quarters of children with severe aortic stenosis originate from BAV, a malformation that causes more complications, deaths, and medical burden in the population than other congenital heart malformations combined. Follow-up of two large groups of patients in North America showed cardiovascular event rates: 25% in the mean 44-year-old group and 40% in the mean 52-year-old group; the proportion undergoing cardiac surgery: 22% at 9 years of follow-up and 27% at 20 years of follow-up; if any of the three were present: age >30 years, moderate or greater aortic stenosis, moderate or greater aortic valve insufficiency Two, the likelihood of a patient’s first cardiac event at 10 years of follow-up is about 65%.
In 95% of diastolic aortic valves, the size of the two leaflets is asymmetric, and in only about 5% of patients are the two leaflets equal in size, so-called “true diastasis. In asymmetric bivalvularization, 80% of the patients have fusion of the left coronary valve and the right coronary valve, forming a large valve, while the rest have fusion of the right coronary valve with the noncoronary valve and very few have fusion of the left coronary valve with the noncoronary valve. In asymmetric bivalvularization, 75% of patients have a “ridge” within the large flap (Figure 3). In addition, BAV may be combined with other cardiac anomalies such as aortic constriction, disrupted aortic arch, ventricular septal defect, and pulmonary valve diastasis.
Figure 3 Leaflet fusion in diastasis
There is a familial tendency for BAV to develop, with a survey showing a 24% prevalence of aortic valve disease in families with more than one person with the disease. Clinical studies have also shown that the incidence of BAV is 9% in first-degree relatives of patients with BAV. Therefore, the American Heart Association guidelines for adult congenital heart disease recommend echocardiographic screening of first-degree relatives of patients with BAV.
The clinical presentation of BAV is highly variable, ranging from a newborn with severe aortic stenosis presenting with heart failure to an asymptomatic older adult found to have severe valvular disease or aortic pathology. However, there should be three basic symptoms of aortic valve disease. The first is panic and shortness of breath after activity and inability to engage in physical activities appropriate to one’s age; the second is angina pectoris, i.e., episodes of dull pain in the forehead, back, anterior neck, or left upper extremity induced by activity, satiety, emotional excitement, and cold, which then resolves on its own when the triggering factors disappear; and the third is episodic, transient loss of consciousness.
Modern diagnostic ultrasound techniques have been able to detect severe fetal aortic stenosis due to BAV. In fetal life, due to the characteristics of its circulation, the right ventricle can carry the systemic circulatory load, but the left heart burden increases immediately after the child is born and the heart function may be impaired. In children, because the implanted prosthetic valve cannot grow with the child’s physical development and the aortic valve is often not calcified at this time, most physicians consider aortic valve balloon dilatation to be the treatment of choice. The indications for balloon dilatation are: 1. symptomatic; 2. asymptomatic but with a transaortic valve pressure difference greater than 60 mm Hg; and 3. asymptomatic with a pressure difference greater than 50 mm Hg and electrocardiographic ST-T changes or regular sports participation. If severe aortic valve insufficiency develops after balloon dilation, valve replacement surgery may be required.
Aortic stenosis due to BAV in adults is usually due to leaflet calcification, a lesion that is caused by endothelial cell damage, inflammatory response, and lipoprotein deposition. Aortic valve sclerosis usually appears at the age of about 20 years and calcification at the age of about 40 years. The lesions are progressive, and the transvalvular pressure difference increases by 20 mmHg approximately every 10 years, significantly faster than in those with trivialized aortic stenosis. When surgery is performed elsewhere in the heart, aggressive valve replacement is warranted for combined BAV with mild stenosis, as this stenosis can worsen rapidly in a shorter than expected time frame. If the patient has smoking, hypertension, or hyperlipidemia, the transvalvular pressure difference will increase even more rapidly. Therefore, those patients with BAV should be strongly advised to quit smoking and control blood pressure and lipids. Age is the most important factor in the progression of aortic stenosis, i.e., the older the age, the more severe the stenosis. Pregnancy accelerates the progression of aortic stenosis in patients with BAV.
In adult patients with BAV combined with aortic stenosis, the incidence of serious complications from percutaneous balloon dilation is usually around 10% and the effect usually lasts only 6-12 months, so such patients should not receive this treatment. The results of pharmacologic therapy for severe aortic stenosis are extremely poor. In a group of patients in the United States with severe aortic stenosis who were not operated on for various reasons, the follow-up results were only 36% event-free survival at 2 years and 3% at 6 years. A new technique, transcatheter implantation of the aortic valve, has emerged. Its efficacy is better than drug therapy, but significantly inferior to conventional aortic valve replacement, and its cost is enormous (300,000 to 400,000 RMB), making it suitable only for elderly patients who are in very poor health, have many comorbidities, and are too risky for surgery.
The incidence of aortic valve insufficiency in BAV varies widely from study to study. The rate of pure aortic valve insufficiency in a group of surgical specimens was 13%. Two major groups in North America reported the incidence of some degree of aortic regurgitation at 47% and 21%, respectively, with the ultimate need for surgical intervention at 3% and 6%, respectively. Most of these patients are young men (male to female ratio of approximately 10:1) with a large amount of leaflet tissue, often combined with leaflet prolapse. The cause of aortic valve insufficiency may be the leaflet’s own pathology or due to aortic root dilatation and aortic annulus enlargement. Of course, infective endocarditis occurs in BAV, and pathogenic microorganisms and local inflammation can damage the aortic valve, leading to aortic valve insufficiency.
Approximately half of adult BAV patients have combined aortic dilatation. Aortic lesions are an important part of BAV and include aortic dilatation, aortic aneurysm, and aortic coarctation. Progressive aortic dilatation is more common in patients with BAV than in patients with tricuspid aortic valve lesions, in both pediatric and adult patients. Previously, the cause of aortic pathology was thought to be related to abnormal blood flow over the aortic valve, including significantly enhanced blood flow impingement in aortic valve insufficiency and post-stenotic dilatation due to high velocity blood flow in aortic stenosis (Figure 4). However, present-day findings suggest that abnormalities in the histology of the aortic wall are a more prominent cause of this lesion for the following reasons.
1, patients with BAV with normal valve function have larger aortic root and ascending aorta diameters than those with tricuspid aortic valves with normal valve function.
2, more lesions can be seen in the ascending aortic wall tissue of patients with diaval valvularization than in those with trivalvularization.
3. a decrease in the amount of protofibrin in the aortic wall and an increase in the amount of the original protein-degrading enzymes in patients with valvular diastasis.
4. aortic valve replacement can still result in dilatation of the ascending aorta in patients with valvuloplasty, but not in patients with tricuspidity.
5. Both the aortic valve and the ascending aorta are derived from cells of the neural crest during embryonic development, and abnormalities in these cells may of course lead to abnormalities in the organs from which they differentiate. If an association of aortic lesions with BAV does exist, surgical treatment should reflect this association.
The incidence of aortic coarctation in patients with BAV is approximately 5%, compared with 15% in patients with Stanford type A. The average age of coarctation in patients with BAV is also 10 years earlier than in patients with tricuspid aortic valves. In addition, aortic coarctation often occurs in patients with BAV who have normal valve function, especially when these patients also have coexisting aortic constriction. Some patients who have undergone aortic valve replacement also develop aortic coarctation at a site other than the previous surgical aortic incision.
Figure 4 Ascending Aortic Aneurysm
The indications for aortic valve replacement surgery in patients with BAV are the same as for general aortic valve lesions. However, a more aggressive approach should be taken for combined aortic dilatation. The current American Heart Association guidelines for valvular disease recommend that patients with BAV who have an aortic root or ascending aorta greater than 50 mm in diameter or an annual growth rate greater than or equal to 5 mm should undergo ascending aortic or aortic root repair or replacement surgery; if a patient undergoes aortic valve replacement surgery for aortic stenosis or insufficiency, the threshold for ascending aortic or aortic root repair or replacement during the same period In a Canadian cohort of patients with a normal ascending aortic diameter at the time of their first aortic valve surgery, the rate of avoidance of aortic aneurysm, aortic coarctation, or sudden death at 15 years was 86%. If the aortic diameter was 40-44 mm, the waiver rate dropped to 81% and was only 43% at 45-49 mm.
Replacing the aorta at the same time as the aortic valve still increases the surgical risk to the patient for any cardiac surgeon, no matter how experienced and skilled he may be. Of course, the benefits of elective aortic surgery in patients at risk for aortic coarctation are clear, and the cost of avoiding the serious disease of aortic coarctation (treated nonoperatively, with a mortality rate approaching 50% at 2 days of onset and 75% at 2 weeks) is slightly higher than the mortality rate of conventional surgery. In addition, if a patient develops an aortic aneurysm or coarctation after aortic valve replacement surgery, reoperation to replace the aortic root and ascending aorta is much more difficult, and of course the surgical risk is correspondingly much higher, especially in the case of emergency surgery. Between October 1996 and December 2008, there were 706 elective aortic root or/and ascending aortic replacements for patients with aortic root aneurysms, ascending aortic aneurysms or/and ascending aortic dilatation at Fu Wai Hospital, with 10 surgical deaths (1.4% mortality), and 246 emergency aortic root or/and ascending aortic replacements for Stanford type A coarctation during the same period, with death in 18 cases (7.3% mortality rate). The difference is dramatic.
For aortic valve replacement in patients with BAV, the vast majority of physicians take the approach of implanting a mechanical or bioprosthetic valve (stented bioprosthetic valve or stentless bioprosthetic valve). For patients with combined aortic lesions, the standard approach is aortic root replacement (Bentall or Cabrol procedure) or aortic valve replacement plus ascending aortic replacement (Wheat procedure) (Figure 5). The long-term follow-up after surgery has been satisfactory.
Figure 5 Schematic diagram of the different procedures
Of particular interest here is the homologous autologous pulmonary valve graft. In this procedure, the autologous pulmonary valve and main pulmonary artery are removed intact and transplanted to the aortic root, replacing the missing autologous pulmonary artery with a homogeneous allogeneic pulmonary artery, the so-called Ross procedure (Figure 5). The debate about this procedure is very intense, both among physicians and patients. If interested, you can visit the American Heart Association’s Ross procedure forum. The advantages of this procedure are that the aortic valve is living autologous tissue that can grow, is resistant to infection, and does not require warfarin anticoagulation. Its problem is the progressive dilatation of the new aortic root that occurs in some patients after surgery. In addition, the allograft is not active, the procedure results in calcification of the allograft pulmonary artery, and it is necessary to undergo multiple pulmonary valve procedures, turning the problem of one valve (aortic valve) into a problem of two valves (aortic valve plus pulmonary valve). In addition, this procedure is very complex and only a few surgeons are skilled in it. Most doctors currently believe that the Ross procedure is indicated for pediatric and severely infected patients. The incidence of pulmonary valve diastasis is also higher in patients with BAV due to the same embryologic origin of the aorta as the pulmonary artery, as well as lesions in the pulmonary artery wall similar to those in the aortic wall. coronary artery variants (variation in the opening position of the right coronary artery, short left main stem, and small right coronary artery) are more frequent in patients with BAV. Both of these factors are extremely detrimental to the Ross procedure.
Diastolic aortic valve malformations are highly detrimental to cardiac function and can lead to cardiac impairment, sudden death, aortic aneurysm, and aortic coarctation. Echocardiography is highly sensitive and accurate in the diagnosis of this disease, and this test is non-invasive, can be performed repeatedly, and can be performed in general hospitals and by slightly trained ultrasonographers and cardiologists, and the test is inexpensive, so it is not difficult to detect this malformation. As soon as the presence of the disease is detected, treatment can be chosen when the condition requires it, thus preventing irreversible impairment of cardiac function, sudden death and aortic coarctation. patients with BAV, who have normal valve function and also do not have combined aortic pathology, should not have their echocardiograms reviewed regularly for more than two years. If there are any abnormalities, they should not exceed one year. Surgeons treating this disease should never simply treat it as simple aortic valve disease.