1.History review
Tetralogy of Fallot is is a common congenital heart malformation and is the most common of the cyanotic heart malformations, accounting for 50-90% of cyanotic congenital heart surgery and about 12% of all congenital heart surgery. The tetralogy of Fallot (TOF) is a group of defective cardiac conditions that include cone-ventricular septal defects and varying degrees of right ventricular outflow tract (RVOT) obstruction, the course of which varies with the degree of obstruction.These defects were first described by Stensen in 1887, and in 1888, his colleague Fallot [1] performed a detailed description of the four basic lesions of the disease: right ventricular outflow tract stenosis, ventricular septal defect, aortic span, and right ventricular hypertrophy. The pathophysiological complexity of the disease is divided into two categories: simple and complex, the former being tetralogy of Fallot with pulmonary stenosis, and the latter including tetralogy of Fallot with pulmonary atresia, with pulmonary artery agenesis, with complete atrial septal defect or with pulmonary valve agenesis. The first subclavian artery-pulmonary artery bypass was performed by Blalock in 1945 [2] to treat tetralogy of Fallot, and the first intracardiac repair of tetralogy of Fallot was performed by Lillehei [3] in 1954 under human cross-circulation. surgical treatment. Since these pioneering efforts, many more procedures have been invented, including: one-stage repair as opposed to two-stage, outflow tract patching, ductal connection, and atrial repair. The natural prognosis of tetralogy of Fallot is poor, with 25% of natural deaths up to 1 year of age, 40% up to 3 years of age, 70% up to 10 years of age, and 95% up to 40 years of age without surgical treatment [5]. The natural prognosis depends on the severity of the right ventricular outflow tract obstruction, and the vast majority of patients are said to die from hypoxia or heart failure. Therefore, tetralogy of Fallot should be treated surgically as early as possible.
2. Indications for surgery and timing of surgery
Most cases of tetralogy of Fallot are born with satisfactory oxygen saturation in the body circulation without treatment, but hypoxia progresses gradually and surgical intervention is necessary when the oxygen saturation in the body circulation drops to 75-80%. The onset of hypoxic episodes is usually considered an indication for surgery.
Stage I radical surgery is preferred for tetralogy of Fallot and is generally recommended in most centers for typical patients with tetralogy of Fallot, even in severe disease and regardless of age [6, 7]. In recent years, there has been a trend toward younger age groups, partly due to advances in surgical techniques and, more importantly, to a better understanding of the pathophysiology of tetralogy of Fallot. Early surgery is beneficial to protect the function of the right and left ventricles, promote the development and growth of the pulmonary arteries, especially the peripheral pulmonary arteries, reduce the damage of chronic hypoxia to the heart, nervous system and other organs, and promote the normal growth and development of organs, in addition to avoiding and reducing preoperative episodes of hypoxia and sudden death due to ventricular arrhythmias in the late postoperative period. Based on the above understanding, surgery can be performed in small symptomatic infants or neonates, and in cases when no specific surgical indications are available, elective surgery is performed at 1-2 years of age and can also be completed at 3-6 months [8, 9] .
Classical radical surgery for tetralogy of Fallot requires pulmonary artery and left ventricular development of more than 60% of normal, McGoon ratio ≥1.2 (normal ≥2), pulmonary artery index (Nakata index) ≥150 mm2/m2 (normal ≥330 mm2/m2), and left ventricular end-diastolic volume index of 30 ml/m2 (normal 55 ml/m2), and some studies Hennein et al. 1995 reported 30 neonates with tetralogy of Fallot [10] who underwent radical surgery regardless of left ventricular development and pulmonary artery development, all with satisfactory results. However, the extent to which the indications for surgery are relaxed depends on the technical capabilities and equipment of each cardiac center and the experience of the physicians.
For those with severe right ventricular outflow tract stenosis and severe distal pulmonary artery dysplasia, or missing pulmonary artery with large body-pulmonary side branches, as well as infantile coronary artery malformation, it is difficult to perform right ventricular outflow tract patch enlargement, and it is also inappropriate to perform extracardiac pipeline or one with ventricular correction, palliative surgery should be performed first, the purpose of which is to establish body-pulmonary artery shunt and increase pulmonary artery blood flow, and to do second-stage radical surgery when pulmonary artery development improves.
3.Surgical methods
3.1 Palliative surgery: Its purpose is to increase pulmonary blood flow, eliminate and improve symptoms such as cyanosis, expand the pulmonary vascular bed, promote pulmonary vascular development, and prepare for radical surgery [11] . Due to the relaxation of the indications for stage I radical surgery, palliative surgery is currently used only in patients with very poor pulmonary artery development and with other severe intracardiac malformations that are not suitable for stage I radical surgery. The classical or modified Blalock-Taussig (B-T) shunt is the most common, whereas the Waterson anastomosis (ascending aorta-right pulmonary artery) and Potts anastomosis (descending aorta-left pulmonary artery) are now largely abolished due to the disadvantages of more difficult flow control, difficult removal and pulmonary artery tortuosity.B-T shunts can be performed at any age and on any size pulmonary artery, but Due to the small size of the subclavian artery in neonates, most physicians prefer to apply a modified B-T shunt in the neonatal period, which has excellent results due to a low rate of shunt failure and excellent reduction performance [11, 12].
3.2 Radical surgery for pure tetralogy of Fallot
The radical surgery of TOF has three main steps: closure of the ventricular septal defect, release of the right ventricular outflow tract obstruction, and correction of the combined intracardiac malformation. The stable improvement in outcome of radical surgery for tetralogy of Fallot is mainly attributed to improved myocardial protection techniques, transatrial repair of the septal defect and more careful right ventricular outflow tract reconstruction. The basic principle of radical surgery is to complete the repair of the septal defect and unblock the right ventricular outflow tract with minimal damage to the right ventricle. Outflow tract sparing is accomplished exclusively through the right atrial pathway, and septal defect repair is the best option. In infant cases, resection of the wall bundle is usually not required and only excision of the hypertrophic obstructive muscle bundle is sufficient. After outflow tract patency, pulmonary valve junction dissection can be performed via the tricuspid valve, and if poorly revealed, a direct pulmonary trunk incision is feasible to complete pulmonary valve junction dissection [13]. The right ventricular/left ventricular pressure (RV/LV) ratio is expected to exceed 0.7 at the end of the procedure, and the outflow tract should be enlarged with a patch [14]. If possible, try to control the right ventricular incision so that it is located over the anomalous left anterior descending branch of the coronary artery; this incision is to facilitate relief of outflow tract obstruction rather than repair of the septal defect. Prolonged pulmonary regurgitation after radical surgery in Law IV patients has deleterious effects on right ventricular function and exercise capacity, and in cases of pulmonary regurgitation, the right and left ventricular ejection fractions are significantly lower than in those with intact pulmonary valve function [15] . When symptoms occur, implantation of a pulmonary valve improves functional status and ventricular function [16]. Therefore, there is a renewed interest in the use of single-valve patches in the right ventricular outflow tract [17, 18] with the aim of minimizing pulmonary regurgitation and thereby reducing its long-term effects on right ventricular function.
Combined surgical and interventional correction (hybrid) of pulmonary stenosis with somatopulmonary collateralization in tetralogy of Fallot improves the success rate of primary treatment and reduces trauma in children [19, 20]. If the cyanosis is not severe, the X-ray shows little pulmonary blood, the oxygen saturation is not significantly reduced, and the echocardiogram shows that the degree of pulmonary artery thinning is not proportional to the above-mentioned manifestations, the possibility of combined body-pulmonary collateral branches should be highly suspected, and cardiovascular imaging should be performed to clarify the diagnosis, and it is very important to clarify whether there is fusion between the body-pulmonary collateral branches and the intrinsic pulmonary artery before surgery, and if there is fusion with the pulmonary artery, interventional occlusion should be performed first, followed by Recent data show [21] that side branch occlusion can reduce intraoperative lung injury and postoperative congestive heart failure and can significantly improve cardiopulmonary function, and if there is no fusion of the giant side branch, it cannot be occluded. aggressive lateral branch fusion is appropriate. In conclusion, combined interventional procedures for congenital heart disease with reduced pulmonary blood supply to the body-pulmonary collateral can improve the success rate of phase I radical treatment and reduce the trauma to the child. For large somatic pulmonary collateral branches that supply blood to the lung segment alone, interventional embolization should be carefully considered to avoid postoperative pulmonary infarction.
3.3 Surgery for complex tetralogy of Fallot
3.3.1 Surgical technique for pulmonary atresia with large main pulmonary collateral artery (MAPCA)
MAPCA is one of the most complex types of tetralogy of Fallot combined with pulmonary atresia. l980, Haworth et al [24] studied the structure of MAPCA and first proposed the idea of “single-source” surgery, in which multiple MAPCAs supplying separate lobes or segments of the lung are surgically connected so that they are fed by a single source of pulmonary blood flow. The idea of “monogenization” was first proposed, in which multiple MAPCAs supplying separate lobes or segments of the lung are surgically connected so that they are supplied by a single source of pulmonary blood. In the past, this was achieved through a staged monogenization approach, in which the peripheral MAPCA was gradually monogenized and eventually converged into the central pulmonary artery (CPA) in the mediastinum and the right ventricular outflow tract was established (peripheral approach), or the right ventricular outflow tract and CPA were established first and the peripheral MAPCA was gradually converged into the CPA (central approach), after multiple surgeries. Only 20-30% of patients can be cured. With the increasing proficiency of surgical methods, interest in one-stage radical treatment of this group of cases has increased. In order to incorporate the maximum number of healthy pulmonary microvascular beds into the pulmonary circulation and restore normal cardiopulmonary vascular physiology as early as possible, and recognizing the higher incidence of progressive disorders of pulmonary vascular and collateral vascular distribution during the peripheral and central approaches, early one-stage monogenization of MAPCA and intracardiac radical treatment for those who are eligible have been advocated in recent years. Since 1995, when Reddy first reported the application of one-stage monogenesis and endocardial repair for this malformation, the procedure has been widely promoted and improved [25], and in 2002 Cho [26] reported the surgical treatment of 495 cases of tetralogy of Fallot with pulmonary atresia, of which 339 were staged or one-stage monogenesis and endocardial repair, with satisfactory results and early and late mortality rates of 4.5%. Similar results were found by Amark et al [27] .
Whether a ventricular septal defect closes without causing right ventricular pressure overload is a great challenge for the surgeon. The total new pulmonary artery index (TNPAI) can be used as an indicator of the postoperative RV/LV pressure ratio, which is calculated by measuring the total cross-sectional area of the pulmonary arteries and MAPCAs and dividing it by the body surface area. If the intraoperative TNPAI is greater than 200 mm2/m2 or pulmonary artery pressure is 25 mmHg or less the ventricular septal defect can be safely closed, but postoperative ventricular pressure exceeds 90% of the body circulation pressure and the ventricular septal defect needs to be opened or the ventricular septal defect patch opened [28] .
3.3.2 Tetralogy of Fallot with pulmonary valve defect
Pulmonary valve agenesis accounts for approximately 5% of cases of tetralogy of Fallot [29], in which the pulmonary valve leaflets are undeveloped and have an annular ridge, with stenosis mostly in the pulmonary annulus, marked dilatation of the main or right and left pulmonary arteries, and even compression of the bronchi, resulting in intractable bronchitis, respiratory distress [22], and congestive heart failure after birth. Many infants require preoperative mechanical ventilation or even extracorporeal membrane pulmonary oxygenation, often requiring emergency surgical intervention. The most effective approach is currently reconstruction of pulmonary valve function and complete intracardiac correction [30]. In the absence of respiratory symptoms, elective surgery can be performed at 6 months of age, and implantation of a pulmonary valve is not necessary. The mortality rate in critically ill neonates and small infants undergoing stage I pulmonary angioplasty and intracardiac repair remains high, with 1999 McDonnell [31] and colleagues reporting a 21.4% mortality rate for early surgery, one late death, a 77% one-year survival rate, and a 72% 10-year survival rate. Due to improvements in surgical technique and perioperative management this year, Alsoufi et al [32] reported 4.8% (3/62) perioperative mortality and 93 ± 4% and 87 ± 5% 5-year, 5-year, and 10-year survival rates, respectively, in a group of 62 pulmonary valve defect surgeries.
3.3.3 Complete atrial septal defect with tetralogy of Fallot flap
In 2% of cases of tetralogy of Fallot with complete AV septal defect, due to the fact that this malformation has both right ventricular pressure overload and volume overload in both ventricles, patients tend to have cyanosis and often right congestive heart failure occurs. In complete radical treatment, it is difficult to avoid left ventricular outflow tract obstruction and to correct atrioventricular regurgitation. The patch of the ventricular septal defect should be cut in a comma shape, with the narrow end of the patch sewn to the right ventricular surface of the inflow tract and the wide end sewn to the aortic valve orifice and the aortic septum into the left ventricle, and the patch in the subaortic valve region should be wide enough to avoid left ventricular outflow tract stenosis. Due to the difficulty of revealing the atrial transection and the severe aortic riding that hinders the surgical operation, some authors have proposed a right atrial-right ventricular transection, but this transection is still difficult except in a few cases. 1998 0’Blenes reported [33] ll cases of this compound malformation with intracardiac repair via right atrial and right ventricular incisions applying the two patch method, of which 6 cases had transvalvular annular right ventricular outflow tract patch, 3 cases of right ventricle to pulmonary artery extracardiac conduit, and 2 cases of funnel resection only. One postoperative death was due to low cardiac output syndrome, and autopsy revealed a very small development of the right ventricular sinus, with no late death and a mean postoperative follow-up of 43 months, with 10 cases of class I and II cardiac function. 17 cases of this malformation were reported in the Prifti E group [34], with 17.6% of operative deaths, and no arrhythmias occurred with malformation correction via the atrial route, and arrhythmias occurred with surgery via the right ventricular route (4 of 11), with a postoperative follow-up of 36 months, no post-discharge deaths, recovery of cardiac function grade 1-2, significant reduction in right ventricular pressure gradient 63 +/- 16 mmHg —- 17 +/- 6 mmHg.
3.3.4 One-sided pulmonary artery agenesis
In contrast to an abnormal origin of one pulmonary artery, a one-sided pulmonary artery agenesis is defined as the absence of an anatomical side of the pulmonary artery. Pulmonary artery agenesis in combination with tetralogy of Fallot is uncommon and often occurs on the left side, and even more rarely in the right pulmonary artery. With the improvement of surgical techniques, the mortality rate has decreased significantly in recent years. Some authors [35] have suggested that radical surgery for tetralogy of Fallot in combination with a pulmonary artery defect on one side requires a well-developed healthy pulmonary artery and often requires a transannular patch, preferably with a flap patch material, to ensure satisfactory hemodynamic correction. The affected pulmonary artery can be grafted to the pulmonary artery for those with an abnormal origin of one pulmonary artery.
Treatment outcome.
The mortality rate of correction of tetralogy of Fallot continues to decrease nowadays. The results of direct visualization orthoprosthesis have been satisfactory. Most reports show an early mortality rate of 1-5% for correction of tetralogy of Fallot [36] [37] . Many series have suggested that radical surgery for tetralogy of Fallot up to 1 year of age does not affect early outcomes due to improved surgical techniques, especially avoidance of excision of excessive right ventricular outflow tract muscle bundles, improved cardiopulmonary diversion techniques and postoperative monitoring. The main causes of early death: left ventricular hypoplasia, bilateral pulmonary artery or peripheral pulmonary artery hypoplasia, one pulmonary artery agenesis, pulmonary atresia, pulmonary artery agenesis, and combined complete endocardial cushion defects.
Long-term results are good [38], with survival rates of 93%, 92%, 92%, and 87% at 1 month, 1 year, 5 years, and 20 years in a group of 814 completely radical cases by Kirklin et al. Risk factors for late mortality: older age at cure, severe preoperative left ventricular degeneration; prolonged right ventricular hypertension exceeding 60 mmHg, and distant high incidence of arrhythmias, especially ventricular arrhythmias.
The most common indications for reoperation are long-term complications related to the right ventricular outflow tract, such as severe pulmonary regurgitation, residual outflow tract obstruction, and ductal failure [39] . Due to poor tolerance of postoperative residual shunts in tetralogy of Fallot, reclosure of residual ventricular septal defects is recommended when the pulmonary circulation-to-body circulation flow ratio is greater than 1.5.
In conclusion, the timing of surgery for the correction of simple tetralogy of Fallot is well established, i.e., radical surgery should be performed in neonates and small infants for symptomatic ones, and early surgery for asymptomatic ones, which can be elective at 1-2 years of age. Surgery in infants and young children is performed via a right atrial transection instead of a right ventricular transection. Minimize damage to the right ventricle during surgery to avoid pulmonary regurgitation and to reduce persistent postoperative right ventricular hypertension. How to improve the outcome of complex tetralogy of Fallot is the direction of future efforts.