# Stents in Congenital Heart Disease: State of the Art and Future Scenarios
## 3 Congenital Heart Disease
### Defects Amenable to Stenting
#### 3.2.1 Hypoplastic Left Heart Syndrome  
Hypoplastic left heart syndrome is the most severe form of CHD, consisting of a critical underdevelopment of the left-sided cardiac structures, including the left ventricle, the mitral valve, the aortic valve, and the ascending portion of the aorta [37] (Figure 3a). This pathology is the most common defect among those characterized by single-ventric physiology [38], comprising 1.4-3.8% of congenital heart defect cases [39], meaning 20,000 diseased newborns worldwide in 2022 [40]. The origin of this condition can be associated with obstruction of the outflow or inflow in the left ventricle, among which a defective aortic valve is the most common [41]. The consequence is a reduced flow in the left ventricle leading to left-sided hypoplasia. Moreover, a pressure unbalance at the foramen oval occurs, causing blood flow to revert its direction from the left to the right side [41; 42]. The physiological blood flow during gestation is affected by the presence of HLHS since the left ventricle does not have sufficient pumping capability to supply oxygen-rich blood to the body. As a consequence, the right heart is exploited as a single pumping chamber, feeding both the pulmonary and the whole systemic circulation (both upper and lower body) in parallel. In this condition, the total cardiac output flows through the right atrium; just a small amount (10%) of blood flow enters pulmonary circulation, while the greater part feeds the systemic circulation through an enlarged ductus arteriosus, which has typically elongated morphology [41]. The blood flow in the aortic arch is reversed to feed the brain and the coronaries, not sufficiently perfused by the left ventricle outflow. Whereas during pregnancy, systemic circulation is not impaired significantly, the spontaneous closure of the ductus arteriosus leads to a rapid onset of hypoxia and tissue ischemia metabolic acidosis [41]. Neonatal death would typically occur within 7-10 days after birth if the pathology is left untreated [37]. However, medical advancements in the last four decades have allowed for the establishment of a multi-stage palliative surgery, which has highly improved the outlook for these patients. There are two different approaches for the treatment of HLHS: a purely open-heart surgical repair and a _Hybrid_ one, combining transcatheter stenting and surgical features. In the first case, within a few days of life, the newborns undergo the Norwood operation, in which the right ventricle is rerouted to the systemic circulation by anastomosing the main pulmonary artery to the ascending aorta (neoaorta) to overcome the dependence of the systemic circulation from the ductus arteriosus. Pulmonary blood flow is derived from either a systemic-pulmonary arterial shunt or a right ventricle-pulmonary arterial shunt, and pulmonary venous return is guaranteed through atrial septectomy. However, the whole body is perfused with mixed blood, since only one pumping chamber is present, with an increased workload of the right ventricle. In order to restore a circulation where pulmonary and systemic circulations are in series, two subsequent surgical stages (Glenn procedure and Fontan operation at 4-6 and 18-24 months of age, respectively) are performed, creating a connection between the two systemic vena cavae and the pulmonary arteries. Figure 3: Characteristic anatomy of the heart in the presence of CHDs treatable through stenting, as (**a**) hypoplastic left heart syndrome, (**b**) pulmonary atresia, (**c**) aortic coarctation, and (**d**) pulmonary artery stenosis. Black solid arrows highlight the main aspects that differ from the physiological state, while red arrows show the location of the stent implantation. RA = right atrium, RV = right ventricle, LA = left atrium, LV = left ventricle, SVC = superior vena cava, IVC = inferior vena cava, Ao = aorta, PA = pulmonary artery, DA = ductus arterious. Created with BioRender.com.