Hypoplastic left heart syndrome (HLHS) is a spectrum of congenital heart defects that is characterized by hypoplasia of the left ventricle and multilevel obstruction of systemic cardiac output from the level of the mitral valve to the proximal descending thoracic aorta. Other closely related functional single right ventricle lesions include double-outlet right ventricle with mitral atresia and unbalanced atrioventricular septal defect. A common physiology of HLHS is that the systemic circulation is driven by the right ventricle via a patent ductus arteriosus. Without intervention, HLHS is fatal.
In most cases, there is no cure for HLHS; however, surgical palliation, also referred to as staged reconstruction, can be undertaken in a series of surgical procedures that ultimately result in the right ventricle supporting the cardiac output to both the systemic and pulmonary circuits that are connected in series. Staged reconstruction, including a Norwood plus right ventricle–to–pulmonary artery shunt (RVPAS) or modified Blalock-Taussig shunt (MBTS) procedure at the time of birth, a stage II superior cavopulmonary connection, and a completion Fontan, remain the mainstay of the treatment of functional single right ventricle lesions.
Cardiac transplantation is an alternative therapy; however, because of the shortage of neonatal donor organs, the need for immunosuppression, and the limited durability of the transplant, this modality is not sufficient for the entire population of infants born with HLHS each year.
A recent multi-institutional, prospective, randomized, controlled trial (Single Ventricle Reconstruction or SVR trial) in 2005 compared the RVPAS to the MBTS.  This trial has continued to yield a wealth of valuable insight into the management and outcomes of infants with HLHS. In this review, the anatomical and clinical considerations of HLHS, the medical and surgical management, and the outcomes of these patients, with special emphasis on the new knowledge gained from the SVR trial, is summarized.
History of the Procedure
The term hypoplastic left heart syndrome was introduced by Noonan and Nadas in 1958 to describe the morphologic features of combined aortic atresia and mitral atresia.  This followed Lev's description in 1952 of congenital cardiac malformations associated with underdevelopment of the chambers on the left side and a small ascending aorta and arch. 
The first successful palliation of hypoplastic left heart syndrome (HLHS)was reported by Norwood et al in a series of infants who underwent surgery from 1979-1981.  The procedure has been technically refined over the years, but the essential components remain (1) atrial septectomy, (2) anastomosis of the proximal pulmonary artery to the aorta with homograft augmentation of the aortic arch, and (3) aortopulmonary (modified Blalock-Taussig shunt [MBTS]) shunt or right ventricle–to–pulmonary artery shunt (RVPAS) to provide restricted pulmonary blood flow. Staged surgical palliation of HLHS with a Fontan operation using the right ventricle as the systemic ventricle was first reported in 1983 by Norwood et al.  More contemporary experience has demonstrated an improvement in short- and long-term survival rates.
Norwood’s strategy of creating a functional single right ventricle circulation initially involved an aortic-to-pulmonary artery connection with patch aortic arch reconstruction to form a neoaorta and controlled pulmonary blood flow with a RVPAS.  The RVPASs used in these initial efforts were either 8- or 12-mm nonvalved polytetrafluoroethylene (PTFE) grafts, and all of these patients died because of pulmonary overcirculation or right ventricular failure. Norwood then achieved better results with either an MBTS or an aortopulmonary shunt, usually a 4-mm PTFE graft, yielding initial survival that allowed progression to a second-stage operation. This early experience strongly favored the systemic-to-pulmonary artery shunt; however, it now seems evident that it was the large size of the RVPAS rather than the RVPAS strategy itself that resulted in failure of this approach. 
Since Norwood’s initial experience, the MBTS has been adopted as the preferred source of pulmonary blood flow in the Norwood procedure. [7, 8, 9, 10, 11] Kishimoto and colleagues from Osaka, Japan revisited the use of RVPAS in the Norwood procedure in the current era. [12, 13, 14] The presumed advantage of the RVPAS as compared with the MBTS was the increased systemic diastolic blood pressure and the potential for improved coronary perfusion. 
Subsequent to these early descriptions of the use of the RVPAS, several small nonrandomized case series demonstrated improved hemodynamics, branch pulmonary artery growth, and overall results with the RVPAS that extended into the later stages of surgical palliation for HLHS. [16, 17, 18, 19, 20, 21]
Hypoplastic left heart syndrome (HLHS) is a relatively common form of congenital heart disease, occurring in 7-9% of neonates in whom heart disease is diagnosed in the first year of life. Without surgical intervention, HLHS is fatal, accounting for 25% of cardiac deaths in the first week of life.
Although the etiology of hypoplastic left heart syndrome (HLHS) is unknown, Lev has postulated that premature narrowing of the foramen ovale leads to a faulty transfer of blood from the inferior vena cava (IVC) to the left atrium during fetal life.  Thus, altered intrauterine hemodynamics may be the physiologic cause of HLHS. Other authors have postulated that the embryologic cause is severe underdevelopment of the left ventricular outflow in the form of isolated aortic valve atresia. This aortic atresia results in abnormal development of the remaining cardiac structures, resulting from the associated blood flow patterns.
A recent linkage analysis of left ventricular outflow tract malformations (aortic valve stenosis, coarctation of the aorta, and HLHS) identified a significant peak on 2p15 for HLHS, although the exact gene(s) involved were not identified.  In an earlier report, these authors also identified a possible role for Notch1 signaling in left ventricular outflow tract malformation (including HLHS).  A recent study has identified seasonal variation of the incidence of HLHS, with a preponderance in summer months compared with other types of congenital heart defects. 
At birth, systemic and pulmonary circulation depends on the cardiac output from the right ventricle via the main pulmonary artery and the patent ductus arteriosus. The amount of systemic and pulmonary flow depends on cardiac output of the right ventricle and the systemic and pulmonary vascular resistances. Obligatory mixing of pulmonary and systemic venous blood occurs in the right atrium and mixed saturated blood is ejected by the right ventricle. Since the mitral valve is either atretic or stenotic, the major outflow from the left atrium is into the right atrium via an atrial septal defect. Sometimes, a restrictive atrial septal defect is present and obstructs pulmonary venous return into the right atrium.
Hypoplastic left heart syndrome (HLHS) is often diagnosed in patients during the newborn period because of tachypnea and cyanosis within 24-48 hours of birth. When the ductus arteriosus begins to close, diminished systemic perfusion rapidly occurs, with pallor, lethargy, and diminished pulses. Cardiac examination reveals a dominant right ventricular impulse, a single second heart sound, and a nonspecific soft systolic murmur at the left sternal border. Ductal closure results in diminished systemic perfusion with the development of metabolic acidosis and renal failure.
The presence of hypoplastic left heart syndrome (HLHS) is an indication for therapy. Without intervention, HLHS is essentially universally fatal within the first month of life. As survival rates for both staged repair and transplantation have improved, the continued role of comfort-measures-only therapy can be questioned. Certainly, both pediatric and adult patients routinely undergo therapy for conditions with far worse prognosis than HLHS.
Hypoplastic left heart syndrome (HLHS) refers to a constellation of congenital cardiac anomalies characterized by marked hypoplasia or absence of the left ventricle and severe hypoplasia of the ascending aorta.
Pathologic findings by Bharati et al in a series of 230 patients with HLHS included 105 with aortic atresia and mitral stenosis (45%), 95 with aortic and mitral atresia (41%), and 30 with severe aortic and mitral stenosis (13%).  The dilated and hypertrophied right ventricle is the dominant ventricle and forms the apex of the heart. The tricuspid valve annulus is invariably dilated, and significant anomalies in morphology have been described in 5-7% of patients. Clinically significant tricuspid regurgitation has been reported by both Barber and Chang in 8-10% of patients studied and has been identified as a significant risk factor in short-term and long-term survival. [30, 31]
In 95% of these infants, the ventricular septum is intact and the left ventricular cavity is only a small slit with thick endocardial fibroelastosis. The ascending aorta is usually very small, ranging in size from 1-8 mm as measured using 2-dimensional echocardiography; mean diameter is 3.8 mm, and, in 55% of patients, the ascending aorta is smaller than 3 mm. The portion of ascending aorta between the atretic valve and the innominate artery serves only as a conduit for the retrograde flow of blood into the coronary arteries. The main pulmonary artery is very large and is the origin of a large ductus arteriosus that carries blood from the right ventricle into the aorta. A localized coarctation of the aorta is present in 80% of patients.
Other than the presence of a lethal chromosomal anomaly, other anomalies, or an extremely poor clinical condition, no absolute contraindications to surgical repair are recognized. However, several factors have been noted to convey higher surgical risk. Patients older than 1 month undergoing the Norwood procedure, patients with severe obstruction to pulmonary venous return, patients with significant noncardiac congenital conditions (eg, prematurity, low birth weight, chromosomal anomalies), and patients with the anatomic subtype of HLHS with aortic atresia are at increased risk. Details of the outcomes for patients who are at standard-risk and those who are at high-risk are outlined in Outcome and Prognosis.
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