The term ‘hypoplastic left heart syndrome’ was coined by Noonan and Nadas in 1958 to describe a very characteristic underdevelopment of the left side of the heart. Classically, it comprises a very small or diminutive ascending aorta in association with severe aortic stenosis or aortic atresia and mitral stenosis or atresia. There is variable underdevelopment of the left ventricle and usually associated hypoplasia of the aortic arch with coarctation of the aorta. Physiologically, the systemic circulation is supported by the right ventricle (RV) with blood flow through a patent ductus arteriosus (PDA) supplying the systemic circulation. The majority of patients die within a few hours or days of birth when the ductus arteriosus closes and the systemic circulation is stopped (Figure 20.1). Whilst hypoplastic left heart syndrome is rare, a 2 to 3 per cent of all congenital defects, untreated it is responsible for about 30 per cent of neonatal cardiac deaths. No definite cause for the condition has been found, although in a minority of cases some chromosomal abnormalities are associated with this condition (e.g. Jacobsen syndrome)
Whilst hypoplastic left heart syndrome represents the more severe end of the spectrum of hypoplasia of the left heart, there can be a variable degree of hypoplasia of the left ventricle, left ventricular outflow tract and mitral valve in association with aortic arch anomalies. Much of the decision making as to possible surgery for this condition relates to deciding whether or not the left side is big enough to support the systemic circulation; in other words, can the heart be septated into left and right ventricles?
The management of hypoplastic left heart syndrome was revolutionized in the 1970s and 1980s by the introduction of prostaglandin into paediatric cardiology practice in order to maintain patency of the ductus arteriosus. At the same time, intensive care management of very sick neonates improved dramatically, and it was possible to keep these children alive for possible surgical interventions. The hypoplasia of the left ventricle and aorta means that the systemic circulation cannot be supported in the usual way by the left heart, and the systemic perfusion depends on blood flow through a PDA. This duct can be kept open with prostaglandin and the child either resuscitated or, if diagnosed antenatally, prevented from collapse.
In the 1980s, Bill Norwood in Boston Children’s Hospital pursued a surgical programme to attempt to palliate these babies and developed the three-stage Norwood procedure. This procedure and its derivatives are used by most congenital heart centres now. Other methods of treatment such as the hybrid procedure and cardiac transplantation are also used but only in a small number of centres.
Classically, the Norwood procedure has three components. The first stage, performed in the first few days of life, comprises a reconstruction of the systemic outflow tract from the heart by connecting the pulmonary artery to the aorta. Blood flow to the lungs is achieved by a modified Blalock-Taussig (BT) shunt in the classical Norwood procedure (Figure 20.2); more recently, many centres are using a direct RV-to-PA conduit (Figure 20.3). At the same procedure, an open atrial septectomy is performed so that there is no restriction of blood flow from the pulmonary veins into the RV. The circulation thus comprises systemic and pulmonary venous return, mixing in a common atrial chamber, and passing through the tricuspid valve into the right ventricular outflow tract and then to the systemic circulation. This systemic circulation also supplies the pulmonary blood flow, either by the BT shunt or by the RV-to-PA conduit.