Hypoplastic left heart syndrome (HLHS) represents 1-2% of all congenital heart diseases, and is the most common lethal cardiac defect in neonates . It is characterised by a variable degree of underdevelopment of the left heart structures, with hypoplasia or absence of the left ventricle, mitral stenosis or atresia, aortic stenosis or atresia and hypoplasia of the aortic arch. The result is an inability of the left side of the heart to maintain the systemic circulation. A duct-dependent circulation exists, with the right ventricle (RV) supporting the systemic circulation via right to left flow through the ductus arteriosus. Initial survival is dependent on continued ductal patency, unrestrictive atrial mixing to allow the pulmonary venous return to reach the systemic circulation and a balance between systemic (SVR) and pulmonary vascular resistance (PVR) to achieve adequate systemic and pulmonary blood flow.
The incidence of HLHS is approximately 2 in every 10,000 births . The precise aetiology is unknown. Research has indicated a genetic component with heritability seen in some families. There is an association with other genetic syndromes including Turners’, Jacobsen’s, Smith-Lemli-Opitz’s, Trisomy 13 & 18, and chromosomes 17 & 18 deletion, amongst others. Extra cardiac anomalies are rare in HLHS however, their presence, or the presence of genetic syndromes in association with a diagnosis of HLHS, usually carry a worse prognosis.
In regions with foetal ultrasound screening programmes, the diagnosis of HLHS is often prenatal allowing for parental counselling, including options of elective termination or no intervention (comfort care), and allowing postnatal management planning. Neonates are often born in reasonably good condition, and diagnosis by echocardiography may follow detection of a murmur, weak/absent femoral pulses (if there is significant arch hypoplasia) or cyanosis if clinically present. As the ductus arteriosus closes in the first week of life, rapid decompensation is likely, and neonates may present in extremis with cardiorespiratory failure and shock. Without intervention, 90% will die in the neonatal period.
Once considered universally lethal, outcome has improved with primary cardiac transplantation or staged surgical reconstruction to create a viable circulation. With surgical intervention, 50-70% of neonates born with HLHS are now expected to survive to adulthood (Feinstein, 2012). Limited availability of suitably small donor organs and the high risk of mortality whilst awaiting a transplant dictates that most HLHS patients will undergo staged palliation. For all but the most borderline of cases, palliation is to a functionally univentricular pathway. The final stage is the creation of a total cavopulmonary univentricular circulation, the Fontan circulation, where pulmonary blood flow is supplied by passive systemic venous return, and the right ventricle is the single systemic ventricle. Right ventricular dysfunction is common, with myocardial ischaemia, chronic overload, and the decreased ability of the morphological right ventricle to sustain systemic pressures all contributing factors. In addition, the Fontan circulation and associated elevated systemic venous pressures may impact on the function of other organ systems, such as the development of hepatic dysfunction. As cardiac function deteriorates, heart transplantation may re-emerge at a later stage as the only option for prolonged survival.
HLHS patients represent a high-risk population for morbidity and mortality under anaesthesia. Each reconstructive stage presents unique anatomical and physiological considerations for the anaesthetist. A clear understanding of the associated physiology and possible complications is of paramount importance in ensuring optimal patient care. Surgery should ideally be performed in centres with the necessary expertise and the availability of appropriate intensive care support.