Hypoplastic left heart syndrome is the incomplete formation of the chambers, valves, and or vessels of the left side of the heart. Infants with HLHS are also born with additional defects including ventricular septal defect, patent ductus arteriosus, and narrowing other parts of the aorta.
During development blood flow is needed to enable the structures of the cardiovascular system to fully develop. If blood flow through a particular structure is decreased due to a congenital abnormality, then not only does it develop poorly, but all the structures downstream develop poorly. Thus if there is congenital mitral stenosis or mitral regurgitation, then associated hypoplasia of the left ventricle, aortic valve and aorta results. Similarly on the right side of the heart. If there is tricuspid stenosis or tricuspid atreasia or even Ebstein’s anaomaly, flow is decreased resulting in a myriad of hypoplastic events downstream and hence the hypoplastic right heart sysndrome. There are varying severities and degree of involvement in each of these syndromes – sometimes incompatinble with life, sometimes amenable to surgery and sometimes mild enough not to be even detected.
With poor systemic supply of oxygenated blood from the left heart, the ductus arteriosus usually remains open so that blood, albeit deoxygenated blood from the pulmonary circulation, is able to provide some blood to the systemic circulation. Mixing of deoxygenated and oxygenated blood therefore occurs. Infants are born with extremely low oxygen saturation rates as well as respiratory distress. HLHS can be fatal within hours if untreated. Standard fetal ultrasound is used to diagnose HLHS. This defect is not entirely curable. Various surgical treatments exist in stages including cardiac transplantation if HLHS is severe. The Norwood procedure is commonly performed as the first stage followed by a second procedure which establishes adequate connection for the systemic and and pulmonary circulations.