Hypoplastic left heart syndrome (HLHS) is a rare but severe cardiac malformation that is invariably fatal without treatment. It is characterized by the failure of the left side of the heart to develop normally. This includes:
- A small or hypoplastic left ventricle
- Atresia or stenosis of the ventricular outflow and inflow tracts
- Narrowing of the proximal aortic segment
- Atresia or stenosis of the mitral and aortic valves
The Pathology of HLHS
The infant with HLHS has a left ventricle which is completely unable to pump oxygen-rich blood reaching the left atrium to the rest of the body through the aorta. Both structural and functional abnormalities make it impossible for a normal circulation to be maintained. Instead, oxygenated blood from the lungs reaches the left atrium and is shunted to the right atrium through the patent foramen ovale (an opening in the interatrial septum which is normally present in fetal life and at birth). From here it mixes with the deoxygenated venous blood that reaches the right atrium. Passing into the right ventricle, the blood is pumped back to the lungs. However, some blood reaches the systemic circulation via the patent ductus arteriosus which connects the pulmonary artery to the aortic arch.
The ductus arteriosus is a fetal vessel that allows blood from the pulmonary trunk to bypass the lungs and enter the systemic circulation instead. It usually closes soon after birth as the pulmonary vascular resistance drops with lung expansion. However, in infants with HLHS this is fatal because it deprives the body of its only source of oxygen-rich blood, passing from the pulmonary trunk into the aorta. The infant usually develops cardiogenic shock, around the fourth day of life.
Understanding the pathology of abnormal circulation in HLHS helps to plan the patient’s management. Immediate treatment of a symptomatic infant with a hypoplastic left ventricle depends primarily on keeping the ductus arteriosus patent, supplying blood from the right ventricle to the systemic circulation. This is achieved by continuous infusion of prostaglandin E1 which inhibits ductus closure. In addition, diuretics, inotropics and correction of metabolic acidosis are all to be employed judiciously to keep such infants alive and out of shock while they are being prepared for surgery. Special formula may be prescribed for such infants to prevent malnutrition, as they tire quickly during feeding.
In addition, the closure of the foramen ovale puts additional stress on the circulation by closing off a shunt between the atria. This may require enlargement of or creation of the opening by balloon or blade septostomy or open septectomy.
Definitive surgical management is by a staged procedure (the Norwood procedure).
- In the first stage, the pulmonary trunk is fused longitudinally with the narrow aorta, to form a larger new aorta. The pulmonary arteries arising from the trunk are separated and connected to the new aorta, or, in a new modification, to the right ventricle, through a shunt vessel, to supply a lower blood volume to the lungs.
- In the second stage, a bidirectional Glenn shunt is created at 4-6 months, fusing the superior vena cava to the right pulmonary artery (a cavopulmonary shunt). This prevents the venous blood from the upper part of the body from mixing with the oxygenated blood from the left atrium. It is directed straight to the lungs instead, improving the oxygen saturation and preventing volume load on the right ventricle.
- In the third stage, the Fontan procedure is performed at around 1.5 to 3 years, creating a total cavopulmonary shunt. This involves fusing the inferior vena cava to the right pulmonary artery by forming a lateral tunnel inside the right atrium, using a prosthetic baffle and a small part of the lateral right atrial wall. A later modification uses an extracardiac shunt to route the inferior vena cava blood straight to the pulmonary artery, not involving the right atrium at all.
The Fontan circuit thus created, allows oxygenated blood to travel from the left atrium through the right atrium, without mingling with venous desaturated blood, into the right ventricle. This then supplies the whole body through the new aorta. In exchange, the pulmonary circulation is perfused from the great veins, without the participation of the right ventricle.
Even after palliation staged surgery the patient will need lifelong monitoring and will be at risk of complications throughout life. However, survival rates have dramatically improved, and the surgeries have improved the quality of life for these survivors, with constant refinement and modification of the surgical technique.
Cardiac transplantation is also considered an option in infants with HLHS. This procedure has a higher mortality rate at present, besides the difficulty of obtaining suitable hearts for transplant.