Hypoplastic Left Heart Syndrome

Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (also known as HLHS), is a rare congenital heart defect in which the left side of the heart is severely underdeveloped.

Causes

While many authorities believe the cause of HLHS is unknown, recent research indicates that HLHS may be due to genetic factors. There is evidence associating it with GJA1.

Presentation In babies with HLHS, the aorta and left ventricle are very small, and the aortic and mitral valves are either too small to allow sufficient blood flow or are atretic (closed) altogether. As blood returns from the lungs to the left atrium, it must pass through an atrial septal defect to the right side of the heart. In a healthy human, the left side of the heart receives oxygen-rich blood from the lungs and pumps it out to the rest of the body; with these structures underdeveloped, they cannot circulate blood to other organs, and the right ventricle must pump blood to both the lungs, as it would normally, and to the rest of the body, a situation which cannot be sustained for long. In cases of HLHS, the right side of the heart often must pump blood to the body through a patent ductus arteriosus. As the ductus arteriosus usually closes within eleven days after birth, blood flow is severely restricted and eventually cut off, leading to dangerously low circulation and eventually to shock.

Treatment

Without treatment, HLHS is fatal, but with intervention, an infant may survive. A pediatric cardiothoracic surgeon may perform a series of operations or a full heart transplant. In the meantime, the ductus may be kept open to allow bloodflow using medication containing prostaglandin. Because these operations are complex and need to be individualized for each patient, a cardiologist must assess all medical and surgical options on a case-by-case basis. The two methods for treatment of HLHS are transplantation and a 3-stage surgical procedure. The 3-stage procedure is a palliative procedure (not a cure), as the child's circulation is made to work with only two of the heart's four chambers. * The first step is the Norwood procedure. In this procedure, the right ventricle is used to pump blood into the systemic circulation. Since the right ventricle is no longer directly pumping blood to the lungs, a shunt is required in order to pass deoxygenated blood through the lungs. Either the subclavian artery can be connected to the pulmonary circulation (Blalock-Taussig shunt), or a shunt is made directly from the right ventricle to the pulmonary circulation (Sano shunt). During this time the baby may be medically fragile and have feeding problems because the heart is working very hard. There is a considerable degree of venous mixing in the right ventricle, leading to lower oxygenation saturations. In addition, the Blalock-Taussig shunt and the Sano shunt both expose the lungs to systemic arterial pressures, leading in the long term to pulmonary hypertension and eventually to heart failure. * The second stage, the bi-directional Glenn procedure or Hemi-Fontan (see also Kawashima procedure) relieves some of the above problems. In this operation, the superior vena cava is ligated from the heart and connected to the pulmonary circulation. At this time, the Blalock-Taussig or Sano shunt is taken down. At this point, the lungs are no longer exposed to systemic arterial pressures, but much lower venous pressures. Although venous blood from the upper half of the body is no longer mixing with oxygenated blood in the right ventricle, there is still venous mixing from the lower half of the body, leading to some degree of oxygen desaturation. * The final procedure, the Fontan (Fontan procedure) completes the repair of the hypoplastic left heart. Although there are several variations, the functional effect is to redirect venous blood from the lower body (through the inferior vena cava) away from the right atrium to the pulmonary artery. Now, there should not be any mixing of oxygenated and deoxygenated blood in the right ventricle. The right ventricle performs the traditional job of the left, supplying the body with oxygenated blood, while the passive systemic venous pressure performs the traditional job of the right, passing deoxygenated blood to the lungs. The Norwood Procedure is generally performed within a week of birth, the second stage at 3-6 months of age, and the Fontan at 18 months to 4 years of age. There are two types of Fontan: the Lateral Tunnel Fontan, and the Extracardiac Fontan. When the Fontan Procedure was first being done for children with HLHS, the only Fontan was the Lateral Tunnel Fontan. This requires actual cutting in the heart itself to create a "tunnel" by which the blood can travel passively to the lungs. Within the last decade, doctors have created an Extracardiac Fontan. This operation creates a tunnel outside the heart itself which reduces the chances of Fontan patients developing scar tissue on the heart which might later cause arrythmias.

Prognosis

While infants successfully treated for HLHS have a good chance of survival, they may experience chronic health problems for the rest of their lives. The 3-stage surgeries were developed in the early 1980's with no survivors prior to that time. Therefore, the earliest survivors are in their early 20's and the long term prognosis is unknown. However, the advances in surgical and medical techniques have helped increase the survival rate dramatically since the surgeries were first developed. As is true for patients with other types of heart defects involving malformed valves, HLHS patients run a high risk of endocarditis, and must be monitored by a cardiologist for the rest of their lives to check on their heart function.

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