Hypoplastic left heart syndrome (HLHS) is a rare and complex congenital cardiovascular malformation. The incidence is approximately 1 to 27 per 100,000, accounting for about 1.4% of congenital cardiovascular malformations. The common feature of these malformations is stenosis or atresia in a part of the left heart circulation, leading to dilation and increased pressure in the left atrium, pulmonary veins, and pulmonary arteries. As a result, blood flow to the right ventricle increases, and survival depends on the presence of a patent ductus arteriosus. In 1952, Lev described congenital cardiovascular malformations characterized by hypoplasia of the left heart and hypertrophy of the right heart as hypoplastic left heart syndrome. In 1958, Noonan and Nadas referred to cardiac vascular malformations involving obstruction and hypoplasia of the left heart as hypoplastic left heart syndrome. This condition is associated with genetics, with a family history and a recurrence rate among siblings of .%. Twins may also be affected simultaneously. Some believe it is caused by a single gene mutation. The pathogenesis remains unclear, but it may be due to an abnormally large ductus arteriosus during fetal development, allowing a significant amount of blood to flow through the patent ductus arteriosus into the descending aorta, or premature closure of the atrial septum and foramen ovale, reducing blood flow to the left heart and aortic arch, resulting in hypoplastic left heart syndrome.

bubble_chart Pathological Changes

The main pathological features of this condition are: ① Atresia or severe stenosis of the main stirred pulse valve and hypoplasia of the ascending stirred pulse, with approximately 35-80% of cases accompanied by coarctation of the stirred pulse. ② Atresia or hypoplasia of the mitral valve. ③ Hypoplasia of the left ventricle. ④ Hypertrophy of the right heart, manifested as enlargement of the right atrium and right ventricle, especially with abnormal dilation of the pulmonary stirred pulse. ⑤ Presence of a large stirred pulse duct, atrial septal defect, or patent foramen ovale.

I. Atresia of the main stirred pulse

a. With hypoplasia or stenosis of the mitral valve

b. With atresia of the mitral valve

II. Atresia of the mitral valve

III. Stenosis of the mitral valve

a. Normal main stirred pulse orifice

b. With stenosis of the main stirred pulse valve

IV. Hypoplasia of the stirred pulse arch

V. Atresia or interruption of the stirred pulse arch

Noonan classifies it into two categories:

I. Atresia or severe stenosis of the main stirred pulse

II. Atresia of the mitral valve

Hemodynamics: In hypoplastic left heart syndrome, the right atrium receives blood from both the superior and inferior vena cava and blood from the left atrium through an atrial septal defect. Therefore, systemic and pulmonary venous blood mix in the right atrium and are pumped by the right ventricle into the pulmonary stirred pulse and left and right pulmonary stirred pulses, and then enter the descending stirred pulse through a large stirred pulse duct, retrograde perfusion of the ascending stirred pulse and coronary stirred pulse. The shunts at the atrial septal defect and stirred pulse duct are prerequisites for the completion of systemic and pulmonary circulation in children with hypoplastic left heart syndrome. Moreover, the shunt volume of the atrial septal defect and the oxygen saturation of the stirred pulse are directly proportional to the size of the atrial septal defect. After birth, due to a significant decrease in pulmonary vascular resistance and an increase in pulmonary blood flow, and often accompanied by coarctation of the stirred pulse and closure of the stirred pulse duct, systemic vascular resistance is relatively high. The imbalance in systemic and pulmonary blood flow ratios results in pulmonary congestion, pulmonary edema, and congestive heart failure, while inadequate perfusion of organs leads to hypoxemia and acidosis, ultimately resulting in death.

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The infant may be a normal full-term baby, but progressive cyanosis, tachypnea or dyspnea, and heart failure may appear within a few hours after birth. No specific murmurs are heard on cardiac auscultation. The second heart sound in the pulmonary stirred pulse area is accentuated and single. The pulse is weak. It is accompanied by acidosis, hypoglycemia, hypoxemia, and shock. If the atrial septal defect has a large shunt, cyanosis may not be obvious, and hypoxemia is milder. About 90% of infants will die within 1 month after birth if diagnosis and treatment are deficient.

bubble_chart Auxiliary Examination

Chest X-ray: Shows enlargement of the right atrium and right ventricle, a spherical heart shadow, pulmonary congestion, and signs of pulmonary edema.

Electrocardiogram: Tall P waves, right axis deviation, and right ventricular hypertrophy.

Echocardiogram: Two-dimensional imaging reveals a massive right atrium and right ventricle, with a small and thick-walled left ventricle, hypoplastic mitral valve, and a narrow ascending aorta and arch.

Right heart catheterization: Can demonstrate atrial level shunting and confirm the presence of a patent ductus arteriosus, but atrial shunting may not be detectable when systemic pressure and oxygen saturation approach pulmonary levels.

Cardiac angiography: Pulmonary angiography can reveal flow through the patent ductus arteriosus into the aorta and show aortic coarctation. Retrograde aortic angiography shows hypoplasia of the ascending aorta and arch, possibly accompanied by aortic coarctation or interruption. Left atrial angiography can assess the development of the mitral valve.

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Preoperative management: Upon definitive diagnosis, administer prostaglandin E1 at 0.01 μg/(kg·min) via continuous intravenous infusion, control oxygen concentration at around 21%, aiming to keep the ductus arteriosus open, use a ventilator to avoid hyperventilation, maintain the carbon dioxide partial pressure in the ductus arteriosus blood above 4.0～5.33 kPa (30～40 mmHg) to promote pulmonary vasoconstriction, increase resistance, reduce pulmonary blood flow, thereby increasing the blood flow into the descending aorta through the ductus arteriosus. Maintain normal levels of water, electrolytes, and blood pH, aiming for an oxygen saturation of 80～85%.

Surgical treatment is the only effective method. Due to the high pulmonary vascular resistance in early neonates, the mortality rate of radical corrective surgery is very high, hence staged surgery is often performed.

The initial stage [first stage] surgery is performed under extracorporeal circulation combined with hypothermia or deep hypothermic circulatory arrest, and extracorporeal circulation can be established via the pulmonary artery or ductus arteriosus and right atrial cannulation. The basic principles of the surgery are as follows: ① Remove most of the atrial septum to create a large atrial septal defect or a condition similar to a single atrium, eliminate the pressure difference between the left and right atria, allowing smooth flow of left atrial blood into the right atrium for full mixing, improving oxygen saturation. ② Aortic arch plasty and relief of aortic coarctation: Make a longitudinal incision along the lower edge of the aortic arch from the origin of the right innominate artery to the upper part of the descending aorta, and transversely cut the pulmonary artery at the level of the sinus of Valsalva, suture the distal incision of the pulmonary artery, and use an artificial fabric (Gortex or PTFE) patch to anastomose the upper and lower edges of the aortic arch incision, allowing right ventricular blood to flow smoothly into the aorta and coronary arteries. However, artificial fabric anastomosis is prone to bleeding or folding, affecting blood flow. Therefore, JoNas et al. moved the pulmonary artery incision down to the level above the pulmonary valve, and transversely cut at the bifurcation of the pulmonary artery, using the obtained pulmonary artery wall to replace the artificial fabric for aortic arch enlargement and reconstruction, and connecting it to the right ventricular outflow tract. Lau reported a longitudinal incision on the posterior wall of the pulmonary artery, reconstructing a 4mm pulmonary artery, and anastomosing the pulmonary artery to the aortic arch. ③ Ligate the ductus arteriosus, reconstruct a suitable systemic-pulmonary shunt pathway, earlier more commonly performing a subclavian artery to right pulmonary artery anastomosis (Blalock-Taussig surgery), in recent years advocating for a central shunt, i.e., making a 3～4mm shunt between the aorta and pulmonary artery.

Postoperative management: If excessive pulmonary blood flow occurs early, it can be reduced by increasing pulmonary vascular resistance and pressure. Administer 21% oxygen, maintain carbon dioxide partial pressure above 4.0～5.33 kPa (30～40 mmHg). Treat metabolic acidosis, increase positive end-expiratory pressure during assisted ventilation. If excessive pulmonary blood flow is found to be due to an overly large reconstructed pulmonary artery, a pulmonary artery banding can be performed. If pulmonary blood flow is too low, administer pure oxygen hyperventilation, reduce carbon dioxide partial pressure below 4.0 kPa (30 mmHg), promote pulmonary vasodilation, reduce resistance, adrenaline drugs can increase systemic circulation resistance, thereby increasing pulmonary circulation shunt ratio, increasing pulmonary blood flow. However, if systemic circulation resistance is less than 8.67 kPa (65 mmHg), and carbon dioxide partial pressure is less than 2.67～3.33 kPa (20～25 mmHg), it indicates an anastomotic stenosis, conservative treatment prognosis is poor.

Routine review and cardiac catheterization are often performed six months postoperatively, if an atrial septal defect is found to be too small or aortic stenosis, balloon catheter dilation can be performed again.

Physiological Correction Surgery: The first-stage surgery can be performed 12 to 18 months after the initial operation. Surgical methods: ① Resect the atrial septum and use an intra-atrial patch to direct left atrial blood through the tricuspid valve into the right ventricle, repairing the atrial septal defect. ② Ligate or suture the main and pulmonary stirred pulse shunt sites, preserving the right ventricular outflow tract or the anastomosis between the proximal pulmonary stirred pulse and the main stirred pulse arch, allowing the right ventricle to continue supplying blood to the systemic circulation. ③ Anastomose the right atrial appendage to the right pulmonary stirred pulse, with the anterior wall potentially enlarged using a pericardial patch or connected via a valved external conduit, known as the modified Fontan procedure. The success of the surgery critically depends on the pulmonary stirred pulse pressure.

Approximately 1/5 of cases can achieve satisfactory therapeutic results through physiological correction, but the right ventricle bears the pressure of the systemic circulation for a long time, and its long-term effects remain to be followed up.

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