hypoplastic left heart syndrome

**Definition/anatomy:** Spectrum of left-sided underdevelopment: hypoplastic or absent LV, mitral stenosis/atresia, aortic stenosis/atresia, and a hypoplastic ascending aorta and aortic arch (often with coarctation). Requires an atrial communication (PFO/ASD) so oxygenated pulmonary venous blood can reach the right side, and a PDA to supply the systemic circulation.

**Pathophysiology:** Pulmonary venous return (oxygenated) crosses the atrial septum to the right atrium and mixes with systemic venous blood. The RV then supplies both circulations: pulmonary arteries directly, and the systemic circulation via the PDA (right-to-left shunt), with retrograde flow into the aortic arch and coronary arteries. This is a "ductal-dependent systemic circulation." Mixing causes cyanosis. Lower body perfusion depends on the PDA, so when it closes (24–48 h), the infant develops circulatory collapse, metabolic acidosis, and shock. Falling pulmonary vascular resistance after birth increases pulmonary blood flow at the expense of systemic flow.

**Etiology/associations:** Mostly multifactorial/sporadic, with some genetic contribution. Associated with Turner syndrome (45,X), Jacobsen syndrome (11q deletion), and other chromosomal abnormalities. It accounts for a small percentage of congenital heart disease but a disproportionate share of early neonatal cardiac deaths.

**Clinical presentation:** Normal at birth, then in the first days of life: tachypnea, poor feeding, lethargy, ashen/gray cyanosis, weak or absent peripheral pulses, cool extremities, hypotension, and shock as the PDA closes. Exam: single loud S2 (aortic component absent), possible nonspecific murmur, hepatomegaly (heart failure). Metabolic (lactic) acidosis.

**Diagnosis:** Prenatal fetal echocardiography (often detected on the 18–22 week anatomy scan, four-chamber view). Pulse oximetry screening for critical CHD at 24 h. Echocardiography is diagnostic. CXR: cardiomegaly with increased pulmonary vascular markings/pulmonary edema. ECG: right axis deviation, RV dominance, diminished left forces. Hyperoxia test: no significant rise in PaO2 with 100% O2 (fixed shunt).

**Management:** - Prostaglandin E1 infusion immediately to keep the ductus open (watch for apnea). - Avoid excess supplemental oxygen (a pulmonary vasodilator that steals systemic flow); avoid hyperventilation. Maintain a balanced Qp:Qs. - Stabilize acid-base status and perfusion; intubate if needed. - Staged surgical palliation: (1) Norwood procedure in the neonatal period (reconstruct the aortic arch using the native pulmonary artery as the neoaorta, with a systemic-to-pulmonary shunt such as a Blalock-Taussig or Sano shunt for pulmonary blood flow); (2) bidirectional Glenn (SVC to PA) at about 4–6 months; (3) Fontan (IVC to PA) at about 2–4 years, producing a single-ventricle circulation with the RV as the systemic ventricle. - Alternative: cardiac transplantation.

**Complications/long-term:** Arrhythmias, RV failure, protein-losing enteropathy and plastic bronchitis (Fontan complications), hepatic congestion/fibrosis (Fontan-associated liver disease), thromboembolism, and neurodevelopmental delay. Untreated, it is uniformly fatal within the first weeks of life.

**Board pearls:** - Ductal-dependent systemic circulation: HLHS, critical coarctation/interrupted aortic arch, critical AS. Contrast with ductal-dependent pulmonary circulation (tricuspid atresia, pulmonary atresia, critical PS, and severe tetralogy of Fallot). - Neonate who was well and then collapses at days 2–7 with weak pulses and shock: think PDA closure in a ductal-dependent lesion, and give PGE1. - Do not confuse with the 5 Ts of cyanotic CHD (Tetralogy of Fallot, Transposition, Truncus arteriosus, Tricuspid atresia, TAPVR), because HLHS is a classic cause of neonatal shock with differential perfusion.