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Hypoplastic left heart syndrome (HLHS) is a severe cyanotic congenital heart defect characterized by underdevelopment of the entire left-sided heart structures, resulting in the inability of the left ventricle (LV) to support systemic circulation. It is uniformly fatal without surgical intervention.

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![The left side of the heart is very poorly formed and cannot support the main circulation (round the body). The left ventricle and aorta are abnormally small (hypoplastic). This is amongst the most severe forms of heart defect. Most babies are very ill in the early days of life and need urgent surgery to survive. The first stage of surgery is called a 'Norwood' operation.

Cardiology : Hypoplastic Left Heart Syndrome HD. Rch.org.au. Published 2025. Accessed July 11, 2025. https://www.rch.org.au/cardiology/heart_defects/Hypoplastic_Left_Heart_Syndrome_HD/](attachment:b6b17967-9dc3-47b7-9a23-bc06f8220b51:10a_Hypoplastic_left_heart_syndrome.jpg)

The left side of the heart is very poorly formed and cannot support the main circulation (round the body). The left ventricle and aorta are abnormally small (hypoplastic). This is amongst the most severe forms of heart defect. Most babies are very ill in the early days of life and need urgent surgery to survive. The first stage of surgery is called a 'Norwood' operation.

Cardiology : Hypoplastic Left Heart Syndrome HD. Rch.org.au. Published 2025. Accessed July 11, 2025. https://www.rch.org.au/cardiology/heart_defects/Hypoplastic_Left_Heart_Syndrome_HD/

Key Anatomic Features

Structure	Anomaly
Left ventricle (LV)	Severely hypoplastic or absent
Mitral valve	Hypoplastic, stenotic, or atretic
Aortic valve	Hypoplastic, stenotic, or atretic
Ascending aorta	Very small; systemic output dependent on ductus arteriosus
Left atrium	Small; receives pulmonary venous return

Pathophysiology

Systemic blood flow depends on right ventricle (RV) via:
- Patent ductus arteriosus (PDA) → retrograde flow into hypoplastic aorta
Pulmonary venous return must pass through patent foramen ovale (PFO) or atrial septal defect (ASD) to reach the RA
Closure of PDA or restrictive atrial shunt leads to cardiogenic shock and death

Clinical Presentation

Typically presents within the first few days of life as ductus closes
Initial period may appear normal due to PDA
Features:
- Profound cyanosis
- Tachypnea, respiratory distress
- Poor perfusion, lethargy, metabolic acidosis
- Shock, cardiac arrest (if untreated)
- Single second heart sound, weak or absent pulses

Complications

Interstage mortality
Arrhythmias, heart failure, thromboembolism