What is hypoplastic left heart syndrome? 

Hypoplastic Left Heart Syndrome (HLHS) is a severe, life-threatening congenital heart defect where the left side of the heart is severely underdeveloped. In a baby with HLHS, the left ventricle (the main pumping chamber), the mitral valve, the aortic valve, and the ascending aorta are all too small to function. This means the heart cannot pump oxygen-rich blood to the rest of the body. Without immediate medical intervention and a series of complex surgeries, HLHS is fatal within the first days of life. 

HLHS is rare, affecting approximately 1 in every 5,000 births in the UK. Because the left side of the heart is effectively missing its pumping capacity, the right side of the heart must do all the work, pumping blood to both the lungs and the body. However, the right side can only reach the body through a small fetal blood vessel called the ductus arteriosus. When this vessel naturally closes shortly after birth, the baby’s circulation collapses. Modern paediatric cardiac surgery in the UK has transformed HLHS from a non-survivable condition into one where many children now live into adulthood. 

What We will cover in this Article 

• The anatomical differences between a healthy heart and HLHS. 

• Why HLHS is a “duct-dependent” emergency at birth. 

• The three stages of surgical palliation: Norwood, Glenn, and Fontan. 

• Recognising early symptoms: Cyanosis and respiratory distress. 

• 2026 clinical data on survival rates and the “Fontan circulation.” 

• Long-term challenges, including liver health and exercise tolerance. 

The Anatomy of HLHS 

In a healthy four-chambered heart, the left side is the “high-pressure” system responsible for sending oxygenated blood to the brain and organs. In HLHS, this system is structurally inadequate. 

The condition is defined by four main structural failures: 

1. Hypoplastic Left Ventricle: The chamber is tiny and lacks the muscle to pump. 

2. Mitral and Aortic Atresia/Stenosis: These valves are either closed shut or so narrow that blood cannot pass through them. 

3. Aortic Hypoplasia: The main artery (aorta) is often the size of a thin piece of string. 

4. Atrial Septal Defect (ASD): A hole between the upper chambers is actually necessary in HLHS to allow blood to reach the right side of the heart. 

The “Duct-Dependent” Window 

A baby with HLHS may appear healthy at the exact moment of birth. This is because, in the womb, the ductus arteriosus allows blood to bypass the left side of the heart. Once the baby takes its first breaths, this vessel begins to close. 

As the ductus closes, the baby will develop: 

• Cyanosis: A blue or greyish tint to the lips, skin, and nails. 

• Respiratory Distress: Rapid, grunting breaths. 

• Cold Extremities: The legs and hands feel cold because blood isn’t reaching them. 

• Weak Pulses: It becomes difficult to feel a pulse in the baby’s groin. 

To prevent collapse, UK clinical teams administer Prostaglandin (PGE1), a continuous medicine that keeps the ductus open until the first surgery can be performed. 

The Staged Surgical Pathway 

There is no surgery that can “fix” the left ventricle. Instead, surgeons perform three operations to reroute the blood so the right ventricle can do the entire job. 

Stage	Procedure Name	Timing	Goal
Stage 1	Norwood Procedure	First 10 days	Builds a new aorta and provides a stable blood source.
Stage 2	Bidirectional Glenn	3 – 6 months	Connects upper body veins directly to lung arteries.
Stage 3	Fontan Procedure	2 – 4 years	Connects lower body veins to lungs; stops blood mixing.

Long-Term Outlook and the “Fontan Life” 

While the surgeries are life-saving, they create a unique circulation where blood flows to the lungs without a pump. This is known as the “Fontan circulation.” 

As these children grow into adults, they face specific challenges: 

• Exercise Tolerance: They may tire more easily than their peers because they lack a second pump for the lungs. 

• Liver Health: The “passive” flow to the lungs can cause back-pressure in the liver, necessitating regular liver scans. 

• Protein-Losing Enteropathy (PLE): A rare but serious complication where the body loses protein through the gut. 

• Neurodevelopment: Many children require extra support in school due to the impact of early surgeries and low oxygen levels on brain development. 

To Summarise 

Hypoplastic Left Heart Syndrome is one of the most complex heart defects, where the left side of the heart is too small to function. While it remains a critical condition requiring three major open-heart surgeries, the advancements in UK paediatric cardiology mean that many babies born with HLHS today can grow up, attend school, and lead meaningful lives. Success depends on early detection often at the 20-week scan and lifelong specialist care to protect the single working ventricle. 

If your baby has a blue or grey tinge to the lips, is struggling to breathe, or is cold to the touch, call 999 immediately. 

If your child with HLHS becomes unusually lethargic or stops producing wet nappies, call 999 immediately. 

Authority Snapshot (E-E-A-T Block) 

This article was reviewed by Dr. Stefan Petrov, a UK-trained physician (MBBS) with postgraduate certifications in ACLS and BLS. Dr. Petrov has extensive clinical experience in general medicine and emergency care, having worked in hospital wards and intensive care units where the stabilization of complex congenital heart defects is a primary focus. His expertise ensures this information on HLHS is accurate, safe, and aligned with current NHS and British Congenital Cardiac Association (BCCA) standards.