Hypoplastic left heart syndrome (HLHS) is a rare congenital heart defect in which the left side of the heart does not fully develop. This restricts the pumping of oxygenated blood around the body.

One in every 3,841 babies born each year in the United States has this condition. It predominately affects males.

Symptoms may include difficulty breathing, a pounding heart, a weak pulse, and a bluish or ashen skin color.

Treatment involves a series of three surgeries to improve heart function. Without treatment, the condition is fatal. With surgery, two-thirds of children will survive to 5 years of age.

Keep reading to learn more about the symptoms, causes, diagnosis, treatment, and outlook for hypoplastic left heart syndrome.

HLHS is a rare type of congenital heart defect, which means it is present at birth. Another term for the syndrome is “underdeveloped left heart.”

When babies are born with a healthy heart, the right side receives oxygen-poor blood from the veins and pumps it to the lungs, where it becomes oxygen-rich blood. From the lungs, this blood flows into the left side of the heart, which pumps it into the aorta. The aorta is the largest artery in the body. It transports oxygenated blood to smaller arteries that reach all parts of the body.

Both sides of the heart consist of an upper chamber, the atrium, and a lower chamber, the ventricle. The atria receive incoming blood, while the ventricles pump outgoing blood.

In babies with hypoplastic left heart syndrome, the left side of the heart cannot pump oxygen-rich blood into the body in the usual way.

During the first few days of life, the oxygen-rich blood bypasses the left side of the heart and enters the heart through two openings that fail to close. This results in the right side of the heart pumping the blood into the body. Once these two openings close, it is difficult for oxygen-rich blood to circulate in the body.

According to the Centers for Disease Control and Prevention (CDC), the malfunction of the left side of the heart stems from the following developmental issues:

  • The left ventricle is too small: This structure, which is responsible for pumping oxygenated blood into the aorta, may not be large enough to function.
  • The aortic valve is too small or does not form: This structure controls the blood flow from the left ventricle to the aorta and may not be large enough to function. It may also be missing altogether.
  • The mitral valve is too small or does not form: This structure facilitates the blood flow between the left atrium and left ventricle. The left atrium receives oxygenated blood from the lungs and sends it to the left ventricle. The mitral valve may be too small to function, or it may be missing.
  • The ascending part of the aorta is too small or does not fully develop: This is the first part of the aorta and may be too small to function or not form at all.
  • An atrial septal defect is often present: This is a hole in the heart between the right and left atria.

Due to the two openings in the heart that are present at birth, babies may not show symptoms during the first few days of life. However, after a few days, when these openings close, symptoms will rapidly occur. They may include:

  • pounding heart
  • weak pulse
  • bluish or ashen skin color
  • problems breathing

The causes of most heart defects, including hypoplastic left heart syndrome, are not known. Some defects stem from changes resulting from a combination of genetic and other factors. These other factors could include a pregnant person’s contact with certain:

  • environmental agents
  • foods and beverages
  • medications

Doctors can diagnose hypoplastic left heart syndrome during pregnancy or after the baby’s birth.

During pregnancy

During pregnancy, an ultrasound may indicate the presence of the condition. If this happens, a fetal echocardiogram can confirm a diagnosis. An echocardiogram is an ultrasound of the heart that can reveal structural or functional abnormalities.

After birth

After birth, diagnosis involves:

  • physical examination, which can show a heart murmur or manifestation of symptoms
  • echocardiogram
  • pulse oximetry screening, which measures the oxygen level in the blood

Treatment may include medications, nutritional changes, and surgeries.


Some babies and children may need medications to:

  • lower blood pressure
  • strengthen the heart
  • help the body remove excess fluid

Following diagnosis, the first medication a doctor will administer is a low dosage of prostaglandin E1, also known as PGE1. This helps maintain circulation.


Babies may become tired when feeding, which results in insufficient nutrition intake for weight gain. When this happens, a doctor may prescribe a high calorie formula or recommend a feeding tube.


Soon after birth, babies may undergo three surgeries, in a set order, to increase blood flow. This is the primary treatment. They do not provide a cure, but they do help with heart function.

Babies who undergo these surgeries may have lifelong complications. In some cases, a heart transplant is necessary. However, currently, 20% of babies in line for a heart transplant will die waiting for a donor.

Stage 1: Norwood procedure

This procedure happens within the first 2 weeks of life. It involves creating a new aorta to connect to the right ventricle.

It also involves the insertion of a tube from the right ventricle or aorta to connect to the pulmonary artery, which is the artery going to the lungs. This enables the right ventricle to pump blood to the lungs and the rest of the body.

Stage 2: Bidirectional Glenn procedure

Doctors perform this surgery when an infant is 4–6 months old. They will create a connection between the pulmonary artery and the superior vena cava, the vein that drains oxygen-poor blood from the upper part of the body into the heart.

This decreases the workload on the right ventricle because it permits blood returning from the upper part of the body to flow into the lungs without going through the heart.

Stage 3: Fontan procedure

This procedure occurs when an infant is between 18 months and 3 years old. It connects the pulmonary artery with the inferior vena cava, the primary vein that returns oxygen-poor blood from the lower part of the body to the heart.

According to 2019 data from the Society of Thoracic Surgeons Congenital Heart Surgery Database (STS CHSD), the mortality rates for each procedure are as follows:

ProcedureMortality rate
Stage 1: Norwood procedure15%
Stage 2: Bidirectional Glenn procedure1.8%
Stage 3: Fontan procedure1.0%

The first surgery involves the highest risk of complications and death, but survival rates are higher after the second and third procedures.

Special care to foster growth and development is necessary for babies and children who survive these procedures, as they have higher risks of:

Regular follow-up care at a center that specializes in the condition is necessary.

As children with congenital heart defects and their families face many challenges, families may wish to contact organizations that offer various help, such as:

Without treatment, the life expectancy is a few days to a few weeks.

With surgical intervention, up to 72% of children survive 5 years.

Hypoplastic left heart syndrome (HLHS) is a congenital heart defect that causes underdevelopment of the structures on the left side of the heart. Consequently, the heart cannot successfully pump oxygenated blood to the body.

As with most congenital heart defects, the cause is unknown.

Doctors can diagnose HLHS before birth with an echocardiogram. After a child is born, doctors may use a physical exam, echocardiogram, and pulse oximetry screening to diagnose the condition.

While medications may help the heart work better, and extra nutrition may foster weight gain, the main treatment regimen involves a series of three surgeries.

Survival rates are shown to increase over time, with roughly two-thirds of children reaching the age of 5 years with surgery.