[Skip to Content]

Hypoplastic Left Heart Syndrome (HLHS)

What Is Hypoplastic Left Heart Syndrome (HLHS)?

Hypoplastic left heart syndrome (HLHS) is a heart problem where the left side of the heart doesn't grow to its usual size and strength.

Babies who have hypoplastic left heart syndrome are born with it.

What Happens in Hypoplastic Left Heart Syndrome (HLHS)?

With hypoplastic left heart syndrome, the left side of the heart can’t do its job. The left side of the heart is supposed pump blood to the body, but with hypoplastic left heart syndrome, it can't because it's too small or too weak. The left side of the heart includes the left ventricle, the left atrium, the left mitral valve, the aortic valve, and the aorta. 

Since the left side doesn't work correctly, a newborn with HLHS has to rely on several other parts of the heart. These include the right ventricle and the patent ductus arterious. Babies with HLHS are also born with another heart problem, an atrial septal defect, which in this case shouldn't be fixed right away. 

  • The right ventricle normally pumps blood to the lungs, where the blood picks up oxygen. From the lungs, the blood returns to the heart. Then the left side of the heart sends blood, now full of oxygen, to the body.

  • The ductus arteriosus connects two major arteries — the aorta and the pulmonary artery — that carry blood away from the heart. The ductus arteriosus normally shrinks and closes on its own in the first few days of life.

  • An atrial septal defect (ASD) is an abnormal opening in the dividing wall between the upper filling chambers of the heart (the atria). 

In a newborn, the right ventricle can pump blood to both the lungs and the body. But, that all changes when the ductus arteriosus closes. At that point, both sides of the heart are supposed to start doing their normal jobs.

In a child with HLHS, if the ductus arteriosus closes, there's no way of getting oxygen-rich blood to the body. And, the same goes for the atrial septal defect. So, both the ductus arteriosus and the atrial septal defect must be kept open until the heart is repaired.

What Problems Can Happen With Hypoplastic Left Heart Syndrome (HLHS)?

A newborn with hypoplastic left heart syndrome may not look sick right away. But problems can include:

  • a smaller than usual left atrium (the chamber receiving oxygen-rich blood from the lungs)
  • a small and weak left ventricle (which pumps oxygen-rich blood to the body)
  • a missing or tight mitral valve (which prevents backward flow of blood from the left ventricle to the left atrium)
  • a narrow aorta (the main blood vessel carrying blood from the heart to the body)
  • a missing or tight aortic valve (which normally prevents blood from flowing from the aorta back into the left ventricle)

What Are the Signs & Symptoms of Hypoplastic Left Heart Syndrome (HLHS)?

Starting a few hours to a day or so after delivery, a newborn with untreated hypoplastic left heart syndrome will have:

  • trouble breathing
  • blue or grayish coloring of the skin and nails
  • difficulty feeding
  • lethargy (very little activity)
  • weak pulses in the arms and legs
  • few wet diapers

Without treatment, the baby's blood pressure will fall too low to meet the body's needs, a condition called "shock."

What Causes Hypoplastic Left Heart Syndrome (HLHS)?

Most cases of hypoplastic left heart syndrome happen in the developing heart during early pregnancy. Some might be due to a combination of genes and things in the baby's and mother's environment during this early stage.

Who Gets Hypoplastic Left Heart Syndrome (HLHS)?

Babies with certain genetic problems have a greater chance of having hypoplastic left heart syndrome, but no single gene has been found to cause it.

There is a very small chance of hypoplastic left heart syndrome in every pregnancy. There is a higher chance in a second pregnancy if the first baby had hypoplastic left heart syndrome. 

How Is Hypoplastic Left Heart Syndrome (HLHS) Diagnosed?

Hypoplastic left heart syndrome may be seen on fetal (before birth) ultrasound scans. A fetal echocardiogram (a detailed ultrasound scan of the fetal heart) provides more information and tells the delivery team how to prepare for care after birth.

Common tests used to learn more about a newborn's heart include:

How Is Hypoplastic Left Heart Syndrome (HLHS) Treated?

Treatment for hypoplastic left heart syndrome includes:

  • helping the baby's heart pump enough blood 
  • using a medicine called prostaglandin to keep the ductus arteriosus open so the right ventricle keeps pumping blood to the body
  • surgery that directs enough blood to the body

Ventilator Support

A ventilator sometimes is used to help the baby breathe and let the intensive care team adjust oxygen levels, which can help keep blood flow to the body and lungs in balance.


Doctors usually treat hypoplastic left heart syndrome with a three-step plan:

  1. The Norwood procedure: Usually done within the first 2 weeks of life. Surgeons build a new aorta and place a tube to the lung blood vessels called a shunt. This redirects the right ventricle's output to go to both the body and the lungs instead of going only to the lungs.

    For some babies, especially those born early or who have problems in other organs (besides the heart), the Hybrid procedure is done first. This helps to stabilize the baby before the Norwood procedure. The Hybrid procedure is less risky because it doesn't require open heart surgery.
  2. The bidirectional Glenn procedure (also called the hemi-Fontan procedure): Usually done between 4–6 months of age, this operation allows blood returning from the upper part of the body to flow directly to the lungs without passing through the heart. Now the right ventricle only has to do one job, pumping blood to the body.
  3. The Fontan procedure: Most often done between 18–36 months of age, this operation channels blood from the lower half of the body to the lungs. Now, blue blood returning from the entire body flows to the lungs without passing through the heart. When it reaches the lungs, the blood becomes oxygen-rich (red). The heart pumps this oxygen-rich blood to the body.

Treatment helps the baby stay alive and grow, but doesn't completely correct the heart problems. In some cases, a heart transplant might be needed.


The baby will get medicines, including prostaglandin, through tubes called intravenous (IV) catheters. The catheters usually are placed in the stub of the baby's umbilical cord.

Cardiac Catheterization

Several cardiac catheterization techniques can help with hypoplastic left heart syndrome, such as inserting a wire mesh tube called a stent into the ductus arteriosus to keep it open.

Looking Ahead

Surgery does not cure hypoplastic left heart syndrome, so problems can happen that require more surgery. Follow-up appointments with a pediatric cardiologist (a doctor who specializes in heart conditions) are essential.

Reviewed by: Gina Baffa, MD
Date reviewed: January 2018