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What is hypoplastic left heart syndrome?

Hypoplastic left heart syndrome (HLHS) is a spectrum of heart diseases in which the left-heart structures are underdeveloped. These structures include the mitral valve, the left ventricle, the aortic valve, and the aorta. HLHS is a congenital heart defect that begins during pregnancy. The cause of HLHS is unclear, and there are no known risk factors for this disease. It is not preventable.

Smiling baby, with red cheeks and wearing a crown of flowers, looks at the camera.

An expectant parent’s guide to HLHS and other single ventricle defects

It’s sometimes difficult to remember every detail of a fetal cardiology consult, so we’ve prepared everything you need to know in “An Expectant Parent’s Guide to Hypoplastic Left Heart Syndrome and Other Single Ventricle Defects” — written by our Fetal Cardiology coordinator, Terra Lafranchi, NP-C, and available in English and Spanish.

Hypoplastic Left Heart Syndrome | Symptoms & Causes

What are the symptoms of hypoplastic left heart syndrome?

In most cases, children with hypoplastic left heart syndrome appear healthy at birth.

When a baby is not diagnosed during pregnancy, symptoms may develop within the first few hours or days of life:

  • rapid breathing or shortness of breath
  • rapid heartbeat or pounding heart
  • poor suckling and feeding
  • cold extremities (poor perfusion)
  • blue color of the skin, lips, and nailbeds (cyanosis)
  • weakness

What causes hypoplastic left heart syndrome?

HLHS is the result of under-development of sections of the fetal heart during the first eight weeks of pregnancy.

The cause of HLHS is complex and involves many factors. In some families, there is a genetic predisposition or tendency towards developing HLHS, but most families have no prior history of congenital heart disease. HLHS comes in many different forms. Some patients have a primary problem with their left ventricular heart muscle, some with the aortic valve, and some with the mitral valve. Many have problems with all three. These details matter to us at Boston Children’s, and we customize the care to meet your baby’s needs.

Meet Jack

After a pre-natal ultrasound, Jack was diagnosed with HLHS. Now 17, Jack is an example of the innovation of the fetal cardiology team at Boston Children’s.

Hypoplastic Left Heart Syndrome | Diagnosis & Treatments

How is hypoplastic left heart syndrome (HLHS) diagnosed?

Most pregnant women have fetal ultrasounds during the course of their pregnancies. If a basic (level one or two) ultrasound shows the possibility of heart abnormalities, a more detailed cardiac ultrasound — fetal echocardiogram — can correctly diagnose your baby’s heart defect.

What happens when a baby is born with HLHS?

Most babies with HLHS appear healthy right after birth. If your baby was not diagnosed during pregnancy, it will become apparent within the first few hours to days that your baby has a heart defect. Some combination (not necessarily all) of the following medical tests are used to diagnose or evaluate HLHS:

Based on the results of these tests, your child’s doctors will determine the best approach to treatment.

What are the treatment options for HLHS?

Fetal cardiac intervention

A small number of babies with hypoplastic left heart syndrome may benefit from intervention before birth. Our highly trained cardiologists can use a needle or tiny balloon catheter to open abnormal heart valves or other obstructions. Learn more about the Fetal Cardiology Program.

Medical treatment after birth

If fetal intervention is not an option for your baby, treatments after birth will focus on stabilizing your baby and may include:

  • medication to keep the patent ductus arteriosus (PDA) open
  • intravenous fluids through a tube inserted into a vein
  • a feeding tube if your baby has difficulty feeding
  • breathing assistance provided by a ventilator to ensure adequate oxygen
  • a balloon atrial septostomy procedure to create an opening in the wall between the upper chambers of the heart to improve mixing of oxygen-rich blood and oxygen-poor blood

Shortly after birth, your child’s care team will take images of your baby’s heart using an echocardiogram to see every detail of your baby’s heart — the way the blood flows, the size and function of the ventricles, and how well the valves work.


Your child will usually need a series of three operations. The goal of these surgeries is to enable the fully-functioning right ventricle to do the work normally done by two ventricles and to separate the oxygen-rich blood from the oxygen-poor blood.

Biventricular repair

In some cases, a baby’s single ventricle heart can be converted into two functioning ventricles (biventricular circulation) through our Complex Biventricular Repair Program. This may be an option for children with borderline HLHS. In some situations, this repair can be the initial procedure, but in others, a series of procedures may be used to recruit the small left ventricle before converting the heart to biventricular circulation.

Home monitoring between stages 1 and 2

During the period between stage 1 and stage 2 surgeries, your baby’s heart circulation requires extra monitoring to prevent complications. Our Home Monitoring Program will give you goals for your baby’s growth and oxygen saturation levels and provide monitoring equipment for your baby. You’ll also get information on when to call.

What is the long-term outlook for children with HLHS?

The long-term outlook for children with HLHS continues to improve. Your child’s ultimate lifespan and quality of life depends on many factors. Every year, medical advancements further extend and improve the quality of life for those born with HLHS.

Your care team will have an ongoing relationship with your family and can help you create a long-term care program for your baby. The team can also advise you and your child on daily life issues such as exercise, activity levels, and nutrition.

How we care for HLHS

At Boston Children’s Hospital, we have a team of specialists ready to evaluate your baby starting in the Maternal Fetal Care Center. This team includes cardiologists who specialize in fetal assessments, and a team of nurses and other specialists who will make sure your baby is in the best possible condition when born. They will help you plan for every step along the way. In some cases, we are able to offer a fetal cardiac intervention even before the baby is born.

Boston Children’s heart surgeons treat a high volume of some of the most complex pediatric heart conditions in the world, with survival rates of more than 98 percent. Our approach to treating HLHS is different for every child, based upon their individual needs. We are one of the few hospitals in the country to offer complex biventricular repair, an approach pioneered at Boston Children’s.

Hypoplastic Left Heart Syndrome | Research & Innovation

Our areas of innovation for HLHS

The Boston Children’s Benderson Family Heart Center offers a unique combination of technical expertise, a state-of-the-art cardiac intensive care unit, and a continuous drive towards innovation that allows us to help when others cannot. We are here to serve as a resource to you. Some of our innovations include:

  • The stage 1 operation (or Norwood procedure) was initially developed and first performed at Boston Children’s Hospital.
  • We were the first center to perform a successful fetal cardiac intervention and have done more fetal interventions for babies with HLHS than any other center in the world.
  • In our Congenital Heart Valve Program, we have pioneered surgical techniques for repairing the tricuspid valve in babies with HLHS and tricuspid regurgitation.
  • In our Complex Biventricular Repair Program, we have learned how to recruit (grow) the smaller ventricles to make two pumping chambers instead of one.

Hypoplastic Left Heart Syndrome | Programs & Services