What Is Hypoplastic Left Heart Syndrome (HLHS)?

Hypoplastic left heart syndrome (HLHS) is a rare and complex congenital (present at birth) heart defect that represents an underdevelopment of the entire left side of the heart. HLHS is found in less than 1% of all live births, but it is among the most severe of all heart defects found in children. While this condition is very serious, the specialized physicians affiliated with Children’s Memorial Hermann Hospital provide comprehensive care to children with HLHS.

In a child with a normal-functioning heart, the right side of the heart receives oxygen-poor (blue) blood from the body and then pumps the blood through a large blood vessel called the pulmonary artery to the lungs to pick up oxygen. Then, the left side of the heart receives the oxygen-rich (red) blood from the lungs and -pumps it to the rest of the body through another blood vessel called the aorta.

In a baby with HLHS, most of the structures on the left side of the heart, including the left ventricle and certain valves, are too small and underdeveloped such that the heart cannot effectively pump blood to fulfill the body’s needs.

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What Are the Causes of Hypoplastic Left Heart Syndrome (HLHS)?

The causes of heart defects such as HLHS among most babies are unknown. Some babies have heart defects because of changes in their genes or chromosomes. A combination of environmental exposures and genetic factors are likely responsible, but little is known about the specific cause.

How Is Hypoplastic Left Heart Syndrome (HLHS) Diagnosed?

HLHS may be diagnosed during pregnancy with a fetal echocardiogram, which is a specialized ultrasound of the fetal heart. The affiliated physicians in the Fetal Cardiology Program at The Fetal Center will confirm a diagnosis and prepare a delivery plan for both mom and baby. A multidisciplinary team of specialists will also develop the baby's immediate care plan following delivery.

If HLHS is not diagnosed in utero, and suspicion of a heart defect occurs after the baby is born, a pediatrician will refer the patient to a pediatric cardiologist or neonatologist to determine the diagnosis.

Some babies that are not diagnosed with HLHS before they leave the hospital may not show warning signs for several days. Once signs are present, the infant may have an acute life-threatening event, such as a pounding heart rate, difficulty breathing, a weak pulse or a gray or blue cast to their skin.

How Is Hypoplastic Left Heart Syndrome (HLHS) Treated?

Typically, your child can be delivered without inducement and without a cesarean section (C-section), unless your obstetrician deems it necessary for other reasons.

A delivery plan will be developed carefully by your multidisciplinary team of affiliated physicians at The Fetal Center and the Children’s Heart Institute and will be discussed with you and your obstetrician. Your baby should be delivered at a tertiary care hospital that is well prepared to care for babies with congenital heart defects. There should be a neonatal intensive care unit with the capability to provide specialized care and pediatric cardiology and pediatric cardiovascular surgery services available at that institution.

Immediately following delivery, doctors will evaluate your baby carefully and, in most cases, transfer the newborn to a neonatal intensive care unit, where he or she will be stabilized and assisted with any breathing and blood pressure concerns. Medicine will be given to help stabilize your baby and support him or her until surgery can be performed.

The Fontan Palliation Sequence

STAGE ONE | Norwood Procedure: The Norwood procedure is the first stage in a series of three open heart procedures to palliate HLHS. Typically, a Norwood procedure is done within the first two weeks of life and represents a high-risk, extensive operation. The principle behind the Norwood procedure is that the only good pumping chamber (the right ventricle) must pump blood to the body. There is no natural, permanent connection between the right ventricle and the body. Thus, the Norwood procedure involves dividing the blood vessel that takes blood from the right ventricle to the lungs and rerouting it to take blood to the body.

By dividing the blood vessel to the lungs, the blood supply to the lungs is taken away. Part of the Norwood procedure involves restoring some blood flow to the lungs. There are two methods generally used:

Modified Blalock-Taussig (MBT) shunt – a small tube from the main blood vessel to the body (aorta or branch of the aorta) that connects to the blood vessel to the lungs (pulmonary arteries)

Sano shunt - tube goes from the pumping chamber (right ventricle) to the blood vessel to the lungs (pulmonary arteries)

Both methods are accepted. The decision to use one versus the other is individualized based on the infant’s anatomy and the surgeon’s preference.

STAGES TWO & THREE: the blood which returns from the body is directly routed to the lungs without passing through the heart. (There is no ventricle pumping blood through the lungs as there is in a normal heart.)

Bidirectional Glenn Procedure and Fontan Procedure:

Among doctors and nurses, a Fontan refers to the procedure where the two big veins that bring blood back from the body (one from the upper body and one from the lower body) are connected directly to the lungs. This is done at two separate operations. The upper vein is usually connected at about 6 months of age), and the lower vein is usually connected between 2 ½ and 5 years of age. The first operation is called a bidirectional Glenn procedure, named after Dr. Glenn, who first performed the procedure. The second operation is referred to as a Fontan procedure, named for Dr. Fontan.

In a bidirectional Glenn procedure (Glenn), the large vein which brings blood without oxygen back from the upper body is connected directly to the lungs. This blood is oxygenated by the lungs, and then is returned to the heart. The blood which comes back from the lower body goes directly to the heart itself, and then gets re-pumped to the body without going to the lungs. Since unoxygenated blood from the lower body gets mixed in the heart (with the oxygenated blood from the upper body), and then is pumped to the body, the blood that goes to the body is not fully oxygenated. The usual oxygen saturation for a child with a Glenn is around 80%. Children tolerate this level quite well, even though it is lower than a normal child’s oxygen saturation.

In a Fontan procedure, the large vein which carries unoxygenated blood to the heart form the lower body is connected directly to the artery that carries blood to the lungs. At the end of this operation, all of the unoxygentated blood that comes from the body goes to the lungs, and all of the blood that comes from the lungs goes to the body. The outcome is a normal blood flow pattern; however, the child’s heart is not structurally normal.

There are a number of reasons why the procedure is staged; that is, the upper vein is connected to the lungs at one operation, and the lower vein is connected at a different operation. Over time, this has shown to be a safer and better strategy for children, rather than doing the upper and lower veins during the same operation. Recovering from a Fontan can be very hard, and the heart has to be in the best shape possible in order for the Fontan to be successful.

A Glenn is not as hard to recover from as the complete Fontan, and a child whose heart is not in perfect shape will have a safer surgery with a Glenn than with a Fontan. A Glenn does not require additional work by the heart. Thus, when the Fontan surgery is performed, the heart is entering the operation in better condition.

Although the Fontan procedure is the best that can be offered for single ventricle patients at this time, it is far from perfect. With a Fontan, there is no ventricle to pump blood through the lungs. Only the “pressure” within the veins drives blood through the lungs. If there is any significant resistance to flow through the lungs, a Fontan will not work.

Why Choose the Children’s Heart Institute?

At Children’s Heart Institute at Children’s Memorial Hermann Hospital, patients with congenital or acquired heart disorders receive hands-on specialized care 24/7 from a team of affiliated physicians and specialty-trained nurses who aim to deliver the best possible outcomes.

Children’s Memorial Hermann Hospital was named one of the top children's hospitals nationally in Cardiology & Heart Surgery by U.S. News & World Report. In addition, Children’s Heart Institute is among the top congenital heart surgery programs in North America for patient care and outcomes, according to the Fall 2019 Society of Thoracic Surgeons (STS) Congenital Heart Surgery Database Report of 118 STS participating programs.

In collaboration with various subspecialties, the affiliated team provides comprehensive care for newborns, children and adolescents, with the ability to transition into adult congenital cardiac care. Team members have the experience and skills necessary to offer innovative treatment methods and specialized services, including, but not limited to:

  • Biventricular repairs and biventricular conversions
  • Congenital heart optimization
  • Full repairs for complex congenital heart defects in newborns
  • Hybrid catheterization and surgical procedures
  • Minimally invasive transcatheter pulmonary valve (TPV) therapy
  • Minimally invasive repairs
  • Treatment for adult congenital heart disease
  • Valve repairs and preservation

With the Level IV Neonatal Intensive Care Unit (NICU) and a dedicated Children’s Heart Institute Intensive Care Unit at Children’s Memorial Hermann Hospital, critical heart patients have access to quality, specialized care. By utilizing state-of-the-art techniques, the team at Children’s Heart Institute strives to offer patients with the most complex problems the greatest opportunity for a normal life.

Contact Us

If you have any questions, use the online tool below to help us connect with you. To refer a patient or schedule an appointment, please contact our clinic using the information below.

  • Pediatric Cardiology Clinic
    The University of Texas Health Science Center Professional Building
    6410 Fannin, Suite 370
    Houston, TX 77030
    Phone: (713) 486-6755 (Appointment Line)
  • Pediatric and Congenital Heart Surgery Clinic
    The University of Texas Health Science Center Professional Building
    6410 Fannin, Suite 370
    Houston, TX 77030
    Phone: (713) 500-5746

Patient Stories

  • patient Luke Keaton

    Luke’s Story: Above and beyond care after a hypoplastic left heart syndrome diagnosis

    February 7, 2023

    As soon as possible, she and Troy made an appointment with The Fetal Center at Children Memorial Hermann Hospital for an in-utero echocardiogram, which would provide a detailed image of their baby's heart. Prenatal diagnosis plays a vital role in identifying critical cardiac defects, helps guide ...

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  • patient theodore

    Giving Theodore a fighting chance: This little heart warrior beat the odds after HLHS diagnosis

    February 6, 2023

    Theodore was diagnosed with hypoplastic left heart syndrome (HLHS), a complex congenital heart defect in which the left side of the heart is underdeveloped and cannot pump oxygen-rich blood to the body. Babies with this condition need a minimum of three open-heart surgeries. ...

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  • Ryder

    Ryder’s Story: Our Son’s Courageous Heart Journey

    May 24, 2022

    When Falon was 23 weeks pregnant, she was very excited about her next ultrasound and couldn’t wait to catch a glimpse of Ryder on the ultrasound screen to see how much he had grown since his last anatomy scan. At first, things were going well. Then, the atmosphere in the room abruptly chan...

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  • Alshammari

    16-year-old Avoids Heart Transplant With Innovative Biventricular Conversion Procedure

    October 1, 2021

    Rasheed Alshammari had been on the waiting list for a heart transplant at a California hospital for over a year. When the surgeon who was to perform the transplant left the hospital, Rasheed was removed from the list.

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  • Mason Mims

    Mason Mims: A Normal, Whole Heart and a Bright Future

    August 9, 2021

    Karen and Taylor Mims spent the first six years of their son’s life trying to accept the fact that he was born with half a heart and would eventually require a heart transplant. Now, thanks to an innovative procedure being performed by affiliated pediatric heart surgeons at Children’s...

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  • Cason Cox

    Cason's Story: Boy Born With Half A Heart Undergoes Innovative Procedure Giving Him A Normal Heart

    April 22, 2019

    During her 20-week ultrasound, Savannah’s obstetrician diagnosed that her unborn baby’s heart was abnormally small and referred her to a maternal fetal medicine specialist. The specialist ordered a dedicated fetal echocardiogram (“echo”), a specialized ultrasound of the fe...

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  • Mason Brown

    With A Heart Full of Gratitude: A Former Patient Thanks The PICU

    March 1, 2016

    Erinn Brown was 34 weeks pregnant when she realized something was wrong with her baby. An ultrasound showed that the left ventricle of the baby’s heart was very small and nearly nonexistent. Brown’s doctor referred her to The Fetal Center at Children’s Memorial Hermann Hospital....

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To contact Children’s Heart Institute at Children’s Memorial Hermann Hospital, please fill out the form below.


Thank you for contacting the Children’s Heart Institute at Children’s Memorial Hermann Hospital. We have received your inquiry, and a team member will contact you soon.

If you need more immediate assistance, please call us during business hours at (713) 486-6755.

If you are experiencing a medical emergency, call 911 or go to the nearest emergency room.

The Children’s Heart Institute is a collaboration between the affiliated physicians at McGovern Medical School at UTHealth Houston and Children’s Memorial Hermann Hospital. Typically, patients are seen on an outpatient basis at a UT Physicians clinic with all inpatient procedures performed at Children’s Memorial Hermann Hospital.

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