HLHS, or hypoplastic left heart syndrome, is a serious birth defect in which the entire left side of a baby’s heart is too small to pump blood to the body. It affects roughly 2 in every 10,000 pregnancies. In a healthy heart, the left ventricle does the heavy lifting of sending oxygen-rich blood out to the organs. In a baby born with HLHS, that left ventricle, the aorta (the body’s main artery), and the valves connecting them are all severely underdeveloped or completely closed off.
How HLHS Affects Blood Flow
Normally, the right side of the heart pumps blood to the lungs, and the left side pumps it to the rest of the body. With HLHS, the left ventricle is so small it can’t do its job. The baby survives in the womb because fetal circulation has built-in detours: a small opening between the two upper chambers of the heart (called the foramen ovale) and a temporary blood vessel called the ductus arteriosus that connects the pulmonary artery to the aorta. These detours allow the right side of the heart to handle blood flow for the entire body during pregnancy.
The problem begins after birth. Both of those detours are designed to close on their own within the first few days of life. When they do, a newborn with HLHS loses the only pathways keeping blood flowing to the brain, organs, and limbs. Without medical intervention, the situation becomes life-threatening within hours to days.
Signs in the First Days of Life
Some babies with HLHS appear stable for a short window after delivery while those natural detours remain open. Once they begin to close, symptoms develop quickly:
- Bluish or ashen skin color, especially around the lips and fingernails, from low oxygen
- Rapid breathing or visible difficulty breathing
- Fast heart rate
- Weak pulse, particularly in the arms and legs
These signs can escalate rapidly. Babies diagnosed after birth are typically admitted to a neonatal intensive care unit immediately.
How HLHS Is Diagnosed
Many cases are caught before birth. A standard prenatal ultrasound between 18 and 22 weeks of pregnancy includes a four-chamber view of the fetal heart, and HLHS is often visible at that stage. Key findings include a very small, thick-walled left ventricle with poor squeezing ability, an aorta that is tiny or barely visible, and little to no blood flow through the aortic valve. A left ventricle or mitral valve measuring 5 millimeters or less is considered diagnostic.
Prenatal detection is a major advantage. It allows the medical team to plan delivery at a hospital with a pediatric cardiac surgery program, so treatment can begin within hours of birth rather than after a crisis. When HLHS is not caught prenatally, it is typically identified through an echocardiogram (heart ultrasound) performed after a newborn develops symptoms.
Keeping the Baby Stable After Birth
The first priority is preventing the ductus arteriosus from closing. Doctors give the baby a continuous infusion of a medication that mimics the natural signals keeping that blood vessel open during fetal life. This preserves the detour that lets the right ventricle send blood to the body while the surgical team prepares. Before this medication became available, many babies with HLHS did not survive long enough to reach surgery. Its introduction dramatically changed early outcomes.
In babies with the most severe form of HLHS, where the aortic valve is completely sealed, this temporary blood vessel is the only route supplying blood flow to the brain and upper body. Keeping it open is not optional.
The Three-Stage Surgical Plan
There is no way to rebuild a missing left ventricle. Instead, surgeons restructure the heart over three operations so the right ventricle can take over the job of pumping blood to the entire body. Each surgery builds on the last.
Stage 1: The Norwood Procedure
This is performed within the first week of life. Surgeons connect the pulmonary artery to the aorta, essentially building a larger outflow path so the right ventricle can pump blood to the body. A small tube (shunt) is placed to direct some blood to the lungs for oxygen. After this surgery, the baby’s oxygen levels will still be lower than normal, but they are stable enough for the baby to grow. The immediate goal is for the baby to feed well and gain weight in preparation for the next stage.
Stage 2: The Glenn Procedure
This surgery happens when the baby is 3 to 6 months old. Surgeons connect the large vein that carries oxygen-depleted blood from the upper body directly to the pulmonary arteries, sending it straight to the lungs without passing through the heart first. This takes significant workload off the right ventricle, which has been doing the job of both sides of the heart since the Norwood. Reducing that strain helps prevent the ventricle from becoming dangerously enlarged. Survival outcomes are best when this procedure is done three to six months after the first surgery.
Stage 3: The Fontan Procedure
The final surgery typically occurs between ages 2 and 4. It completes the separation of oxygen-rich and oxygen-poor blood flow. Surgeons connect the large vein carrying blood from the lower body directly to the pulmonary arteries as well, so all oxygen-poor blood now flows passively to the lungs without the heart needing to pump it there. The right ventricle is freed up to focus entirely on pumping oxygenated blood to the body. For this passive flow to work well, the pressure in the lung arteries must stay low.
Survival and Long-Term Outlook
HLHS remains one of the most challenging congenital heart defects to treat. Five-year transplant-free survival for children who undergo the full three-stage surgical series ranges from 55% to 75%, reflecting significant improvements over past decades but also the ongoing seriousness of the condition. The highest-risk period is the interval between birth and the second surgery, when the single ventricle is under the most strain.
Children who complete all three surgeries live with what doctors call Fontan circulation. The right ventricle works as the sole pumping chamber, and blood flows to the lungs passively. This is a functional solution, not a cure. The single ventricle is doing work it was never designed for, and over time, complications can develop.
Long-Term Health Considerations
Living with Fontan circulation means the body operates with lower cardiac output and higher pressure in the veins than normal. Most children are active and attend school, but they typically have reduced exercise tolerance compared to peers. The circulatory setup places ongoing stress on certain organs.
One of the most significant long-term complications is a condition where the intestines lose protein into the digestive tract because of chronic venous congestion. This affects roughly 17% to 20% of HLHS patients after the Fontan procedure, higher than for other single-ventricle defects. It causes swelling throughout the body, low protein levels in the blood, and often requires repeated hospitalizations. Symptoms tend to appear on average about two to two and a half years after the Fontan surgery.
The liver is also vulnerable. Years of elevated venous pressure can gradually cause liver scarring. Cardiac teams monitor liver function as part of routine follow-up care for Fontan patients. Some individuals eventually need a heart transplant if the right ventricle weakens significantly over time. Children with HLHS require lifelong cardiology care, with the frequency and intensity of monitoring evolving as they grow.