Hypoplastic Left Heart Syndrome
A brief summary of HLHS and the single-ventricle pathway by Michael Schram
What HLHS is
Hypoplastic means underdeveloped. In hypoplastic left heart syndrome, the whole left side of the heart is too small to function. Sometimes these parts are merely tiny. Sometimes they're essentially absent. More simply put, HLHS babies are "born with half of a heart". In Barrett's case his Left ventricle was diagnosed as "present, but too small to function" putting him towards the center of the spectrum for an HLHS diagnosis.
The two halves of the human heart can essentially be viewed as two individual pumps with two separate jobs.
- The right pump takes used, oxygen-poor blood coming back from the body and sends it to the lungs to pick up oxygen.
- The left pump takes the freshly oxygenated blood coming back from the lungs and pushes it out to the entire body.
So in the case of a baby with HLHS - the body's pump (left half) isn't functional, but the lung's pump - the right ventricle - is healthy and strong.
It happens in roughly 1 in 4,000 births, about a thousand babies a year in the U.S, and is an extremely rare birth defect.
Why the baby is completely fine before birth
A baby in utero doesn't use its lungs. The placenta does the oxygenating. And fetal anatomy provides two temporary shortcuts into every fetus so blood can skip the lungs on its way around. Both shortcuts are designed to close after birth. They're not needed once the lungs inflate and the plumbing rearranges itself. One of these shortcuts, the "ductus arteriosus", starts shutting within hours after birth and is typically sealed within a day or two.
For a typical newborn, that's routine housekeeping. For a baby with HLHS, that connection isn't a shortcut. It's the only road to the body. When it closes, blood flow to the organs collapses. Without rapid surgical intervention after birth, babies born with HLHS will pass away within hours to days. Babies born with HLHS are administered a drug called Prostaglandin E1 which temporarily keeps the ductus arteriosus open longer, allowing time to prepare the baby for surgery.
The Three Surgeries (Norwood, Glenn, Fontan)
HLHS cannot be corrected, because you cannot manufacture a left ventricle. There's no repair to make; the part isn't there, or is too small to function. So surgeons do something more radical: they take the one good pump (right ventricle) and reassign it, then rearrange the pipes until the lungs don't need a pump at all.
This trio of staged palliative surgeries is known as the "single-ventricle pathway" because by the end of the third surgery the patient's body is functioning off of single-ventricle heart.
The Norwood (~day 3–14 of life)
This is the largest and most complex of the 3 surgeries, and it is absolutely critical -an HLHS patient cannot survive for long outside the womb without a successful Norwood procedure. It involves 3 major steps:
#1. Promote the right ventricle. The pulmonary artery — the big pipe leading to the lungs — is divided, and its base is sewn onto the baby's undersized aorta, which is then widened with a patch. The result is a brand-new, full-size aorta, driven by the right ventricle. The right ventricle is now the body's pump. Permanently.
#2. Build a new road to the lungs. The pulmonary artery just got repurposed, so the lungs need a new supply. Surgeons install a small Gore-Tex tube called a "Shunt" which supplies blood directly to the lungs, fulfilling the original role of the now repurposed right ventricle from step #1.
#3. Open up the mixing. The wall between the two upper chambers is removed, so oxygen-rich blood returning from the lungs can cross into the right side, where the working pump lives.
After a successful Norwood, the right ventricle is doing two jobs from one squeeze - pumping to the body and to the lungs. This new anatomy is very taxing on the heart and body, but it is a life-saving intervention. The two additional surgeries goals are centered around alleviating the stress and taxation that is now placed upon the functional single-ventricle heart.
The Glenn (~3-6 Months)
The vein bringing blue blood back from the head and arms (top half of the body) is disconnected from the heart and sewn directly onto the lung artery. The shunt from Stage 1 comes out. Blood from the upper body now reaches the lungs without passing through the heart at all. No pump. It gets there on venous pressure alone — the leftover gentle push from the body's circulation.
Why they wait 3-6 months: at birth, the lungs' vessels are still tight and high-resistance, because before birth they never had to carry much. Over the first months they relax. Only then is passive flow possible. This is why the Glenn can't simply be done at birth. It's also why this entire strategy quietly depends on the lungs staying healthy.
The Fontan (~2-4 Years)
Same logic as the Glenn, applied to the bottom half of the body. The vein returning blue blood from the belly and legs is routed to the lung artery, usually through a Gore-Tex tube run alongside the heart.
Now all the blue blood, top and bottom, goes straight to the lungs, bypassing the heart entirely. The heart handles red blood only, and only pumps it to the body.
With this surgery complete the child is now living with a single-ventricle heart providing oxygen saturation levels often above the 90% mark.
A Single-Ventricle Heart
After the Fontan, the child has a circulation that works: one pump for the body, and lungs fed passively by returning blood.
A single-ventricle heart is a palliative solution to an otherwise fatal defect - so there are a host of potential health complications stemming from living with a single-ventricle heart including livelong elevated blood pressure, limited exercise capacity, lifelong cardiology and medications, and the potential for additional necessary surgeries or organ failures.
That said - The staged pathway didn't exist before the early 1980s. Before it, HLHS was uniformly fatal. Which means the oldest survivors are only in their early forties - nobody knows what a seventy-year-old Fontan looks like, because there isn't one yet. Results have improved with essentially every passing decade.