Q&A: Hypoplastic
Left Heart Syndrome
By
Frank Pigula, MD Associate in Cardiac Surgery
What is Hypoplastic Left Heart Syndrome (HLHS)?
HLHS is a severe form of congenital heart disease that affects
the left side of the heart. Important components
of the left heart, such as the mitral valve, the left ventricle,
and the aortic valve, may be tiny. Likewise, the ascending aorta
can be very tiny, sometimes as small as 1 to 2 mm. Finally, the
hypoplastic aortic arch ends with a coarctation where the patent
ductus arteriosus connects the pulmonary artery to the aorta.
Given the severity of this defect, the diagnosis can be surprisingly
difficult to make, at least until the ductus arteriosus begins
to close. When this happens, cardiac output falls dramatically,
and the baby goes into shock.
Who gets HLHS?
It is estimated that HLHS occurs in 0.16 to 0.18 per 1000 live
births. Males account for 57 to 67 percent of new cases, and the
risk of sibling recurrence has been reported to be 0.5 percent.
No environmental risk factors have been identified.
What is the clinical presentation?
The most common symptom is low saturation as measured by pulse
oximetry. This may be a subtle finding, however, as some children
may initially have near normal saturations. Within hours to days,
however, tachypnea and pallor may become apparent. While these
findings may be subtle initially, patients may rapidly experience
progressive acidosis, cyanosis and eventually cardiopulmonary
collapse.
How is HLHS diagnosed?
As with many forms of congenital heart disease, echocardiography
is the diagnostic procedure of choice in HLHS. Rapid, accurate
and noninvasive, cardiac ultrasound provides the anatomic information
necessary for surgical treatment. Other tests, such as chest x-ray
and EKG, may be unremarkable, and as mentioned, physical findings
may be subtle, at least initially.
What is the state of the art treatment?
Medical HLHS treatment
requires both medical and surgical specialists. Prostaglandin
E1 is absolutely critical, and must be given when the diagnosis
is suspected. Since the systemic circulation is duct dependent,
its closure results in cardiopulmonary collapse and rapid demise.
Once diagnosed, the baby with HLHS must be intensively managed
to maintain an adequate balance between two competing circulations:
the pulmonary and the systemic circulations. In HLHS, total blood
flow coming from the heart can be considered to be a zero sum
game. Thus, when more blood is directed to one circulation, less
is available for the competing circuit. For example, increasing
the inspired oxygen levels in these children tends to reduce the
pulmonary vascular resistance, thereby increasing pulmonary blood
flow. However, this occurs at the expense of the systemic circulation,
and these babies become pale, have poor pulses and develop a severe
metabolic acidosis. In practice, these children are usually maintained
on room air, or even hypoxic gas mixtures, to avoid this scenario.
In fact, the management of the resistance to blood flow in the
competing circulatory beds constitutes a mainstay in HLHS management.
Surgical Before
the introduction of the first effective surgical treatment by
Bill Norwood in 1983, 1-month mortality for HLHS was about 95
percent. Since then, great strides have been made in care and
treatment of these children. While a distinct improvement over
all other treatment options, the surgical mortality in the early
surgical experience remained as high as 50 to 60 percent. Since
then, however, improvements in anesthetic techniques and critical
care, as well as increased surgical experience, have dramatically
improved the outcomes that dedicated centers can offer these children.
Today, about 90 percent of babies presenting with HLHS can be
expected to survive their Norwood operation; truly a success given
that just 21 years ago the outlook was hopeless.
Do these children need other surgeries?
Yes. The Norwood procedure is the first in a three-operation strategy
to achieve a stable single ventricle circulation. At the age of
4 to 6 months, these children will return for their second operation,
a bidirectional Glenn operation. This procedure removes the shunt
(placed during the Norwood operation) as the source of pulmonary
blood flow, and replaces it with the superior vena cava (SVC).
During the Glenn operation, the SVC is amputated from the heart,
sewn to the pulmonary arteries and provides the source of all
pulmonary blood flow. These children will typically have pulse
oximetry saturations of 75 to 85 percent. This arrangement is
more stable than the shunted Stage I Norwood operation, and will
palliate the children for two-to-three years. By age 2 to 4, these
children tend to experience decreasing exercise tolerance (a manifestation
of their limited pulmonary blood flow), and the third and final
operation is performed. This surgery, called the Fontan operation,
diverts the blood returning to the heart via the inferior vena
cava to the lungs. This greatly increases the total pulmonary
blood flow, and by directing all the venous blood returning from
the body to the lungs before reaching the heart, separation of
the pulmonary and systemic circulations is achieved.
What's new on the horizon?
Much has been learned over the last two decades in our struggle
to treat HLHS. The tremendous improvements in outcomes represent
diligent efforts by cardiologists, anesthesiologists, intensivists
and surgeons to develop new and better treatments for HLHS and
other forms of congenital heart disease. The story does not end
here, however. At Children's Hospital Boston, we are developing
new modifications to the Norwood operation that have the potential
to further improve the treatment of these children. New techniques
reduce the need for circulatory arrest during Stage I of the Norwood
operation, hopefully reducing the risk of neurologic injury. Modifications
to the shunt, called the Sano modification, may improve early
stability during the critical early days following surgery. And
finally, doctors are making initial forays into the final frontier
of congenital heart disease: tackling this disease before birth
using fetal intervention in the hope of reducing or reversing
the process that results in HLHS.
Children's Advanced Fetal Care Center and other institutions
have developed many successful treatments of HLHS, but our efforts
are a work in progress, and it can fairly be said that the more
we learn, the less we know. However, building on past successes
while developing and incorporating new techniques, we are optimistic
about providing an even better life for children diagnosed with
HLHS in the future.
August
2004
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