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Hypoplastic left heart syndrome (HLHS) is a congenital heart defect that occurs when the left side of the heart does not develop normally. The left ventricle, which normally pumps oxygen rich blood to the body, and the opening of the aorta, the vessel that carries that blood, are too small. The aortic and mitral valves that control the flow of blood into the left ventricle and out to the aorta are narrowed or completely closed.
While the cause of HLHS remains unknown, recent research strongly suggests that genetics may play an important role. Cincinnati Children’s investigators, supported by the National Heart Lung and Blood Institute (NHLBI), examined 38 individuals with HLHS along with several members of their families from three generations to determine whether HLHS is primarily the result of genes passed from parent to child. Results confirmed that HLHS and bicuspid aortic valve (BAV) are seen more often in the family members of individuals with HLHS than in the general population.
In other cases, there is data to suggest that HLHS may evolve in the fetus with severe aortic valve stenosis. For this reason, some centers are performing fetal cardiac interventions in order to improve the degree of fetal aortic stenosis, and hopefully improve growth of the left ventricle.
Congenital heart defects are present in about 1% of live births, according to the American Heart Association. Within that 1%, the incidence of hypoplastic left heart syndrome ranges from 4.8% to 9%. The overall prevalence of HLHS ranges from 1 per 10,000 to 3 per 10,000 births, according to various estimates.
Because the usual pathway of forward blood flow is blocked in HLHS, the oxygenated blood returning from the lungs passes through an opening in the interatrial septum (callout 4 in illustration above), the wall between the left atrium and the right atrium. This opening is called an atrial septal defect (ASD). The right ventricle must then do “double duty” of pumping blood both to the lungs through the pulmonary artery and out to the body through an open passageway between the pulmonary artery and the aorta. This passageway is known as the ductus arteriosus (callout 3 in illustration above) and while it is a normal structure in the fetus, it usually closes within the first days of life. When this happens to a baby with HLHS, the blood and oxygen flow to the body’s vital organs is severely diminished. Without treatment, HLHS is usually fatal within the first few days of life. HLHS accounts for up to 22% of deaths due to heart disease during the first weeks of life.
The mortality rate is higher if the opening between the left and right atria (callout 4 in illustration above) is either very small or absent. This situation occurs in about 6% to 11% of fetuses / infants with HLHS and is known as HLHS with highly restrictive or intact atrial septum (RAS). After birth of these babies, oxygenated blood returning to the left atrium has great difficulty crossing into the right side of the heart and out to the body. Thus, blood oxygen levels in these babies are usually very low, and the babies are very sick and unstable immediately after birth. Persistently high mortality rates, despite surgery within the first few days of life to open the septum, have led to efforts to develop fetal interventions for HLHS with RAS.
Hypoplastic Left Heart Syndrome is one of the most common heart defects picked up by routine pregnancy ultrasound scans performed at 18 to 20 weeks of gestation, but fetal echocardiography is needed for a definitive diagnosis. Fetal echocardiography uses sound waves (ultrasound) to provide detailed information about the anatomy of the heart of the fetus. In a fetus with HLHS, fetal echocardiography would show the structures in the underdeveloped left side of the heart, as well as the right side of the heart. Fetal echocardiography presents no known risk to the fetus or the pregnant woman.
A study reported in Circulation: Journal of the American Heart Association, found that babies diagnosed prenatally with HLHS have improved survival following surgery in the newborn period. (Further details of the surgical treatment are discussed below.) The study involved 88 infants diagnosed with HLHS — 33 prenatally and 55 postnatally — from July 1992 until March 1999 at the University of San Francisco.
If HLHS is detected, a detailed ultrasound of the rest of the fetus is usually performed, since about 66 percent of fetuses with congenital heart defects also have other types of malformations. Knowing about these can help plan treatment. Early detection and diagnosis of HLHS and other anomalies may improve outcome for the newborn by changing the planned delivery to a hospital prepared to treat HLHS and by giving parents the option of fetal intervention.
Treatment for hypoplastic left heart syndrome begins with rapid stabilization immediately after a child is born. Stabilization measures include the insertion of catheters and use of medications to keep open the communication between the left and right atria and optimally balance the flow of blood through the baby’s heart, lungs and the rest of the body. Treatment options would then include a series of operations, usually three, to reconfigure the child’s cardiovascular system to be as efficient as possible despite the lack of an adequate left ventricle. A heart transplant is a less common treatment option.
For a baby with HLHS and RAS, immediate treatment would include placing a catheter in the baby’s heart to create or enlarge the internal communication between the left and right atria and facilitate the flow of blood out of the lungs. This procedure is known as a catheter-based atrial septoplasty.
Mortality is high among infants who have atrial septoplasty after birth. These infants are usually very unstable, with severe hypoxemia (not enough oxygen in the blood) and acidosis (too much acid in the blood), complicating treatment. Even with successful placement of the catheter, mortality ranges from 48 to 64 percent, with many dying while awaiting reconstructive surgery or a heart transplant.
Careful fetal echocardiography — including 2D and color flow Doppler echocardiography of the atrial septum, and pulsed wave Doppler analysis of pulmonary venous flow patterns — can identify risk factors for RAS in the fetus with HLHS (Circulation 112:2974-2979, 2005). If a fetus has been diagnosed as having HLHS with RAS, the Cincinnati Fetal Center and the Fetal Heart Program at Cincinnati Children’s may offer several management option, depending on specific circumstances:
Despite catheter based atrial septoplasty, newborns with HLHS and RAS have a higher mortality rate than those without RAS. This may be due to changes in the pulmonary blood vessels and heart tissue that result from chronic obstruction of blood flow from the lungs during fetal life. Decompressing the left atrium and improving shunting of blood from the left to right atrium could promote more normal development of the heart muscles and vessels and improve overall outcome for newborns.
In utero atrial septoplasty, using fetal echocardiography to guide the placement of the catheters may improve the prognosis for fetuses with HLHS and RAS. However, the benefit of fetal intervention remains unproven. Strategies to provide immediate access to perinatal cardiac intervention, such as delivery of the baby in the cardiac catheterization laboratory, may minimize the severe instability, low oxygen levels and acidosis that occur in these newborns after birth. Surgeons at Cincinnati Children’s have also successfully performed ex-utero intrapartum transfer (EXIT) to extracorporeal support (ECMO) in patients with RAS, and performed atrial septoplasty under conditions of stable oxygenation and systemic blood pressure.
Fetuses with congenital left heart obstructive lesions are first screened with fetal echocardiography to see if there is adequate left to right shunting at the atrial level. Inadequate shunting could indicate HLHS with RAS.
The diagnosis of RAS is based on findings from fetal echocardiography. Recent work at Cincinnati Children’s has shown that several measures of pulmonary venous Doppler (PVD) flow analysis are highly predictive of the need for emergent atrial septoplasty in the newborn period. This could also be applied to fetal atrial septoplasty. In addition, 2D and color flow Doppler imaging should show evidence of minimal to no atrial level (left-to-right) shunting.
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If you have questions regarding a fetal heart condition, or would like more information about the Fetal Heart Program, please contact: Dana McBride Administrative AssistantPhone: 513-636-9931 Toll free 1-800-344-2462 ext. 6-9931 Fax: 513-636-9747 Email: firstname.lastname@example.org
Watch: Fetal Pacemaker for Complete Heart Block.
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