(Radiographics. 2001;21:705-717.)
© RSNA, 2001
Hypoplastic Left Heart Syndrome1
Dianna M. E. Bardo, MD,
David G. Frankel, MD,
Kimberly E. Applegate, MD,
Daniel J. Murphy, MD and
Russel P. Saneto, DO, PhD
1 From the Departments of Radiology (D.M.E.B., D.G.F.), Pediatric Cardiology (D.J.M.), and Pediatric Neurology (R.P.S.), Cleveland Clinic Foundation and Children’s Hospital, 9500 Euclid Ave, Cleveland, OH 44195; and the Department of Radiology, University Hospital, Cleveland, Ohio (K.E.A.). Recipient of a Certificate of Merit award for a scientific exhibit at the 1998 RSNA scientific assembly. Received March 19, 1999; revision requested May 18 and final revision received August 24; accepted August 24. Address correspondence to D.G.F.
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Abstract
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Hypoplastic left heart syndrome (HLHS) is a complex combination of cardiac malformations that probably results from multiple developmental errors in the early stages of cardiogenesis and that, if left untreated, invariably proves fatal. A variety of chest radiographic findings are seen in patients with HLHS, including an enlarged cardiac silhouette (notably a prominent right atrium), pulmonary venous hypertension, an atrial septal defect, and valvular stenosis or atresia. The recent evolution of palliative surgical procedures (modified Norwood procedure, bidirectional cavopulmonary shunt, modified Fontan procedure, aortic valvuloplasty, heart transplantation) has increased the survival rate in children with HLHS. Echocardiography allows accurate assessment of the size and location of the ductus arteriosus, the hemodynamics of the aortic root, the patency and size of the foramen ovale or atrial septal defect, and the presence of a ventricular septal defect to help determine whether surgical intervention is appropriate and, if so, to facilitate planning. Pediatric radiologists now view radiologic images obtained in patients with HLHS before surgical intervention and at important intervals during treatment. Familiarity with the malformations that characterize HLHS and the surgical procedures used to enhance postnatal survival will help pediatric radiologists provide better care for patients with this relatively common pathologic condition.
Index Terms: Heart, abnormalities, 51.1715 • Heart, anatomy, 51.92 • Heart, failure, 51.71 • Heart, flow dynamics, 51.92 • Heart, surgery, 51.451
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LEARNING OBJECTIVES FOR TEST 4
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After reading this article and taking the test, the reader will be able to:
- Identify the malformations that characterize HLHS anatomy along with their probable embryologic origins.
- Describe the hemodynamics of HLHS and discuss the physiologic basis for various findings at early chest radiography.
- Discuss the palliative surgical procedures used to correct HLHS and the resulting radiographic changes.
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Introduction
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Until recently, hypoplastic left heart syndrome (HLHS) was an invariably lethal pathologic condition. HLHS comprises a wide spectrum of cardiac malformations, including hypoplasia or atresia of the aortic and mitral valves and hypoplasia of the left ventricle and ascending aorta. The great vessels are normally related in HLHS. The New England Regional Infant Cardiac Program lists HLHS as the most common form of univentricular cardiac malformation. Along with isolated coarctation and patent ductus arteriosus, HLHS is the fourth most common cardiac malformation to manifest in the 1st year of life behind ventricular septal defect, d-transposition of the great arteries, and tetralogy of Fallot (1). HLHS has a reported prevalence of 0.2 per 1,000 live births and occurs twice as often in boys as in girls (2,3). Left untreated, HLHS is invariably lethal and is responsible for 25% of early cardiac deaths in neonates (4,5). However, the recent evolution of palliative surgical procedures has increased the survival rate in children with these malformations. The pediatric radiologist now sees a larger population of HLHS patients, in whom radiologic findings reflect both the degree of hypoplasia and the results of palliative repair.
HLHS has been described and redefined over the latter half of the 20th century by Lev (6) and by Noonan and Nadas (7) as comprising a wide spectrum of cardiac malformations ranging from hypoplasia to complete atresia of the anatomy of the left side of the heart in patients with normally related great vessels. Through the pioneering surgical work of Norwood, Fontan, and others, the anatomic variations have been defined and practical knowledge gained, leading to surgical techniques capable of providing reconstructive palliation for most affected neonates (5,7). Infants are now surviving into childhood, withstanding what was once a catastrophic diagnosis.
A total of 57 patients identified as having HLHS were seen at our institution between April 1993 and February 1999. There were 37 boys (65%) and 20 girls (35%). Eighteen patients (32%) had a prenatal diagnosis of HLHS. Thirty-seven patients (65%) were diagnosed in the early postnatal period (36 patients at 1–4 days and one patient at 9 days). Two patients (4%) were diagnosed unusually late: one at 9
weeks and one at 11 weeks. Two patients (4%) died prior to or without surgical intervention.
In this article, we briefly discuss cardiac embryology and patterns of cardiac blood flow, review salient chest radiographic features of HLHS in infants and children, and discuss and illustrate the use of other imaging modalities and of palliative surgical procedures (modified Norwood procedure, bidirectional cavopulmonary shunt, modified Fontan procedure, aortic valvuloplasty, heart transplantation) in affected patients.
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Cardiac Embryology
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Cardiac embryogenesis is a continuum of stages that have a delicate affect-effect association. The pivotal cardiac lesion in HLHS and the timing of the occurrence of critical developmental errors are not clearly understood.
Formation of the cardiovascular system occurs in the early weeks of gestation, taking over the distribution of nutrients and expulsion of wastes as the embryo outgrows the capacity to carry out these functions through simple diffusion.
By the 5th gestational week, the cardiac loop has already formed inside the pericardial cavity. The bulbus cordis has divided into the truncus arteriosus, the conus cordis, and the trabeculated portion of the right ventricle. The truncus arteriosus is incorporated into the right ventricle as the conus arteriosus. In the left ventricle, the bulbus cordis forms the walls of the aortic vestibule, the left ventricular outflow tract, just inferior to the aortic valve. During the 5th week, the bulbar ridges form from a proliferation of mesenchymal cells in the walls of the bulbus cordis. At the same time, truncal ridges form and fuse in continuity with the bulbar ridges to form the aorticopulmonary septum.
The partitioning of the bulbus cordis and truncus arteriosus from spiral truncal and bulbar ridges takes place under the stimulus of blood flow from the ventricles to the embryonic pulmonary and systemic circulations. The spiral aorticopulmonary septum and the outflow tracts are thus formed and the positions of the semilunar valves determined (Fig 1).
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Figure 1. Drawing illustrates embryologic heart anatomy. The partitioning of the bulbus cordis and truncus arteriosus is demonstrated. Arrows indicate the direction of normal blood flow, which directly affects the formation of the normal spiral aorticopulmonary septum and the ventricular outflow tracts.
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Subsequently, in the 6th and 7th gestational weeks, the semilunar valves develop from swellings of subendocardial tissue to form the orifices of the aorta and pulmonary trunk. The atrioventricular valves develop from subendocardial tissue at the atrioventricular canals (8,9).
We and others hypothesize that several stages of this process can be miscued. For example, abnormal partitioning of the truncus arteriosus could result in a small aortic outflow tract, hypoplastic valve annulus, and hypoplastic aortic isthmus. Each malformation by itself could serve as the catalyst for the development of the other lesions. The stimulus for normal development of the ascending aorta from the aortic sac is high-pressure systemic blood flow from the left ventricle through the aortic valve (4,9). When normal flow is impeded by an atretic or stenotic aortic valve, the aortic root and ascending aorta receive only low-pressure diastolic retrograde flow via the ductus arteriosus and the aortic arch (10). This prevents the normal growth and development of the ascending aorta, which becomes hypoplastic and whose caliber adapts to that required to accommodate the volume of blood received from the left ventricle. Normal growth and development of the left ventricle and the mitral valve can be affected secondarily, resulting in aplasia or hypoplasia of these structures (2,10).
Fetal aortic valve stenosis has been shown to reduce the growth rate of the left ventricle (10), and the concomitant outflow leads to abnormal development of this chamber (4,5,10). The compensatory left ventricular hypertrophy that results from the increased pressure load on the left ventricle in adults with aortic stenosis does not occur in the fetus. In adults, the diseased aortic valve undergoes pathologic changes over time, leading to left ventricular hypertrophy and, eventually, valvular regurgitation (11). In the fetus with aortic valve stenosis, increased left ventricular pressures may be compensated for by blood that is shunted either from the pulmonic to the systemic circulation via the foramen ovale or from the systemic to the pulmonic circulation via the ductus arteriosus. In cases of associated mitral atresia or stenosis, the left ventricle may be mildly to markedly hypoplastic. However, the left ventricle can retain its normal size even in cases of mitral atresia or stenosis when an adequate ventricular septal defect is part of the cardiac malformation. This is because a right-to-left shunt through the defect impels growth of the left ventricle (12,13).
The aortic arch (derived from the left embryonic fourth aortic arch) and the descending aorta (formed when paired dorsal aortas fuse) develop normally (9). These structures receive blood shunted through the ductus arteriosus, which supplies adequate volume to induce development to normal diameter (14).
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Patterns of Blood Flow
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It is these normal intrauterine hemodynamics that allow maturation of the fetus, even if severe cardiac malformation such as HLHS is present. The remainder of the vascular system, which serves the genitourinary, gastrointestinal, neurologic, musculoskeletal, and dermatologic systems, develops and grows nearly normally because adequate nutrition and waste removal are provided by the hemodynamically normal fetal circulation (15).
Prenatal hemodynamics, specifically patency of the ductus arteriosus, must be maintained after birth to perfuse the coronary arteries and systemic circulation. In patients with HLHS, flow through the patent ductus arteriosus is right to left during ventricular systole, providing flow to the systemic circulation. During ventricular diastole, there is left-to-right flow across the patent ductus arteriosus and retrograde flow through the ascending aorta to perfuse the coronary arteries (14,16).
In HLHS, pulmonary venous blood enters the left atrium, but atrial systole cannot push blood across the atretic or stenotic mitral valve into the hypoplastic left ventricle. Consequently, left atrial blood is shunted across an interatrial communication. The interatrial defect can range from a highly restrictive pinhole-sized foramen ovale to a wide, nonrestrictive atrial septal defect. Most commonly, the foramen ovale restricts left-to-right shunting (4). This restricted flow, coupled with decreased flow through the mitral valve, causes pulmonary venous outflow obstruction, resulting in the typical presentation of a neonate with severe congestive heart failure (Fig 2a). A wider, nonrestrictive atrial septal defect allows a larger-volume left-to-right shunt at the atrial level and increased blood flow to the pulmonary arteries. Normal or increased pulmonary arterial blood flow patterns are seen in these infants (Fig 2b). A mixed pattern of pulmonary veins and arteries is seen in most cases (14,16). In our study, quantitative assessment of blood flow through the interatrial communication on initial echocardiograms was performed in 46 patients. In 36 of these patients (78%), interatrial communications were reported as restrictive (generally <5 mm in diameter), whereas in 10 patients (22%), they were reported as nonrestrictive (generally >7 mm).
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Figure 2a. (a) Pulmonary venous hypertension in a 1-day-old male neonate with HLHS who developed respiratory distress and cyanosis a few hours after birth. Chest radiograph demonstrates an oddly shaped cardiothymic silhouette: The left ventricle does not form the apex of the heart, and the right atrial border is prominent (arrows). Diffuse, markedly increased interstitial and alveolar edema are clearly seen. No skeletal abnormality is detected. A diagnosis of HLHS was made at echocardiography, which demonstrated the severely restrictive interatrial communication responsible for the marked pulmonary venous hypertension. (b) Normal pulmonary vasculature in a 2-day-old male neonate with a wide, nonrestrictive atrial septal defect. The patient had become cyanotic and tachypneic on the previous day. Chest radiograph demonstrates a normal-sized cardiac silhouette with a prominent right atrial border (solid arrows), a finding that suggests enlargement. The right ventricle forms the cardiac apex (open arrow). HLHS was diagnosed at echocardiography, which demonstrated a small left ventricle and a hypoplastic ascending aorta. The patient had a common arteriovenous canal, which did not restrict blood flow between the atria.
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Figure 2b. (a) Pulmonary venous hypertension in a 1-day-old male neonate with HLHS who developed respiratory distress and cyanosis a few hours after birth. Chest radiograph demonstrates an oddly shaped cardiothymic silhouette: The left ventricle does not form the apex of the heart, and the right atrial border is prominent (arrows). Diffuse, markedly increased interstitial and alveolar edema are clearly seen. No skeletal abnormality is detected. A diagnosis of HLHS was made at echocardiography, which demonstrated the severely restrictive interatrial communication responsible for the marked pulmonary venous hypertension. (b) Normal pulmonary vasculature in a 2-day-old male neonate with a wide, nonrestrictive atrial septal defect. The patient had become cyanotic and tachypneic on the previous day. Chest radiograph demonstrates a normal-sized cardiac silhouette with a prominent right atrial border (solid arrows), a finding that suggests enlargement. The right ventricle forms the cardiac apex (open arrow). HLHS was diagnosed at echocardiography, which demonstrated a small left ventricle and a hypoplastic ascending aorta. The patient had a common arteriovenous canal, which did not restrict blood flow between the atria.
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If blood has entered the left ventricle through a stenotic mitral valve, flow may then enter the ascending aorta via a stenotic aortic valve. With atresia of the aortic valve, retrograde flow enters the ascending aorta via the ductus arteriosus and aortic arch (Fig 3). Blood can then be delivered through normal coronary artery ostia (17) to perfuse the coronary arteries. In such cases, the ascending aorta is sometimes referred to as the "common coronary artery" because the caliber of the ascending aorta has been determined by necessity; that is, the volume of blood needed to perfuse the coronary arteries has determined the caliber of the ascending aorta (14).
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Figure 3. Drawing illustrates HLHS, which includes hypoplasia or atresia of the aortic and mitral valves and hypoplasia of the left ventricle and ascending aorta. The great vessels are normally related. Arrows indicate the direction of blood flow. Antegrade flow through the patent ductus arteriosus to the aortic arch and descending aorta occurs during ventricular systole. Retrograde flow through the ascending aorta during ventricular diastole supplies the coronary arteries.
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Neonatal Complications and Treatment
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Affected neonates receive all pulmonary, systemic, and coronary blood flow from the right side of the heart. However, the right side can neither functionally nor physiologically provide the volume of adequately oxygenated blood needed to sustain perfusion (3). The resulting systemic and coronary ischemia leads to cyanosis, severe acidosis, and eventually death in most patients who do not undergo surgery. Three (5%) of our patients survived for 9 weeks or longer without palliation (range, 9 weeks–9 months). Early management involves recreating the prenatal hemodynamics that allowed successful maturation of the fetus. As the ductus arteriosus closes during the first few days of postnatal life, the blood flow to the systemic circulation diminishes to nothing, eliminating blood flow to the cerebral, coronary, and systemic circulations. Thus, patency of the ductus arteriosus must be maintained with intravenous administration of prostaglandin E1 (3). Severe congestive heart failure occurs when adequate blood volume cannot flow from the left atrium into the hypoplastic left ventricle and backs up into the pulmonary venous system. Hypoventilation is induced to increase carbon dioxide tension and thereby maintain the increased pulmonary vascular resistance normally seen in the fetus. Intravenous administration of nitroprusside artificially decreases the elevated systemic vascular resistance seen in affected neonates. The increased pulmonary vascular resistance together with the decreased systemic vascular resistance approximate the fetal physiologic state until surgical intervention can take place (5).
Associated cardiac malformations include pre- and postductal coarctation of the aorta, patent ductus arteriosus, patent foramen ovale, dilated pulmonary artery, ventricular septal defect, dilatation of the right atrium, enlarged right ventricle, and various forms of double-outlet right ventricle (2,3,5). Fibrotic thickening of the endocardium (endocardial fibroelastosis) can occur in either side of the heart and contribute to poor cardiac function (16,18). Additional complicating factors include right ventricular failure and tricuspid valve or common atrioventricular valve regurgitation (3).
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Prenatal Diagnostic Imaging
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Prenatal diagnosis of severe congenital cardiac malformations has been made with four-chamber ultrasonography (US) of the fetal heart performed to evaluate ventricular size and symmetry and the direction of flow within the ascending aorta. In their investigation of flow characteristics in HLHS, Lee et al (14) state that diastolic flow reversal in an extremely narrow ascending aorta is virtually diagnostic for HLHS. There are, however, high false-negative rates in making this diagnosis. Multiple factors, including limiting the study of the fetal heart to a single four-chamber view, the examiner’s ability to recognize the disease, the rarity of the malformation, and the timing of the prenatal examination contribute to false-negative prenatal diagnosis of cardiac malformations (19). Although routine prenatal US is not performed during every pregnancy, it is now used more commonly than ever before. US is certainly used to assess the fetus when risk factors for congenital heart disease are suggested by family history or when an abnormality is suspected. In screening for fetal anomalies, US is generally used during the 16th–18th gestational weeks. Although the critical events of organogenesis are completed in weeks 3–8 of the embryonic period, the growth that occurs in the next several weeks allows improved resolution of fetal anatomy (20).
In HLHS, prenatal US is most effective for diagnostic evaluation of ventricular size beginning in the 18th week of gestation and is most reliable beginning in the 22nd week (Fig 4). Until the 18th week, the ventricles may be the same size, regardless of whether congenital malformation is present. By 22 weeks gestation, ventricular size discrepancies are more apparent on fetal US studies. Therefore, significant cardiac malformations may go undetected if only a single early US examination is performed. Congenital malformations that have historically been considered "missed" when not recognized at prenatal US performed at 18 weeks gestation were likely not apparent and were diagnosed at later prenatal US or during the neonatal period (20,21).
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Figure 4. Echocardiogram (four-chamber view) obtained in a female fetus at 20 gestational weeks demonstrates a markedly hypoplastic ascending aorta (arrowhead) and a hypoplastic left ventricle (LV) (arrow). LA = left atrium, RA = right atrium, RV = right ventricle.
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Postnatal Diagnostic Imaging
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When a prenatal diagnosis of HLHS is made, plans for transporting the patient to a medical facility capable of providing the requisite specialized care can be made well in advance. Echocardiography is used for confirmation of a prenatal diagnosis of HLHS and for early postnatal assessment of affected patients. In the absence of a prenatal US diagnosis of HLHS, echocardiography is the postnatal diagnostic examination of choice.
Echocardiography provides the information required for surgical planning or for recommending no intervention in severely ill infants. Angiographic studies are no longer routinely performed. Critical measurements are made and associated malformations and cardiac function are accurately assessed with echocardiography (14,16,21). The important anatomic variables include size and location of the ductus arteriosus, hemodynamics of the aortic root, patency and size of the foramen ovale or atrial septal defect, and presence of a ventricular septal defect (Fig 5). Awareness of abnormal ventricular wall motion, whether the result of endocardial fibroelastosis or ischemic damage, and valvular competency are essential to understanding the severity of HLHS in a given patient and can affect surgical outcome (4,18,21).
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Figure 5a. (a) Echocardiogram obtained in a 4-day-old male neonate with HLHS demonstrates a severely hypoplastic ascending aorta (ASC AO) that is smaller in caliber than the aortic arch (AO ARCH) and descending aorta. A tight preductal coarctation is also seen. (b) Echocardiogram (four-chamber view) obtained in a different 4-day-old male neonate shows a hypoplastic left ventricle (LV) and a markedly enlarged right atrium (RA). A patent foramen ovale is also seen (arrowhead). LA = left atrium, RV = right ventricle.
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Figure 5b. (a) Echocardiogram obtained in a 4-day-old male neonate with HLHS demonstrates a severely hypoplastic ascending aorta (ASC AO) that is smaller in caliber than the aortic arch (AO ARCH) and descending aorta. A tight preductal coarctation is also seen. (b) Echocardiogram (four-chamber view) obtained in a different 4-day-old male neonate shows a hypoplastic left ventricle (LV) and a markedly enlarged right atrium (RA). A patent foramen ovale is also seen (arrowhead). LA = left atrium, RV = right ventricle.
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Subcostal, apical four-chamber, and parasternal long-axis projections demonstrate the cardiac anatomy (3). The size of the hypoplastic ascending aorta (an internal diameter of <5 mm is diagnostic for aortic atresia) (21) and left ventricle can be accurately assessed. In HLHS, the left ventricle can range from a slitlike potential space to a nearly normal-sized ventricle (14,21). Normal postnatal left ventricular measurements have been investigated to determine which patients with HLHS were appropriate candidates for aortic valvotomy (21) or single-ventricle palliation (12). In one study, the normal mean cross-sectional area of the left ventricle measured with two-dimensional echocardiography on the long-axis view is 2.65 cm2 (±2 SD about the mean; range, 1.8–3.5 cm2) (12). In another study, a left ventricular cross-sectional area of less than 2.0 cm2, also measured on the long-axis view, was indicative of HLHS (21). Both studies point out that left ventricular size in patients with HLHS varies considerably and that no one criterion can be used to determine the course of treatment for a given patient.
Chest radiography may be the first imaging modality to indicate serious congenital heart disease in a neonate exhibiting early clinical signs of distress (eg, tachypnea, dusky complexion). Although chest radiography typically demonstrates enlargement of the cardiac silhouette and pulmonary venous hypertension in a neonate with HLHS, a variety of findings may be seen. The size and shape of the cardiac silhouette vary with the degree of left ventricular hypoplasia, aortic hypoplasia, and compensatory changes in the size of the right side of the heart. The cardiac silhouette can appear hypoplastic, normal, or enlarged (specific or four-chamber enlargement). The differences in the appearance of the cardiothymic silhouette in neonates with HLHS depends primarily on the size of the left ventricle and whether it forms the cardiac apex, the degree of dilatation of the right atrium, the severity of ascending aortic hypoplasia, and the degree of thymic atrophy (Fig 6). Infants with congenital heart disease experience fetal and surgical stresses that may result in thymic atrophy, which is reflected on chest radiographs (22).
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Figure 6a. (a) Preoperative chest radiograph obtained in a 5-day-old male neonate with HLHS demonstrates an enlarged cardiac silhouette. The left ventricle forms the cardiac apex (open arrow), and the right atrial border is prominent (solid arrows), reflecting dilatation of the right atrium. The aortic arch cannot be defined, and the aorta descends on the left side. The superior mediastinal shadow is broad owing to overlying thymic tissue. The pulmonary vasculature is normal. (b) Preoperative chest radiograph obtained in a 4-day-old female neonate with HLHS demonstrates an unusual cardiomediastinal silhouette. The heart is quite small, and the left ventricular silhouette is virtually absent, a finding that reflects severe hypoplasia of this chamber. The superior mediastinal silhouette seems normal but is actually narrow, a finding that signifies mild thymic atrophy. The aortic arch and descending aorta are left-sided. The pulmonary vasculature is somewhat increased because the interatrial communication in this patient is mildly restrictive. (c) On a preoperative chest radiograph obtained in a 1-day-old male neonate, the cardiac silhouette is globular because the cardiac apex is not formed by the left ventricle but rather by the hypertrophic right ventricle (arrowhead). The dilated right atrium forms the prominent, rounded border of the right side of the heart (arrows). Mild pulmonary edema is seen because pulmonic to systemic shunting through the interatrial communication is restricted due to a small foramen ovale. The superior mediastinum is narrow, and the right border of the superior mediastinum is straight, a finding that reflects thymic hypoplasia.
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Figure 6b. (a) Preoperative chest radiograph obtained in a 5-day-old male neonate with HLHS demonstrates an enlarged cardiac silhouette. The left ventricle forms the cardiac apex (open arrow), and the right atrial border is prominent (solid arrows), reflecting dilatation of the right atrium. The aortic arch cannot be defined, and the aorta descends on the left side. The superior mediastinal shadow is broad owing to overlying thymic tissue. The pulmonary vasculature is normal. (b) Preoperative chest radiograph obtained in a 4-day-old female neonate with HLHS demonstrates an unusual cardiomediastinal silhouette. The heart is quite small, and the left ventricular silhouette is virtually absent, a finding that reflects severe hypoplasia of this chamber. The superior mediastinal silhouette seems normal but is actually narrow, a finding that signifies mild thymic atrophy. The aortic arch and descending aorta are left-sided. The pulmonary vasculature is somewhat increased because the interatrial communication in this patient is mildly restrictive. (c) On a preoperative chest radiograph obtained in a 1-day-old male neonate, the cardiac silhouette is globular because the cardiac apex is not formed by the left ventricle but rather by the hypertrophic right ventricle (arrowhead). The dilated right atrium forms the prominent, rounded border of the right side of the heart (arrows). Mild pulmonary edema is seen because pulmonic to systemic shunting through the interatrial communication is restricted due to a small foramen ovale. The superior mediastinum is narrow, and the right border of the superior mediastinum is straight, a finding that reflects thymic hypoplasia.
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Figure 6c. (a) Preoperative chest radiograph obtained in a 5-day-old male neonate with HLHS demonstrates an enlarged cardiac silhouette. The left ventricle forms the cardiac apex (open arrow), and the right atrial border is prominent (solid arrows), reflecting dilatation of the right atrium. The aortic arch cannot be defined, and the aorta descends on the left side. The superior mediastinal shadow is broad owing to overlying thymic tissue. The pulmonary vasculature is normal. (b) Preoperative chest radiograph obtained in a 4-day-old female neonate with HLHS demonstrates an unusual cardiomediastinal silhouette. The heart is quite small, and the left ventricular silhouette is virtually absent, a finding that reflects severe hypoplasia of this chamber. The superior mediastinal silhouette seems normal but is actually narrow, a finding that signifies mild thymic atrophy. The aortic arch and descending aorta are left-sided. The pulmonary vasculature is somewhat increased because the interatrial communication in this patient is mildly restrictive. (c) On a preoperative chest radiograph obtained in a 1-day-old male neonate, the cardiac silhouette is globular because the cardiac apex is not formed by the left ventricle but rather by the hypertrophic right ventricle (arrowhead). The dilated right atrium forms the prominent, rounded border of the right side of the heart (arrows). Mild pulmonary edema is seen because pulmonic to systemic shunting through the interatrial communication is restricted due to a small foramen ovale. The superior mediastinum is narrow, and the right border of the superior mediastinum is straight, a finding that reflects thymic hypoplasia.
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The pulmonary vasculature can demonstrate a variety of flow relationships. The vascular pattern may be normal. Depending on the size of the interatrial communication, pulmonary arterial or venous hypertension or a mixed pattern can be present. When a nonrestrictive atrial septal defect is present, the pulmonary vasculature can be normal or pulmonary arterial flow may be increased as a larger volume of blood is shunted through the defect and enters the pulmonary arteries with each atrial systole. Restrictive interatrial communication (ie, only the foramen ovale is patent) results in pulmonary venous hypertension because pulmonary venous return is obstructed by the atrial septum, leading to high left atrial pressure. Left atrial pressure is increased because blood can neither cross the atretic stenotic mitral valve nor be effectively shunted into the right atrium through the foramen ovale. The result is pulmonary venous hypertension (Fig 2a) (4,14,16).
Angiography is used for presurgical planning only if specific questions remain following echocardiography (3,14). Presurgical evaluation of complex malformations and postoperative assessment of shunt patency are possible with electrocardiographically gated spin-echo and gradient-echo magnetic resonance (MR) imaging (Fig 7) (23).
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Figure 7a. (a) Coronal electrocardiographically gated turbo spin-echo MR image of the heart obtained in a 6-month-old boy with HLHS shows the superior vena cava (black arrow) and a patent bidirectional cavopulmonary shunt (arrowhead). A markedly hypoplastic left ventricular chamber is also identified (white arrow). The great vessels branch normally. (b) Sagittal electrocardiographically gated turbo spin-echo MR image demonstrates the neoaorta constructed with end-to-side anastomosis of the main pulmonary artery to the transverse aortic arch (arrow). The main pulmonary artery (arrowhead) serves as the root of the neoaorta.
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Figure 7b. (a) Coronal electrocardiographically gated turbo spin-echo MR image of the heart obtained in a 6-month-old boy with HLHS shows the superior vena cava (black arrow) and a patent bidirectional cavopulmonary shunt (arrowhead). A markedly hypoplastic left ventricular chamber is also identified (white arrow). The great vessels branch normally. (b) Sagittal electrocardiographically gated turbo spin-echo MR image demonstrates the neoaorta constructed with end-to-side anastomosis of the main pulmonary artery to the transverse aortic arch (arrow). The main pulmonary artery (arrowhead) serves as the root of the neoaorta.
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Palliative Surgical Procedures
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In the 1980s, Norwood et al (4,24,25) described a two-stage palliative surgical procedure for relief of HLHS. Norwood later modified this two-stage approach to the currently used three-stage method of palliation (5). Advancements in surgical technique and a better understanding of the clinical signs and symptoms of HLHS have been made possible in part by improved diagnostic testing.
In this staged surgical approach, the hypoplastic left ventricle is eliminated from the circulation and the right ventricle becomes the systemic pumping chamber. Steps are taken to protect the pulmonary vascular bed and to avoid volume overload on the right ventricle.
The first of the three surgical stages, known as the modified Norwood procedure, has several objectives. A "neoaorta" is constructed from the aortic arch, descending aorta, and main pulmonary artery. The aortic root is anastomosed to the neoaorta. An unobstructed communication is established between the right ventricle—now the systemic ventricle—and the systemic circulation. Coronary arterial flow is maintained. A 3–5-mm shunt from the innominate artery to the main pulmonary artery controls pulmonary arterial blood flow to avoid obstructive pulmonary vascular disease and minimize volume load on the right ventricle (3,5). The distal ductus arteriosus is excised, and the atrial septum is also excised, creating a large interatrial communication and thereby preventing pulmonary venous hypertension and its consequences (Fig 8). If indicated, a subtotal thymectomy is performed for accessibility (5).
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Figure 8. Drawing illustrates the anatomic changes made during the modified Norwood procedure. The hypoplastic left ventricle is bypassed and eliminated from the circulation. The aortic root, aortic arch, and pulmonary artery are anastomosed to create a neoaorta that communicates with the right ventricle and the systemic circulation. Blood flow to the coronary arteries is maintained. A 3-5-mm shunt between the innominate and pulmonary arteries regulates flow to the pulmonary arterial vascular bed. The atrial septum is excised, reducing pulmonary venous outflow obstruction and, consequently, pulmonary venous hypertension. Arrows indicate the direction of blood flow.
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Following the modified Norwood procedure, the right (systemic) ventricle pumps blood through the neoaorta to the coronary and systemic circulations. Blood is delivered to the pulmonary arteries through the shunt connecting the innominate and pulmonary arteries. The right and left atria receive systemic blood from the inferior vena caval and pulmonary venous return. Blood mixes freely through the excised atrial septum.
Fifty-two (91%) of our patients underwent the initial procedure in the three-stage surgical palliation approach developed by Norwood. Ten of these patients (18%) died during the immediate postoperative period (ie, prior to hospital discharge), and two patients (4%) died after discharge.
Postsurgical chest radiographs typically demonstrate stable or decreased cardiac size. The right atrium remains prominent, and the right ventricle usually becomes hypertrophic over time to accommodate the large volume of blood it pumps to maintain systemic perfusion. Obstruction of pulmonary venous outflow (and, therefore, pulmonary congestion) is relieved because the atrial septum has been resected. The shunt connecting the innominate and pulmonary arteries is the source of a new right paratracheal area of increased opacity. The remaining thymus is imperceptible (Fig 9) (5,14,16).
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Figure 9a. (a) Chest radiograph obtained in a 2-month-old girl with HLHS who came to medical attention and surgery late displays flagrant signs of congestive heart failure. The cardiothymic silhouette is moderately enlarged, and the pulmonary vasculature is redistributed with marked pulmonary edema. The right superior mediastinal border is straightened, a finding that suggests thymic hypoplasia. (b) Chest radiograph obtained after the patient had undergone a modified Norwood procedure with a short recovery period shows a slightly smaller cardiac silhouette and a marked decrease in the pulmonary venous vasculature. The lungs are hyperexpanded. The shunt between the innominate and pulmonary arteries projects to the right of the mediastinal border (arrow), making the mediastinal width appear normal.
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Figure 9b. (a) Chest radiograph obtained in a 2-month-old girl with HLHS who came to medical attention and surgery late displays flagrant signs of congestive heart failure. The cardiothymic silhouette is moderately enlarged, and the pulmonary vasculature is redistributed with marked pulmonary edema. The right superior mediastinal border is straightened, a finding that suggests thymic hypoplasia. (b) Chest radiograph obtained after the patient had undergone a modified Norwood procedure with a short recovery period shows a slightly smaller cardiac silhouette and a marked decrease in the pulmonary venous vasculature. The lungs are hyperexpanded. The shunt between the innominate and pulmonary arteries projects to the right of the mediastinal border (arrow), making the mediastinal width appear normal.
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The second surgical procedure—the creation of a bidirectional cavopulmonary shunt—is performed when pulmonary arterial resistance has decreased to normal levels, generally at 3–6 months of age. This is the first step in separating the pulmonary and systemic circulations. Forty of our patients underwent this second procedure, one (3%) of whom died during the immediate postoperative period.
The procedure involves constructing an end-to-side anastomosis of the superior vena cava to the pulmonary arteries. Decreasing the volume of blood delivered to the atria relieves the developing hypertrophy of the right ventricle seen after the modified Norwood procedure. The shunt from the innominate artery to the main pulmonary artery is eliminated (Fig 10) (5,14,16).
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Figure 10. Drawing illustrates the end-to-side anastomosis of the superior vena cava to the pulmonary artery, the initial step toward separating the pulmonary and systemic circulations. The shunt from the innominate artery to the pulmonary artery has been eliminated. A bidirectional cavopulmonary shunt is created when pulmonary arterial resistance has decreased to normal levels, which usually occurs between 3 and 6 months of age. Arrows indicate the direction of blood flow.
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Following the creation of a bidirectional cavopulmonary shunt, chest radiography demonstrates stable, normal cardiac size and normal pulmonary vasculature. The right paratracheal area of increased opacity seen following the modified Norwood procedure is no longer evident due to the elimination of the innominate artery–pulmonary artery shunt (Fig 11).
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Figure 11a. (a) Chest radiograph obtained in a 3-month-old boy with HLHS who had undergone a modified Norwood procedure shows the cardiac silhouette as mildly enlarged with a prominent right atrium (arrows). The pulmonary vasculature is mildly increased. (b) Chest radiograph obtained 5 months later after creation of a bidirectional cavopulmonary shunt demonstrates a stable cardiac silhouette and prominent pulmonary vasculature without edema or pleural effusion.
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Figure 11b. (a) Chest radiograph obtained in a 3-month-old boy with HLHS who had undergone a modified Norwood procedure shows the cardiac silhouette as mildly enlarged with a prominent right atrium (arrows). The pulmonary vasculature is mildly increased. (b) Chest radiograph obtained 5 months later after creation of a bidirectional cavopulmonary shunt demonstrates a stable cardiac silhouette and prominent pulmonary vasculature without edema or pleural effusion.
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The third stage of surgical palliation, known as the modified Fontan procedure, completes the separation of the systemic and pulmonary circulations and is performed when the patient experiences increased cyanosis (ie, has outgrown the capacity to perfuse the systemic circulation with adequately oxygenated blood). A lateral tunnel is created within the right atrium to direct blood from the inferior vena cava to the right pulmonary artery. This serves two purposes: (a) right ventricular stress from volume overload is further reduced, and (b) a greater volume of blood is directed to the pulmonary arteries, thereby relieving cyanosis (Fig 12) (5,14,16). To date, 12 patients have successfully undergone the modified Fontan procedure.
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Figure 12. Drawing illustrates construction of a lateral right atrial tunnel, which directs blood flow from the inferior vena cava to the right pulmonary artery. This completes the separation of the pulmonary and systemic circulations in the modified Fontan procedure, the third and final stage of palliative reconstruction in patients with HLHS. Arrows indicate the direction of blood flow.
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Cardiac size remains stable on chest radiographs obtained in most patients who have undergone this third stage of surgical palliation. Because the volume of blood flowing directly to the pulmonary arterial bed is now increased, the chest radiographs obtained in the vast majority of patients exhibit some degree of chronic pulmonary edema. Persistent pleural effusions (most often right-sided) are common. Increased pulmonary venous return may result in increased right atrial pressure, leading to hepatomegaly or superior vena cava syndrome (Fig 13) (5,16).
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Figure 13a. (a) Chest radiograph obtained in a 3-year-old boy with HLHS who had undergone a modified Norwood procedure and creation of a bidirectional cavopulmonary shunt demonstrates a moderately enlarged, globular cardiac silhouette and slightly prominent pulmonary vasculature but no pulmonary edema. (b) Chest radiograph obtained after the patient had undergone a modified Fontan procedure demonstrates moderate cardiomegaly. Venous pulmonary edema and bilateral pleural effusions (arrows) reflect the increased blood volume delivered to the pulmonary vascular bed.
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Figure 13b. (a) Chest radiograph obtained in a 3-year-old boy with HLHS who had undergone a modified Norwood procedure and creation of a bidirectional cavopulmonary shunt demonstrates a moderately enlarged, globular cardiac silhouette and slightly prominent pulmonary vasculature but no pulmonary edema. (b) Chest radiograph obtained after the patient had undergone a modified Fontan procedure demonstrates moderate cardiomegaly. Venous pulmonary edema and bilateral pleural effusions (arrows) reflect the increased blood volume delivered to the pulmonary vascular bed.
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Not all patients with HLHS undergo this three-stage palliative repair procedure. In patients with critical aortic valve stenosis and a nearly normal-sized, well-functioning left ventricle, a two-ventricle procedure can be attempted. Aortic valvotomy or valvuloplasty can help relieve the aortic valve obstruction, and both ventricles remain part of the cardiac circulation. The left ventricle functions as the systemic ventricle, and the right ventricle functions normally as the pulmonic ventricle. If the procedure is complicated by severe aortic valve regurgitation or some other crisis during the postoperative period, a single-ventricle procedure may be used (3,26). Four (7%) of our patients underwent aortic valvuloplasty. Two of these patients died shortly after the procedure. The other two patients have subsequently undergone the first two steps in surgical palliation (ie, modified Norwood procedure and creation of a bidirectional cavopulmonary shunt).
Transplantation has been used when palliation is required but physiologic or anatomic considerations suggest a poor outcome (5). Significant tricuspid regurgitation correlates with poor outcome in patients with HLHS who have undergone palliative surgery (27). Combined heartlung transplantation was performed in two patients (4%) seen at our institution. Both patients initially underwent the first two steps of the three-stage palliative approach but were poor candidates for the third stage because of progressive right ventricular failure and tricuspid regurgitation or intractable pulmonary arterial hypertension.
Lack of donor organs and lifelong immunosuppression make cardiac transplantation the least viable surgical option. Rarely, a patient will have such a severe malformation that staged palliative repair is not a viable option. The Norwood procedure is sometimes used in these cases to extend patient survival while he or she awaits a suitable donor organ (3).
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Conclusions
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HLHS is a complex combination of cardiac malformations that probably results from multiple developmental errors in the early stages of cardiogenesis. Whereas fetal circulation allows normal growth and development of the fetus, closure of the ductus arteriosus and elevation of pulmonary arterial pressures in the early postnatal period lead to poor perfusion of the coronary, cerebral, and systemic circulations, severe congestive heart failure, and, if left untreated, premature death. A variety of chest radiographic findings are seen in patients with HLHS, and multiple innovative surgical procedures are now available to help prolong the lives of these patients. An awareness and understanding of the spectrum of radiographic findings in HLHS will help pediatric radiologists better assist their colleagues in cardiology and cardiovascular surgery in caring for patients with this relatively common pathologic condition.
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Footnotes
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Abbreviation: HLHS = hypoplastic left heart syndrome
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References
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