Congenital heart disease (CHD) is the most common severe congenital abnormality and a leading cause of infant morbidity and mortality, with an estimated incidence of about 4-13 per 1000 live births. It affects between 0.4% and 0.8% of newborns. Between 1950 and 1994, 42% of infant deaths reported to the World Health Organization were attributable to cardiac defects. In-utero detection of CHD has many important implications. These include the ability to provide prenatal counseling and diagnostic testing for aneuploidy that can provide valuable information for a discussion of pregnancy options, as well as the ability to plan for delivery at an appropriate facility that can provide the highest level of neonatal care. Prenatal detection of CHD has been shown to improve neonatal surgical outcomes and childhood developmental milestones and reduce neonatal mortality. Fetal echocardiography in experienced hands has been reported to detect up to 90% of serious CHD in low-risk populations. The four-chamber view can be reliably obtained in 95% to 98% of pregnancies and theoretically detects >50% of serious cardiac malformations when performed in mid-gestation. Outflow tract abnormalities, also referred to as conotruncal defects, account for 20% of prenatally diagnosed CHD. However, many outflow tract anomalies will not be evident if only the four-chamber view is assessed, as only 30% of cases of abnormalities of the great vessels (i.e. outflow tracts) are associated with an abnormal four chamber view. Table 1 is an example of outflow tract anomalies that are associated with a normal four-chamber view.
In 2018, AIUM in conjunction with other organizations including ACOG, SMFM and ACR, updated 2013 guidelines with regards to the practice parameter for the standard obstetric ultrasound examination. This update now included recommendations to incorporate both the three-vessel trachea (3VV) and three-vessel trachea view (3VT) when technically feasible. This update highlights the importance of these two views, even in the low-risk pregnant patient undergoing the standard fetal anatomical survey. The AIUM practice parameter for the detailed obstetric ultrasound examination was also updated recently. In contrast to the guidelines for the standard obstetric guidelines that recommend the 3VV and 3VT if technically feasible, these guidelines require the 3VV and 3VT, in addition to assessment of the ductal and aortic arches, interventricular septum and superior and inferior venae cavae.
The addition of views of the outflow tracts can increase recognition of CHD from 20-50% to 55-75%, and in some series as high as 90%.The detection of outflow tract anomalies is particularly important as many of these lesions are ductal dependent (that is, require maintaining patency of the ductus arteriosus which normally closes after birth) and thus warrant a high level of neonatal care after delivery. Without prompt intervention including surgical repair, many of these outflow tract lesions can result in death or significant morbidity. This course will examine the normal anatomy of the fetal cardiac outflow tracts, including tips on how to obtain the necessary images, as well as discuss some outflow tract anomalies.
Embryology of the Fetal Heart-The Basics
At 23 days gestational age, the heart begins as a beating primitive tube, and its development is complete by 43 days gestational age. This tube is composed of endothelial cells and has both a caudal and cranial end. Initially, this tube bends into a “horseshoe” shape, and by 23 days gestational age, the free ends of this “horseshoe” fuse into a single beating heart tube. At this point, blood flow is caudal to cranial (inferior to superior), and the future left and right atria first develop as a common atrium at the caudal end of the tube.
The heart tube grows and elongates and develops alternate sections of dilations and constrictions. The most cranial (superior) section is the truncus arteriosus, with the bulbus cordis just inferior. The bulbus cordis section will form the right ventricular cavity, as well as the right and left ventricular outflow tracts (conus cordis). At approximately 28 days gestational age, the truncus arteriosus will divide to form the great vessels of the heart, the aorta and pulmonary artery.
As noted above, part of the bulbus cordis is the conus cordis that will form the right and left ventricular outflow tracts. Abnormalities of this area will result in conotruncal defects. For the right ventricular outflow tract (RVOT), there is a subpulmonary conus (a conus is essentially a ridge of tissue), and for the LVOT, a subaortic conus. During the process of septation, the subpulmonary conus remains, which accounts for the distance between the tricuspid and pulmonary valves. The subaortic conus regresses which allows for the fibrous continuity between the aortic and mitral valves. The conal septum refers to the wall that separates the right and left ventricular outflow tracts. When there is either anterior or posterior deviation of the conal septum, this will often result in obstruction of one of the great arteries. Good examples of anterior and posterior deviation of the conal septum that will result in a defect are tetralogy of Fallot and an interrupted aortic arch, respectively.
To assess the anatomy of the outflow tracts, three different ultrasound planes are incorporated to obtain transverse, oblique and sagittal views. It is helpful to remember that these views are obtained using the four-chamber view as a starting point. In addition, keep in mind that moving from one view to the next often requires only subtle angulation or sliding of the ultrasound probe. Obtaining many of these views is often referred to as the cardiac sweep; as the name implies, one is moving (or “sweeping”) the transducer towards the fetal head and into the superior mediastinum.
The following views with the corresponding ultrasound plane that will be discussed are as follows:
1. Left ventricular outflow tract (LVOT)/five-chamber view: transverse plane
2. Three-vessel view (3VV): transverse plane
3. Three-vessel trachea view (3VT): transverse plane
4. Transverse view of the arterial duct: transverse plane
5. Right ventricular outflow tract (RVOT) short axis view: oblique plane
6. Aortic arch view: sagittal/parasagittal plane
7. Ductal arch view: sagittal plane
Figures 1 and 2 provide a schematic representation to illustrate the anatomic relationship among these different ultrasound planes.
From the four-chamber view, this view is obtained with a slight rotation of the ultrasound transducer, angling slightly toward the fetal right shoulder. The LVOT is parallel to the long axis of the left ventricle, exiting the left ventricle to the ascending aorta with an anterior angle and directed away from the fetal spine. This is in contrast to the right ventricular outflow tract (RVOT) from which the pulmonary artery arises and continues toward the fetal spine as the ductus arteriosus. This view is also referred to as the five-chamber view, as the aortic outflow accounts for this “additional chamber” and demonstrates the left ventriculoarterial connection and the perimembranous and muscular ventricular septa.
Important highlights of the LVOT/five-chamber view include:
1. The continuity of the anterior wall of the aorta wall with the ventricular septum. Discontinuity may serve as a marker in certain defects such as tetralogy of Fallot and common arterial trunk (truncus arteriosus).
2. The continuity of the posterior wall of the aorta with the anterior mitral valve leaflet.
3. Note the wide angle between the ventricular septum and the ascending aorta (figure 3). Loss of this angle is seen in conditions with an overriding aorta such as tetralogy of Fallot.
4. The LVOT should not bifurcate, and if it does, transposition of the great arteries may be suspected.
5. With color Doppler, no blood flow should be visualized from the right ventricle. Otherwise this may suggest the presence of a ventricular septal defect (VSD).
6. The aortic valve should not be thickened and should be shown to open freely. A practical tip is that the leaflets of the aortic valve should normally ‘disappear’ as the ventricle contracts because the valve leaflets open fully against the wall of the aorta.
7. Only the aortic and mitral valve are visualized in the LVOT view. The pulmonary and tricuspid valves are not seen.
8. The LVOT and RVOT cross at nearly a 90 degree angle (figure 4) at the level of the semilunar valves (pulmonic and aortic) and cannot be visualized in the same plane. Inability to confirm crossing outflow tracts may suggest CHD, such as transposition of the great arteries.
Three-vessel view (3VV)
This view, also called the transverse pulmonary trunk view or 3VV-pulmonary artery bifurcation view, is fairly easy to obtain throughout pregnancy. It is considered one of the most important views as most significant heart defects involving the outflow tracts have an abnormal 3VV. Specifically, it can help detect ductal dependent lesions, which require maintaining patency of the ductus arteriosus in the newborn period. From the four-chamber view, a 3VV is achieved by sliding the ultrasound transducer into the upper mediastinum toward the fetal head, maintaining a transverse plane. It is fairly easy to obtain throughout pregnancy, even in patients with an elevated body mass index. One must remember this view does not evaluate the semilunar (i.e. aortic and pulmonic valves). The three vessels in this view, in decreasing order of size, are the pulmonary artery, ascending aorta and superior vena cava. The pulmonary artery should never be smaller than the ascending aorta, as this is often a clue to an abnormality. These three vessels are arranged in an oblique line, with the pulmonary artery the most anterior and leftward, and the superior vena cava (SVC) most posterior and to the right, with the ascending aorta in between. Importantly, this view demonstrates the bifurcation of the pulmonary artery into right and left pulmonary arteries (RPA and LPA). The RPA comes off at a right angle from the main pulmonary artery and runs behind the ascending aorta and the SVC, whereas the LPA follows the same course as the main pulmonary artery. The descending aorta is located posteriorly in the 3VV, to the left of the spine. Figure 5 illustrates the 3VV and the oblique arrangement of these vascular structures.
During assessment of the 3VV, the following should be addressed:
· Are the correct number of vessels present? In some cases, a fourth vessel may be seen to the left of the pulmonary artery. This often represents a persistent left superior vena cava, which is often not a concern if visualized in isolation without any other cardiac lesions noted.
· Is the size of the vessels as expected (pulmonary artery ≥ aorta > SVC).
· Is the alignment of the vessels appropriate?
· Is the location of the descending aorta as expected?
· Does color doppler demonstrate appropriate forward flow?
Ideally, color Doppler should be utilized in evaluation of the fetal heart, and especially when performing a fetal echocardiogram. The angle of the ultrasound beam should be as parallel as possible to the blood flow in the particular vessel of interrogation. Color Doppler is used to confirm forward flow in a vessel; retrograde flow, particularly in the ductus arteriosus can be an extremely concerning finding that may warrant intervention in the immediate postnatal period. Color doppler may also be helpful in distinguishing between the ductal and aortic arch. Pulsed wave Doppler is a useful adjunct to color Doppler that is employed to assess wave patterns across the heart valves as well as the peak systolic velocities across the valves. The ductus arteriosus, a critical structure in fetal life that directs highly oxygenated blood from the pulmonary artery to the aorta, has the highest peak systolic velocity in the fetal cardiovascular system.
Transverse View of the Arterial Duct
From the 3VV, this view is obtained with a slight tilt toward the fetal head (cranial tilt). This view also consists of three vessels similar to the 3VV. However, for clarity, one can also refer to the transverse view of the arterial duct as the 3VV of the ductal arch. This view displays the main pulmonary artery with the ductal arch merging with the descending aorta that resides to the left of the spine. Moving to the right is the ascending aorta and then the SVC. Similar to the 3VV view, the vessels are arranged in an oblique line. In some cases, the trachea, esophagus and azygous vein may be visualized posterior to the SVC. Finally, the thymus gland can be seen in its anterior location in the fetal chest. Figure 6 demonstrates this view.
Three-vessel trachea view (3VT)
This view demonstrates structures in the superior mediastinum as we move cranially from the transverse view of the arterial duct. It is a slightly oblique axial plane just a few millimeters cephalad from the 3VV. The 3VT differs from the 3VV in that the ascending aorta is viewed in cross section but not the aortic arch in the 3VV. In the 3VT, we see the actual transverse aortic arch (and its isthmus) in long axis merging into the descending aorta. The transverse aortic arch is the most superior vessel in the thorax. The pulmonary artery also is seen to the left of the aortic arch and is continuous with the ductus arteriosus. Thus, a key feature of the 3VT is this “V shape” appearance of the ductal and aortic arches converging at the descending aorta (figure 7). This is in contrast to sagittal views of the ductal and aortic arch that cannot be visualized in the same plane. As noted in the 3VV, the arrangement from left to right in an oblique line (anterior to posterior) is similar: PA>aorta>SVC (in cross section). The trachea is located centrally, just anterior to the spine. It is a hypoechoic structure with an echogenic rim that is due to the rings of cartilage that appear bright on ultrasound. The trachea is a reference point that is a marker for a left-sided (i.e. normal) aortic arch. On the 3VT, both arches should be located to the left of the trachea. In the case of a right aortic arch, the “V shape” configuration is located to the right of the trachea, which indicates both the aortic arch and the ductus arteriosus are to the right of the trachea.
A useful checklist of features that should be identified in the 3VT include:
· Ductal arch size > transverse aortic arch size
· Both arches are to the left of the trachea
· The ductal and aortic arch make a “V” shape, joining at the descending aorta
· Using color Doppler, flow in both arches should be toward the descending aorta
· The aortic arch should be seen in its complete course to the left of the trachea, coursing from the right anterior to left posterior position
· The thymus gland can be identified anteriorly, particularly with a high-frequency ultrasound probe
· Are there an appropriate number of vessels?
· Is the alignment of the vessels as expected?
The 3VT may be helpful in detection of the following cardiac abnormalities:
· Ductal dependent lesions: coarctation of the aorta, severe aortic/pulmonic stenosis, interrupted aortic arch
· Aortic arch abnormalities: right sided arch, vascular ring, aberrant head and neck vessels
· Persistent left superior vena cava
· Thymic hypoplasia
Right ventricular outflow tract (RVOT) short axis view
This view is also referred to as a short axis view of the great vessels, or a short axis view of the base of the heart. It is obtained by angling the ultrasound transducer to an oblique plane after a midsagittal view of the fetal chest is acquired. The orientation of this oblique plane is from the right iliac bone to the left shoulder of the fetus. In this view, the aorta appears as a circular structure as it is seen in cross section at the level of the aortic valve, with the main pulmonary artery crossing over the aorta and dividing into the right pulmonary artery and ductus arteriosus (figure 8). The left pulmonary artery is not seen in this view as it is overlapped by the ductus arteriosus. The aortic, pulmonic and tricuspid valves can all be identified in this view.
Sagittal (longitudinal) view of the aortic arch
Once a sagittal view of the thoracic fetal spine is obtained, this view is acquired by sliding the ultrasound transducer to the left, and thus is essentially a parasagittal plane. This view may be difficult to achieve when the fetal spine is anterior (i.e. spine up). Often, slight adjustments are necessary to image both the aortic and ductal arches in their entirety.
In this view, the aorta arises from the central portion of the chest and forms an acute circular curve that is often referred to as a “candy cane” appearance (figure 9). The aortic arch provides the majority of blood supply to the head, neck and upper extremities via three arterial branches that arise from the superior aspect of the aortic arch. These are the brachiocephalic (also referred to as the innominate), left common carotid and left subclavian arteries. The brachiocephalic artery divides into the right common carotid and right subclavian arteries. Color Doppler should be used to confirm forward and laminar flow across the arch.
Certain cardiac anomalies may be suspected when the longitudinal aortic arch view appears abnormal. If a “candy cane” shaped vessel with branching is seen arising from the right ventricle instead of the left, transposition of the great vessels may exist. Coarctation of the aorta, a diagnosis that often goes undiagnosed prenatally, may be suspected if there is narrowing in the region of the aortic isthmus, the area located between the left common carotid and the junction of the ductus arteriosus. Finally, when the ascending aorta appears straight instead of its typical
“candy cane” appearance, this can suggest an interrupted aortic arch, which is characterized by complete separation of the ascending and descending aorta.
Sagittal (longitudinal) view of the ductal arch
From the sagittal view of the aortic arch, continued sliding of the ultrasound transducer to the left will yield the sagittal view of the ductal arch, which may also be assessed parasagitally. The ductal arch, in contrast to the “candy cane” shape of the aortic arch, has been noted to have a “hockey stick” appearance suggested by its wide, angular curvature that is almost perpendicular to the descending aorta (figure 10).
The ductal arch does not give rise to any branches and is seen in its entirety as it connects with the descending aorta. This lack of branching is a key difference as compared to the artery branching associated with the aortic arch. The differences between the aortic and ductal arches are summarized in table 2.
In a parasagittal view of the ductal arch, the right atrium and tricuspid valve can also be seen, which is not the case in the sagittal view. The main pulmonary artery can also be seen wrapping around a cross section of the aorta at the level of the aortic valve, with the aorta posterior to the right ventricle. The pulmonary valve is seen in an anterior and superior position to the aortic valve. The ductal and aortic arches reside in close proximity, and color Doppler will help to distinguish between them.
CARDIAC OUTFLOW TRACT ANOMALIES
Tetralogy of Fallot
Tetralogy of Fallot (TOF) accounts for 3% to 7% of infants with CHD and is the most common malformation in children born with cyanotic heart disease, occurring in approximately 1 in 3600 live births. In the majority of cases of TOF (95%), the four-chamber view is normal, illustrating the importance of including outflow tract assessment as part of fetal cardiac screening. The four main components of TOF are an overriding aorta, a VSD, RVOT obstruction and RV hypertrophy, the latter not usually appreciated prenatally. Recall that the conal septum is the wall that separates the right and left ventricular outflow tracts. In the case of TOF, there is anterior malalignment of the conal septum with the muscular portion of the ventricular septum which results in a VSD and an overriding aorta. The RVOT obstruction combined with the VSD will result in increased aortic outflow, thus the aorta will often appear dilated on ultrasound. A chromosomal abnormality is identified in approximately 45% prenatally and in 25% of live births. Trisomy 21, 13, 18 and the 22q11 microdeletion syndrome (DiGeorge syndrome) have all been implicated. The prognosis is determined by the underlying syndrome.
One of the first ultrasound features noted in TOF is the overriding aorta, often described as the “Y sign” (figure 11). Recall that the pulmonary artery should always be as large (or larger) than the aorta on a 3VV. In most cases of TOF, the pulmonary artery will be smaller (figure 12).
The RVOT obstruction usually is due to pulmonary stenosis, which is the classic form of TOF accounting for approximately 80% of cases. Associated cardiac anomalies include a right aortic arch (25%), persistent left superior vena cava and an atrioventricular septal defect (sometimes referred to as a Tet-canal). In TOF with absent pulmonary valve, usually a ductus arteriosus cannot be identified. Color Doppler will reveal back and forth flow across the pulmonary valve, and in contrast to classic TOF, the aortic root is not dilated and the pulmonary artery (and its left and right branches) will be massively dilated, which can be seen on short-axis and the 3VT views. It is important to use color Doppler to assess flow patterns, particularly when viewing the ductus arteriosus where reversal of flow is predictive of the need for prompt surgical intervention in the immediate postnatal period. The prognosis for short and long-term outcomes with definitive surgical repair is excellent if TOF occurs in isolation, with a greater than 98% survival in the newborn period.
Persistent Truncus Arteriosus
This condition is also referred to as common arterial trunk (CAT). CAT is found in 1.6% of all newborns with congenital heart disease and is reported to occur in about 0.006 per 1,000 live births. Embryologically, the truncus represents the most superior part of the primitive heart tube. During normal development, swellings of the truncus will divide the lumen of the truncus into the ascending aorta and pulmonary trunk. If these swellings fail to divide the lumen, a single vessel will exit the heart. It is reported that this single vessel will arise from both ventricles in 68-83% of cases, with the remainder arising completely from either the RV or LV. Aneuploidy, including trisomy 21, 13 and 18 may be noted in approximately 4.5% of cases of CAT, and the 22q11 microdeletion syndrome has been reported to occur in 30-40% of cases. A right aortic arch is noted in approximately 30% of cases of CAT. With surgical intervention, the prognosis is excellent (> 90%).
The valve of this single vessel, known as the truncal valve, is often dysplastic and color Doppler will demonstrate truncal valve regurgitation in diastole. On occasion, the truncal valve may also be stenotic. In the majority of cases, the truncal valve will have three or four leaflets, although rarely it may have one, two, five or more leaflets.
On ultrasound, an overriding aorta and a VSD (in most cases) will be seen on the five-chamber view. In contrast to TOF where the pulmonary artery can be seen arising from the RV, in CAT a separate pulmonary artery and valve exiting from the right ventricle cannot be identified. Rather, the pulmonary artery can be viewed to arise directly from the overriding large vessel (figure 13). On the 3VV, a single large vessel will be seen that represents the aortic arch, and in greater than 50% of cases a ductus arteriosus has not developed. CAT is classified into four types (known as the Collett/Edwards classification) distinguished by the pulmonary branching pattern:
Transposition of the Great Arteries
Transposition of the great arteries (TGA) is noted in 5-7% of CHD. It has an incidence of 0.315 cases per 1,000 births and occurs twice as frequently in males. In TGA, the aorta arises from the RV and the pulmonary artery from the LV, hence the great vessels are “transposed”. This is described as ventriculoarterial discordance. The outflow tracts will be seen coursing in parallel as they exit the heart and thus views displaying the normal crossing of the great vessels cannot be obtained.
The term TGA is often used interchangeably with D-TGA, where the “D” refers to dextroposition where the aorta is anterior and to the right of the pulmonary artery. D-TGA can be simple or complex, with the former implying an isolated lesion, and the latter indicating an association with other cardiac anomalies. These other cardiac anomalies include VSD’s that occur in approximately 40% of cases of D-TGA, as well as coarctation of the aorta,
LVOT obstruction, atrioventricular valve abnormalities and abnormalities of the coronary arteries. Extracardiac anomalies and associations with aneuploidy are uncommon.
Ultrasound will reveal the great vessels seen in parallel in the same plane. On the 3VT, a large vessel is identified that is positioned anteriorly and superiorly to the pulmonary artery, with the SVC to the right. The pulmonary artery usually cannot be seen on the 3VT, thus giving the appearance of “two vessels”. The five-chamber view will demonstrate bifurcation of a great vessel exiting from the LV, proof that this vessel is indeed the pulmonary artery. Figure 14 provides an example of these ultrasound principles that are concerning for TGA.
In the postnatal period, it is critical to allow mixing of blood such that oxygenated blood can be delivered to the tissues. This is initially accomplished by maintaining patency of the ductus arteriosus. Surgery is usually required within the first 1-2 weeks of life and preferably in the first few days. If a VSD is not present, a balloon atrial septostomy (known as the Rashkind procedure) is required. This procedure opens the atrial septum, thereby enhancing blood mixing and improving oxygen saturation.
L-TGA, also known a congenitally corrected TGA (cc-TGA) is characterized by both atrioventricular and ventriculoarterial discordance, which can be depicted as the following:
right atrium → morphologic left ventricle → pulmonary artery
left atrium → morphologic right ventricle → aorta
Unlike in the case of D-TGA, infants born with cc-TGA are not cyanotic at birth as oxygenated blood from the lungs is able to reach the systemic circulation like normal. However, the right ventricle is not equipped to serve as the systemic pump for a lifetime, and thus surgical correction is often performed.
Double-Outlet Right Ventricle
Double-outlet right ventricle (DORV) accounts for approximately 1-1.5% of children born with CHD and occurs in approximately 0.09 per 1,000 live births. Its main feature is that the pulmonary artery and more than 50% of the aorta arise from the RV. It is sometimes called partial transposition since the aorta arises from the (abnormal) RV but the pulmonary artery arises from the appropriate right ventricle. It is often associated with a VSD which allows for outflow from the LV. It can be best demonstrated on a short axis view showing both the aorta and pulmonary artery arising from the RV and coursing in parallel without ever crossing (figure 15). The five-chamber view will display a lack of continuity of the medial wall of the aorta with the ventricular septum. Atresia or stenosis may be seen in either of the great vessels, thus it is important to assess for this with color Doppler. Pulmonary artery atresia/stenosis tends to occur more frequently than aortic atresia/coarctation in cases of DORV.
There are four types of DORV that are classified according to the positions of the great vessels relative to each other (i.e. whether the aorta is lateral to, anterior to or posterior to the pulmonary artery) as well as the location of the VSD and the presence or absence of pulmonary and less commonly aortic outflow obstruction. DORV may be associated with trisomy 21, 13, and 18 as well as CHARGE or heterotaxy syndromes. There also is an association with maternal diabetes. DORV can appear similar to other conotruncal abnormalities, namely TOF and TGA. However,
both great vessels arising from the same ventricle is the distinguishing factor. In TOF, the great vessels are normally related and mitral valve-aortic valve continuity is preserved, unlike in the case of DORV. In TGA, only one vessel arises from each ventricle.
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