|FOCUS ISSUE - CONGENITAL HEART DISEASE
|Year : 2020 | Volume
| Issue : 3 | Page : 325-331
Aortopulmonary Shunts: Echocardiographic Evaluation
Neeraj Awasthy, Gaurav Kumar, Romila Chimoriya
Department of Pedaitrica Cardiology and CTVS, Max Superspeciality Hospital, Delhi, India
|Date of Submission||17-Aug-2020|
|Date of Acceptance||02-Oct-2020|
|Date of Web Publication||18-Dec-2020|
Dr. Neeraj Awasthy
A204, Palam Apartments, Sector 5, Plot Number 7, Dwarka, New Delhi
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Awasthy N, Kumar G, Chimoriya R. Aortopulmonary Shunts: Echocardiographic Evaluation. J Indian Acad Echocardiogr Cardiovasc Imaging 2020;4:325-31
|How to cite this URL:|
Awasthy N, Kumar G, Chimoriya R. Aortopulmonary Shunts: Echocardiographic Evaluation. J Indian Acad Echocardiogr Cardiovasc Imaging [serial online] 2020 [cited 2021 Apr 13];4:325-31. Available from: https://www.jiaecho.org/text.asp?2020/4/3/325/303942
| Aortopulmonary Shunts|| |
Aortopulmonary shunts are anatomic connections between the aorta (AO) and main or branch pulmonary arteries (PAs) causing left-to-right (L-R) shunts. Occasionally, in cases of severe PA hypertension (PAH), it causes right-to-left (R-L) shunt.
Types of aortopulmonary shunts
- Connection between AO and main PA (MPA):
- Patent ductus arteriosus (PDA)
- Aortopulmonary window (APW).
Connection between AO and branch PA:
- Aortic origin of a PA (AOPA)
- Major aortopulmonary collateral arteries.
| Patent Ductus Arteriosus|| |
Definition and physiology
The ductus arteriosus is a muscular artery that develops from the sixth branchial arch connecting left PA (LPA) to AO during intrauterine life. It is an obligatory part of normal fetal circulation allowing blood to bypass circulation to the lungs and flow directly to descending aorta (DA). Functional closure of ductus arteriosus is caused by smooth muscle contraction due to high levels of oxygen and decreased concentration of circulating prostaglandins after birth. Closure occurs within 24 h in half of the neonates and is universal by 72 h of life. Persistence patency of ductus arteriosus is termed as PDA.
Incidence and frequency
Isolated PDA represents 5%–10% of congenital heart defects and 1 in 2000 live births. However, if we include children with silent patent ductus, the incidence can be as high as 1 in 500 live births. Its incidence is inversely proportional to the preterm infant's gestational age, with a rate of about 20% in premature infants born at 32 weeks of gestation compared to an estimated 80%–90% in extremely low birth weight infants with a gestational age of <26 weeks. Females are more commonly affected than males with a ratio of 2:1.
The ductus arteriosus is a vascular structure that connects the LPA with the descending portion of the aortic arch. The PA end of the PDA is usually immediately to the left of the PA bifurcation. The aortic connection is just distal to the origin of the left subclavian artery. The normal direction of flow in the ductus determines its orientation, and its size (along with pressure differences) determines the amount of flow. Closure of the PDA usually begins at the pulmonary end, accounting for the funnel-shaped configuration which is seen in majority of patients.
| Krichenko Classification (Based on Narrowest Diameter of Vessel)|| |
- Type A: Developed ampulla and constriction at PA end (funnel or conical shape), the most common type (65%)
- Type B: Short ductus with constriction at the aortic end (window type)
- Type C: Ductus with no significant narrowing
- Type D: Multiple areas of constriction
- Type E: Long ductus with narrowing far anterior to trachea at PA end.
Echocardiography is the investigation of choice to confirm the diagnosis and to assess the functional significance of PDA.
The objectives are to detect:
- The presence of a duct
- Detailed definition of ductus
- Size of the duct
- Type of duct
- Ampulla and PA end
- Orientation of ductus
- Orientation with respect to the device closure.
- The hemodynamic significance of a duct
- Direction of shunt
- Pulmonary arterial pressure
- Quantification of shunt.
- Associated defects.
PDA can be visualized with ideally four views:
A high left parasternal view is directed in plane of LPA and angulated to visualize the trifurcation where MPA is seen dividing into three vessels, namely right PA (RPA), LPA, and PDA [Figure 1]. After obtaining the short-axis cut of the great vessel visualizing the PA bifurcation, the transducer is rotated anticlockwise in gradual motion. At one point, the LPA goes away from view and the duct with adjacent DA opens [Figure 2]. This view in neonates and infants also visualizes the origin of the left subclavian artery. In patients with associated coarctation, the posterior shelf is also well visualized.
|Figure 1: Ductal view (high parasternal short-axis view) on two-dimensional echocardiography showing trifurcation where main pulmonary artery is seen dividing into three vessels, namely right pulmonary artery, left pulmonary artery, and patent ductus arteriosus. RA: Right atrium, LA: Left atrium, AO: Aorta, PA: Pulmonary artery, PDA: Patent ductus arteriosus|
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|Figure 2: Ductal view (high parasternal short-axis view) from a child on two-dimensional echocardiography (left) and on color flow mapping (right) showing turbulent mosaic jet of patent ductus arteriosus. MPA: Main pulmonary artery, PDA: Patent ductus arteriosus, DA: Descending aorta|
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- Suprasternal long-axis view – The PDA can be seen connecting the DA and the MPA [Figure 3]. This view can never open up the usual ductus, which arises from the lateral wall of the DA. However, this is the best view for visualizing the vertical duct arising from the undersurface of the transverse arch in patients with pulmonary atresia. The origin of such ductus is well seen, but the insertion point at the PA required further anterior tilt due to its tortuous nature. However, in patients with discordant ventriculoarterial connection, the entire length of the duct can be visualized very well in this view
- Suprasternal short-axis view – This can visualize those rare ductus which arises from the base of the left subclavian artery and descends straight down to insert into the LPA. In right-sided aortic arch, the entire length of the duct can be seen in this view
- Modified ductal view – This view can visualize the duct in its entire length and most closely mimics the lateral angiogram performed during cardiac catheterization. From the usual suprasternal long-axis view, the transducer is rotated anticlockwise. A slight anterior tilt then shows the duct from its ampullary part to its insertion.
|Figure 3: Suprasternal long-axis view, two-dimensional echocardiography (left), and color flow mapping (right) with large patent ductus arteriosus showing long tubular duct (arrow). AO: Ascending aorta, DAO: Descending aorta, PDA: Patent ductus arteriosus|
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Size of duct
- Size of the narrowest part of the duct – The most accurate view for measuring of ductal size is high left parasternal short-axis window also known as ductal view or modified aortic arch view. The ductal size measurements are taken at the narrowest point which in majority of cases is at the site of PA insertion. This measurement is crucial for device selection in transcatheter closure of PDA
- Size of the ampulla of duct – The size of the ampulla is equally important for intervention to determine whether coil placement would be possible. The ampulla can be best measured in the modified ductal view
- Length of the duct – Measurement of length of duct helps in decision-making to determine the adequacy of coil/device/stent placement. The modified ductal view aids in the measurement.
The ductus arises from DA just below the origin of the third arch branch and inserts into the PA immediately to the left of PA bifurcation. This type of ductus is best defined from ductal view and suprasternal modified ductal view. The most common morphology of ductus is the one that narrows at the PA giving funnel shape and has a straighter course. This type of ductus is most easy access by femoral artery route for catheter interventions. Other rare configurations of usual ductus include the following:
- Short duct with narrow aortic end
- Tubular connection with no narrowing
- Tubular connection with multiple narrowings
- Calcified PDA
- Aneurysm of the aortic end of the PDA.
Vertical ducts are most commonly seen in pulmonary atresia with ventricular septal defect (VSD) [Figure 4]. There is a reverse angle of the PDA and absence of aortic isthmus narrowing caused by severe right ventricular outflow tract obstruction in utero with altered fetal flow patterns. It usually arises from the undersurface of arch giving S shape and has a tortuous course seen in suprasternal long-axis view. Because of double curve, it is not possible to visualize the aortic and pulmonary end in the same view. Anterior angulation of the transducer will show the PA insertion of the duct. These ducts are difficult to cannulate from the femoral artery route and may have to be accessed from the ascending AO or from the upper limb arteries.
|Figure 4: Suprasternal long-axis view on two-dimensional echocardiography of a child with pulmonary atresia and ventricular septal defect showing vertical duct (arrow). AO: Aorta, LA: Left atrium, LSCA: Left subclavian artery, PDA: Patent ductus arteriosus|
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Subclavian origin of ductus occurs in patients with right aortic arch with duct-dependent pulmonary blood flow. Suprasternal view demonstrates this ductus having straighter course. It is easy to cannulate this type of ductus arteriosus from femoral artery route.
Hemodynamic significance of ductus arteriosus can be assessed by volume overload of the left atrium (LA) and left ventricle, direction of shunt, and pulmonary arterial pressure.
Left atrial enlargement signifies increased pulmonary venous return because of left-to-right ductal shunting. The pulmonary blood flow increases due to amount of blood running back from DA into lung circulation through PDA (steal phenomenon). In the infant with compliant LA, the LA to AO at the level of the aortic valve (the LA: AO ratio) is measured by M-mode echocardiography in parasternal long-axis view [Figure 5]. M mode is taken from parasternal long axis view at the level of the aortic valve with the transducer parallel to the aortic valve has been used for indirect assessment of the size of the ductal shun. The aortic valve is measured just before its opening at the end of diastole, whereas LA is measured at its maximal volume during systole. The aortic root does not enlarge significantly with even extremely large PDA. In general, a LA:AO ratio >1.3:1 indicates a significant shunt. However, in many instances, LA:AO ratio can be misleading due to unusual transducer position and cardiac rotation giving fallacious interpretation. In Infants with large PDA, the atrium septum bowing predominantly to the right indicates LA dilatation. Left ventricle will enlarge as cardiac output increases with both increased pulmonary venous return and with increased diastolic runoff from the systemic circulation. The best method to determine the presence of volume overload of the left ventricle is M-mode measurement of diastolic dimensions of the Left Ventricle and comparing it with normal range for the patient's weight and height. Due to diastolic runoff from DA to main PA, the pulsations in DA in subcostal views are increased.
|Figure 5: Parasternal long-axis view with M-mode showing left atrium to aorta ratio. LA: Left atrium, AO: Aorta|
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Direction of shunt and pulmonary arterial pressure
Color Doppler imaging of duct
Color Doppler flow mapping has improved the detection of tiny ducts which would have been missed otherwise in two-dimensional echocardiography. On color flow mapping, small duct with normal PA pressure (PAP) is displayed as a high-velocity jet with mosaic flow area layered along the left lateral border of MPA [Figure 6]. With large duct, and low pulmonary vascular resistance, the duct jet appears as predominantly red flow with minimal aliasing (color reversal). The R-L jet through the ductus arteriosus is pure blue flow area indicating equivalent PA and DA pressures. With suprasystemic PAP as in duct-dependent systemic blood flow, a restrictive duct will show turbulent high-velocity R-L flow in systole and diastole in the DA.
|Figure 6: Parasternal short-axis view with restrictive patent ductus arteriosus. (a) Parasternal short-axis view with color flow mapping showing turbulent left-to-right flow across restricted duct. (b) Continuous-wave Doppler tracing of ductus showing high-velocity continuous flow above the baseline. AO: Aorta, DA: Descending aorta, RV: Right ventricle, PDA: Patent ductus arteriosus. S: Systolic, D: Diastolic|
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Doppler examination of the main pulmonary artery
Doppler sample volume positioned in distal MPA shows continuous disturbed flow directed toward transducer. If PAP is normal, velocity of signals will be high. Similarly, with elevated PAP, velocity of the flow signals will be decreased. Doppler sample volume placed in the midportion of MPA, it is possible to record diastolic flow signals below the baseline. These signals arise as jet flow from PDA strikes the closed pulmonary valve in diastole and swirls back on itself giving flow signals away from transducer (swirling effect).
Continuous-wave Doppler examination of ductus arteriosus
Continuous-wave Doppler examination of PDA jet shows different flow patterns:
- Isolated L-R PDA shunt – Continuous positive flow with a peak velocity in late systole [Figure 7]
- Isolated R-L PDA shunt – Continuous negative flow with a peak velocity in early systole [Figure 8]
- Bidirectional PDA shunt and severe PAH-R-L shunt in systole and L-R shunt in late systole extending to late diastole.
|Figure 7: Ductal view from a child with large patent ductus arteriosus on color flow mapping (left) laminar flow toward the transducer (red flow) is seen suggestive of left-to-right shunt from aorta to pulmonary artery. Continuous-wave Doppler tracing (right) of duct showing high-velocity continuous flow above the baseline suggestive of left (Lt)-to-right (Rt) shunt|
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|Figure 8: Continuous-wave Doppler tracing of duct with suprasystemic pressures suggestive of right-to-left shunt|
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By the use of continuous-wave Doppler, direction of shunt in relation to cardiac cycle and pulmonary arterial pressure (systolic blood pressure minus Doppler peak gradient across duct = systolic pulmonary arterial pressure) can be detected accurately. However, PA systolic pressure measurement via Doppler has some limitations such as sample volume position, signals arising from adjacent LPA, and shape of ductus arteriosus.
Evidence of aortic runoff
Doppler examination of the AO shows evidence of a diastolic runoff or steal of blood from AO. Large PDA with low Pulmonary artery diastolic pressure blood flows from AO into PA in diastole. Color flow mapping shows flow reversal in DA in diastole up to the level of ductus arteriosus. In suprasternal notch view, Doppler sample volume place in DA shows forward flow signals in systole below the baseline and above the baseline in diastole indicating flow up the DA toward ductus and MPA. The retrograde diastole flow signals are usually M shaped with peak in early diastole [Figure 9]. However, the finding of retrograde flow in blood in DA is not specific for PDA as it can be found in any defect in which diastolic runoff of blood from AO occurs.
|Figure 9: Continuous-wave Doppler tracing of aorta shows diastolic flow reversal in descending aorta due to aortic runoff|
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Doppler quantification of magnitude of ductal shunt
Sampling of flow velocities at right ventricular outflow or pulmonary valve provides systemic blood flow and sampling at left ventricular outflow provides pulmonary blood flow.
Echocardiographic criteria for hemodynamically significant PDA includes:
- Nonhemodynamically significant PDA:
- High flow velocity during systole and diastole with a maximal velocity above 2 m/s at the end of diastole
- This is called restrictive continuous transductal flow.
Moderately hemodynamically significant PDA:
- Doppler flow velocity maximal diastolic velocity of < 2 m/s
- This is called unrestrictive pulsatile transductal flow.
Large hemodynamically significant PDA:
- Maximal end-diastolic velocity ≤ 1 m/s
- Unrestrictive pulsatile transductal flow.
Hemodynamically significant PDA can also be identified if the peak diastolic velocity is < 50% of peak systolic velocity and the flow pattern is pulsatile.
Limitations of echocardiographic imaging of the duct include:
- Limited acoustic windows
- Long and tortuous duct morphology
- Poor acoustic windows as in adults with thick chest, chest deformity, and airway disease.
| Aortopulmonary Window|| |
APW first described by Elliotson is a rare cardiac abnormality with an incidence of 0.2%–0.6% of all congenital heart defects. It is characterized by abnormal communication between ascending AO and PA in the presence of separate semilunar valves and separate arterial trunks.
Faulty septation of the aortic sac by septum aortoplumonary is an underlying cause of the malformation. APW can occur as an isolated defect or may be associated with other cardiac defects in 50%–60% of the cases. The associated defects are most commonly aortic arch anomalies (coarctation, Type A interrupted aortic arch), tetralogy of Fallot (TOF), anomalous origin of coronary arteries, tricuspid atresia, aortic and pulmonary atresia, and transposition of great arteries (TGA).
Classification of aortopulmonary window
- Type I: Proximal defect (usually large) located in between the origin of MPA and ascending AO with little inferior rim above semilunar valves
- Type II: Distal defect (usually small) between ascending AO and origin of RPA with the absence of rim at PA bifurcation
- Type III: Large defect extending from semilunar valves to PA bifurcation. It is also referred to anomalous origin of one of the branches of PAs from AO
- Type IV: Intermediate defect which is a smaller central defect with adequate superior and inferior rims.
Echocardiography remains the cornerstone for diagnosis of APW. These defects are generally overlooked when other causes for L-R shunt are identified which are present in more than half of the patients with APW. Hence, meticulous evaluation is required for diagnosis of APW.
Objectives are to identify:
- Presence of the defect
- Type and size of defect
- Hemodynamic effects of defect
- Associated cardiac anomalies.
| Identification of Defect|| |
Parasternal short-axis view at the level of great vessels
This is the most ideal view for diagnosis of APW. It allows visualization of both MPA and AO to identify the communication between these structures [Figure 10]. It also aids in defining the origin of RPA to rule out the anomalous origin of RPA from ascending AO. Color Doppler aids in confirmation by color flow across the defect and differentiates it with artifactual dropout. T artifact at the edge of defect distinguishes it from normal dropout. Large defect flow is laminar, low velocity, and bidirectional across the defect. Smaller defects have continuous high-velocity L-R shunt across the defect. The size and type of the defect can also be assessed from this view.
|Figure 10: Echocardiography of an infant with aortopulmonary window (Type 1). Parasternal short-axis view at the level of great vessels on two-dimensional echocardiography (left) showing large defect (arrow) in between ascending aorta and main pulmonary artery. On color flow mapping (right), there is a laminar flow between ascending aorta and main pulmonary artery. AO: Aorta, PA: Pulmonary artery|
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Subcostal coronal view of left ventricular outflow tract
This view allows visualization of defect and assesses distance from the semilunar valves, and the presence of rims can also be identified. The origin of the left coronary artery along with associated anomalies such as TGA, VSD, and TOF can also be assessed.
This view confirms the diagnosis and extent of involvement can be evaluated. The arch anomalies can also be ruled out.
The following parameters can be seen:
- Dilated LA/left ventricular
- Increased pulmonary venous return
- Increased flow across mitral and aortic valve
- Pan diastolic forward flow in branch PAs
- Diastolic flow reversal in DA.
It can provide a more robust spatial orientation of the defect, and AOPA can be assessed.
| Anomalous Origin of Pulmonary Branches from the Ascending Aorta|| |
Anomalous origin of the PA from the ascending AO (AOPA) is commonly referred to as hemitruncus. It is a rare disorder accounting for 0.1% of all congenital heart diseases. The RPA arises directly from the ascending AO in 70%–80% of cases. The RPA arises from the ascending AO just above the aortic sinuses, whereas the MPA and the other pulmonary branch arise in their normal position.
AOPA may occur as an isolated defect or may be commonly associated with TOF, PDA, right aortic arch and aortopulmonary defect, atrial septal defect, isthmic hypoplasia, and interrupted aortic arch.
The most common views to diagnose AOPA are apical four chamber followed by parasternal short axis view and suprasternal view. There are reported cases where origin site is close to the innominate artery or sometimes arises from the innominate artery itself.
Echocardiography remains the ideal investigation of choice for diagnosis of AOPA. The most common views to diagnose AOPA are apical four chamber followed by parasternal short axis view and suprasternal view. The features specific for AOPA are:
- Presence of two concordant ventricular outflow tracts
- Absence of usual MPA trunk bifurcation
- Origin of either right or LPA directly from AO with MPA continuing with contralateral PA branch.
Subcostal coronal view with anterior tilt, parasternal short-axis view, and suprasternal long-axis view show the origin of one-branch PA from ascending AO.
Besides these features associated, cardiac anomalies should also be seen carefully. Furthermore, it should be differentiated from APW, tricuspid atresia, and discontinuous PAs with one PA supplied from duct or collateral as seen in VSD with pulmonary atresia.
| Multiple Aortopulmonary Collaterals|| |
In certain cardiac conditions such as TOF with pulmonary atresia or TOF with pulmonary stenosis, pulmonary circulation is supplied entirely by collateral arteries that serve both as a nutritive function and respiratory function. These systemic arterial collaterals are divided into three types according to their origin:
- Bronchial systemic arterial collaterals:
- These originate from bronchial arteries and anastomose to PAs within the lung (intrapulmonary anastomoses).
Direct systemic arterial collaterals:
- These originate from descending thoracic aorta, enter the hilum, and distribute into intra PAs (hilar anastomosis).
Indirect systemic arterial collaterals:
- These originate from the internal mammary, innominate and subclavian arteries and anastomoses to proximal PAs outside the lung (extrapulmonary anastomosis).
The ideal view for demonstrating aortopulmonary collaterals is a suprasternal long-axis view. The collateral is seen as a high-velocity jet arising from DA [Figure 11]. Besides suprasternal view, it can be also seen in subcostal view by profiling the DA along its long axis.
|Figure 11: Suprasternal long-axis view on two-dimensional echocardiography (left) and color flow mapping (right) of aorta showing small aortopulmonary collaterals arising from descending aorta. DA: Descending aorta, MAPCA: Multiple aortopulmonary collaterals|
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Conflicts of interest
There are no conflicts of interest.
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