Fetal Ultrasound: The 3 Vessel Tracheal View


Reviewed by R Kamble,Alpana Joshi,P Mistry

THE 3 VESSEL TRACHEA VIEW (3VT VIEW)

Antenatal diagnosis of congenital anomalies of the fetal heart is important for proper perinatal and neonatal management. The four-chamber view is the basis of the cardiac scan in a screening examination for congenital heart disease (CHD). The technique is simple and easy to reproduce and with careful standardization of the elements that constitute a normal view, up to 60% of CHD can be diagnosed.

Nevertheless, it has certain limitations in the detection of anomalies involving the ventricular out?ow tracts and/or the great arteries; some anomalies can be missed by screening with a four-chamber view alone. Therefore scanning of the out?ow tracts is important. However, this examination is not easy to teach or learn, and is often time-consuming. In order to improve the antenatal detection of CHD involving the ventricular out?ow tracts and great arterial trunks, Yoo et al proposed the three-vessel view, a transverse view of the fetal upper mediastinum. It can be easily obtained by moving the transducer cephalad from four chamber view.

The 3 vessel view is obtained in a transverse plane of the fetal upper thorax. One has to move the transducer cephalad from 4 chamber view to obtain this 3 vessel view. This view demonstrates the main pulmonary trunk in an oblique section and the ascending aorta and the superior vena cava in transverse section (fig.1). The 3 vessel plane is helpful in assessment of conotruncal abnormalities in which 4 chamber view is usually normal, thus are missed on 4 chamber view. 

3 vessel trachea view is more cranial to 3 vessel view. This view demonstrates the aortic arch and ductal arch merging together into the descending aorta with an acute angle between them (fig.2). Both arches are located to the left of the spine and trachea, an important anatomic landmark. The superior vena cava is seen to the right of aortic arch (Fig. 3). The oblique line orientation of the three vessels is maintained with the ductal arch assuming more anterior position, aortic arch in the middle and SVC more inferior. Color Doppler reveals anteroposterior flow in both arches (Fig. 4). Normally there should not be any vessel crossing posterior to trachea.

       
 

Fig  2:  Drawing of three vessel trachea (3VT) view showing the aortic arch (Ao arch0 and ductal arch (DA) merging together into the descending aorta (D Ao) to the left of spine and trachea. SVC is to the right of trachea

 
   

Fig  3:  The three vessels and trachea view. SVC, Superior vena cava; AO, aorta; PA, pulmonary artery; TR, trachea

 
 

 

 

 

 

 

Fig 4:  Three vessels and trachea view showing anteroposterior flow in aorta (AP) and pulmonary artery (PA) and in opposite direction in SVC.

 

The abnormalities detected in the upper mediastinum are as follows:

  1. Abnormal vessel size
  2. Abnormal vessel arrangement and alignment
  3. Abnormal vessel number
  4. Abnormal location of the transverse aortic arch in relation to the trachea
  5. Abnormal functional behavior of the arterial vessels (reversed or turbulent flow) using color Doppler

ABNORMAL VESSEL SIZE

The size of the vessel reflects the amount of blood flow passing through it. Normally, the structures in the 3VT view, in descending order of size from left to right, are: pulmonary artery, transverse aortic arch (TAoA) and SVC. The ductus arteriosus and the TAoA are similar in size.

Preferential flow into the left side of the heart (larger TAoA and smaller pulmonary artery) are seen in pulmonary stenosis, Tetralogy of Fallot, some cases of double outlet right ventricle (DORV). The pulmonary artery is characteristically small in tetralogy of Fallot (Figure 5).

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Fig 7:  Three vessels and trachea view of the heart of a fetus with coarctation showing small aorta as compared to pulmonary artery (PA).

 

Preferential flow into the right side of the heart (larger pulmonary artery and smaller transverse arch) is seen in fetuses with obstruction of the left outflow tract aortic atresia / stenosis, mitral valve atresia (Figure 6), coarctation of the aorta (Figure 7), some cases of double outlet right ventricle, absent pulmonary valve syndrome (APVS). These anomalies are characterized by a variable degree of discrepancy in the size of both TAoA and pulmonary trunk in the 3VT view.

Dilatation of both arterial vessels is found in polyvalvular dysplasia in which there is stenosis/insufficiency across the valve.

Dilatation of SVC is observed in IVC interruption with azygos or semi azygos continuation, anomalous pulmonary venous return-supracardiac type, vein of Galen aneurysm.

Although abnormal vessel size can be quantified using nomograms, the qualitative evaluation allows correct recognition of many of the critical major defects involving the great arteries. 

ABNORMAL VESSEL ARRANGEMENT AND ALIGNMENT

In this there is complete distortion of the right-left order (Figure 8). In corrected transposition of the great arteries, the TAoA is located to the left of and anterior to the main pulmonary artery. In double outlet right ventricle, the aorta may be displaced anteriorly and more to the right than normal. 

     

 

Fig 8  : [A]  Note normal SVC, AORTA, PA arrangement and [B] the complete distortion of Right to Left order. SVC, PA , AORTA  in a case of complex univentricular heart (C) 

 

In the fetus with a normal heart and normally related great arteries, the pulmonary artery, as it courses cranially in the superior mediastinum, forms a leftward convexity or traverses the thorax in a straight line (Figure 9). In contrast, in the fetus with TGA the vessel arising from the anterior ventricle (the aorta) forms a rightward convexity as it courses in the superior mediastinum to the level of the 3VT view (Figure 10).

The abnormal rightward convexity of the great vessel arising from the outflow tract of the anterior ventricle may be an aid in the diagnosis of TGA and may be the initial clue to the diagnosis of TGA. It is relatively simple to obtain. The 3VT view is in fact a misnomer in some cases of TGA as only two vessels (the aorta and the superior vena cava) are seen, this is because of the pulmonary artery and ductus arteriosus being below the transverse arch in a more caudal plane.

Fig 9:  Note the convexity of the great vessel arising from anterior ventricle (here the pulmonary artery) to the left in normal heart.

 

                                                                              

 

ABNORMAL VESSEL NUMBER

Two or four vessels are seen instead of normal three vessels. Two vessels instead of three are seen in many conditions like, transposition of great arteries (TGA) (Fig 10), pulmonary atresia (Fig 11), truncus arteriosus (Fig 12), interruption of aortic arch. Four vessels are seen in persistent left SVC (Fig 13). 

ABNORMAL LOCATION OF THE TRANSVERSE AORTIC ARCH IN RELATION TO THE TRACHEA

Abnormal visualization of the trachea, between the pulmonary artery and the transverse arch, is seen in right aortic arch.

RIGHT SIDED AORTIC ARCH

In normal conditions, the left aortic arch crosses the left bronchus in the upper chest. Right aortic arch is defined by the aortic arch that crosses the right bronchus instead of left bronchus. In fetal echocardiography, a right aortic arch is diagnosed when the transverse aortic arch is located to the right of trachea. A right aortic arch occurs in about 1 in 1000 of the general population.

A right aortic arch is associated with three main subgroups of arch abnormalities

  1. Right aortic arch with right ductus (fig. 14B)
  2. Right aortic arch with left ductus (fig 14C.)
  3. Double aortic arch (fig. 14D)

Right aortic arch can be a part of a complex cardiac malformation, but can often be an isolated finding.

ULTRASOUND FINDINGS

  1. Right aortic arch with a right ductus arteriosus (right V-sign): In this group, the ductus arteriosus is right sided. Both the aorta and pulmonary arteries merge together in a V-configuration to the right of trachea with no vascular ring (Fig 14-B). The brachiocephalic vessels arise in mirror image branching to the normal left aortic arch. In most cases this condition is associated with cardiac malformation, mainly “conotruncal anomaly”.
  1. 2.     Right aortic arch with left ductus arteriosus (U-sign): The aortic arch is right sided and the pulmonary trunk & ductus arteriosus are to the left of trachea. In this condition trachea is seen between the transverse aortic arch (right) and the ductus arteriosus (left) (fig.15). This forms a U-configuration. This is considered a loose “vascular ring”. This condition is commonly an isolated finding. 

  1. Double aortic arch: The aortic arch has a course to the right of the trachea but bifurcates directly at the level of the trachea to have one arch to the right and one to the left in the upside down N type configuration, with the oblique bar of the N representing the second arch. Behind the trachea both arches fuse into the descending aorta. The esophagus and trachea are entrapped between right and left aortic arches.

ABNORMAL FUNCTIONAL BEHAVIOR OF THE ARTERIAL VESSELS (REVERSED OR TURBULENT FLOW) USING COLOR DOPPLER

Reversed flow in one of the arterial vessels was defined as a marker of ductal dependent CHD and denotes a worse spectrum of prognoses for the newborn. The reversal of flow in the TAoA is observed in hypoplastic left heart syndrome (Fig 16). Reversed flow in the pulmonary trunk is seen in hypoplastic right heart syndrome (Fig 17). Pulmonary atresia with intact ventricular septum (PAIVS)

 

  1. Double aortic arch: The aortic arch has a course to the right of the trachea but bifurcates directly at the level of the trachea to have one arch to the right and one to the left in the upside down N type configuration, with the oblique bar of the N representing the second arch. Behind the trachea both arches fuse into the descending aorta. The esophagus and trachea are entrapped between right and left aortic arches.

ABNORMAL FUNCTIONAL BEHAVIOR OF THE ARTERIAL VESSELS (REVERSED OR TURBULENT FLOW) USING COLOR DOPPLER

Reversed flow in one of the arterial vessels was defined as a marker of ductal dependent CHD and denotes a worse spectrum of prognoses for the newborn. The reversal of flow in the TAoA is observed in hypoplastic left heart syndrome (Fig 16). Reversed flow in the pulmonary trunk is seen in hypoplastic right heart syndrome (Fig 17). Pulmonary atresia with intact ventricular septum (PAIVS)

 

Turbulence is associated with increased velocity of blood flow through a valve. It is seen in the aortic stenosis and in pulmonary stenosis. The aliasing effect is easily detected using color Doppler. Aliasing is observed in absent pulmonary valve syndrome as there is to and fro flow seen through pulmonary valve (Fig 18).

 

 

Fig  18: The MPA is normal in size, the branches right and left pulmonary arteries are dilated and ballooned. On Color Doppler study, there is typical to and fro turbulent flow pattern seen, stenotic forward flow (B) and regurgitant reverse flow (C) seen across rudimentary pulmonary valve. (D) Spectral Doppler demonstrating the same, stenosis gradient across PV is 25 mm of Hg and regurgitation gradient 15 mm of Hg. This is a case of Absent pulmonary valve syndrome (APVS) SP spine; MPA main pulmonary artery; LPA left pulmonary artery; RPA right pulmonary artery; PV pulmonary valve

 

OTHER MISCELLANEOUS CONDITIONS SEEN IN 3VT VIEW

  • Aberrant right subclavian artery.
  • Assessment of thymus
  • Aortic isthmus Doppler velocimetry in IUGR fetuses.

ABERRENT RIGHT SUBCLAVIAN ARTERY:

Left aortic arch with an aberrant right subclavian artery (ARSA) is the most common variant of the aortic arch occurring in 0.5 – 1.4 % of normal population. The aortic arch normally gives rise to three vessels, brachiocephalic, left common carotid artery, left subclavian artery. Whereas in this condition four vessels arise from the aortic arch: right common carotid artery, left common carotid artery, left subclavian artery and the right aberrant subclavian artery. It courses from the left side of upper chest, behind the esophagus and the trachea, to the right arm. This aberrant vessel is also called retroesophageal right subclavian artery or Lusorian artery.

ULTRASOUND FINDINGS

The three vessel trachea view is the most important view to demonstrate ARSA. It is seen as a vessel from the junction of aortic arch and ductus arteriosus with a course behind the trachea towards the right shoulder (Fig 19). The PRF should be reduced to demonstrate it. When ARSA is suspected it should be confirmed by demonstrating arterial flow on pulsed Doppler. The azygos vein, which courses behind the trachea before it enters the SVC can be confused with ARSA and spectral Doppler can help differentiate ARSA from an azygos vein by demonstrating arterial spectrum. 

 

Fig 19:  Three vessels and trachea view with color Doppler mapping. Arrow indicates the expected site of origin of the aberrant right subclavian artery, traversing behind trachea. AOA, aortic arch PA,pulmonary artery; SP, spine; SVC, superior vena cava; TR, trachea

 

 

 

 

3D ultrasound is of help in the multiplanar display. In most cases however, color Doppler is sufficient for the diagnosis.

First observation of ARSA in fetuses with trisomy 21 was reported by Chaoui et al. An association in the range of 14% to 20% of ARSA with trisomy 21 fetuses has been reported. In most cases, additional trisomy 21 markers are noted on ultrasound. ARSA has also been reported in other aneuploidies.

ASSESSMENT OF THYMUS:

On ultrasonography thymus is best imaged on the 3VT view (Figure 20), On this ultrasound plane the thymus appears as a roughly oval structure slightly less echogenic than the adjacent lungs, with some tiny echoic spots in its texture. Often, a clear echoic linear interface is also visible separating the thymus from the lungs, especially if the insonation angle is favorable. It has been shown that, among fetuses with conotruncal anomalies, thymus hypoplasia/aplasia has a sensitivity of 90%, a specificity of 98.5%, a positive predictive value of 81.8% and a negative predictive value of 99.2% for the identification of those with microdeletion 22q11. Another study found a significantly small thymus in fetuses with trisomy 21 and those with trisomy 18 but not in those with trisomy13.

 

 

Normal thymus on USG

However it is not always easy to identify the thymus so it was thought that it may be useful to have an anatomical landmark to aid visualization of thymus. Paladini proposed “The thy-box” which appears to represent such a landmark for enhancing visualization of the fetal thymus in normal and abnormal conditions. On the three-vessel view, if the position of fetus is favorable, i.e. with an anterior or oblique approach, both mammary arteries can be easily visualized, provided that the pulse repetition frequency of the color or power Doppler is adjusted to the relatively low velocity of these vessels (Figure 21). In this way the thymus box, or so called thy-box, is depicted, with the thymus highlighted on both sides by the two internal mammary arteries, on the front by the sternal plate, and on the back by the three vessels and the trachea. The thy-box (i.e. the internal mammary arteries) can be visualized consistently from 16 weeks’ gestation onwards with two-dimensional (2D) color Doppler, until the late third trimester.

In cases of thymus aplasia, the course of the internal mammary arteries is rather difficult to trace, because the ductal and aortic arches are displaced anteriorly, just behind the sternal plate. But in cases of thymus hypoplasia, these vessels can be seen to converge anteriorly, losing their normal parallel course.

AORTIC ISTHMUS DOPPLER VELOCIMETRY IN IUGR FETUSES.

In the fetal circulation, the aortic and pulmonary arches are positioned in parallel. The aortic isthmus (AoI), located between the origin of the left subclavian artery and the aortic end of the ductus arteriosus, is the sole link between the two arches.

            Preliminary studies have indicated that aortic isthmus (AoI) evaluation may serve as a potential monitoring tool for IUGR fetuses and a long-term predictor of neurodevelopmental outcome. These results suggest that abnormal AoI impedance indices occur prior to cardiac decompensation.

Evaluation of aortic isthmus can be done in longitudinal aortic arch (LAA) view and the three vessels and trachea (3VT) view. However, there is greater ease and a shorter time interval for assessment of the AoI in the 3VT view. With advancing gestation, the 3VT is more accessible than the LAA view. The V shape shows the convergence of the AoI and the arterial duct, helping to identify where the range gate should be placed (Fig 22).  The AoI flow velocity waveform has a typical shape and is easily recognizable in most instances. The spectral pattern resembles that of great vessels and forward diastolic flow is always demonstrated. The PSV at AoI after 34 weeks varies from 80-100 cm/sec and PI varies from 2.5 to 3.0. There is always forward flow noted at aortic isthmus. Studies indicate that aortic isthmus Doppler becomes abnormal (absent or reverse forward flow) before abnormal ductus venosus Doppler; hence helping in decision making of time of delivery. 

Assessment of AoI velocity is also helpful in predicting constriction of ductus arteriosus especially monitoring in diabetic mothers in whom incidence of ductal constriction is high and in patients with polyhydramnios who are on indomethacin therapy to reduce amniotic fluid volume.

 

Fig 23 (A):  The 3VT view showing turbulence at aotic isthmus (arrow). There is almost no forward flow seen in pulmonary artery (PA). Forward flow seen through aorta(AO)

 

POINTS TO REMEMBER

  • The three vessels in three vessel view are arranged in an oblique line, pulmonary artery is the most anterior; the SVC posterior and aorta in between.
  • In three vessel trachea view, both aortic and ductal arches are located to the left of spine and trachea, an important landmark as no vessel is seen to the right of trachea or posterior to trachea.
  • These views can be easily obtained by moving the transducer cephalad from four chamber view.
  • Some forms of CHD are dependent on the patency of the arterial duct (ductus dependent circulation) in the immediate neonatal period (such as hypoplastic left heart syndrome, severe coarctation of the aorta, interruption of the aorta, pulmonary atresia with intact ventricular septum, severe pulmonary stenosis) and in some conditions the ductal patency is important for ‘mixing’ (such as transposition of the great arteries and total anomalous pulmonary venous drainage). It is observed that 3VT view provides important clues to the diagnosis of most of these critical anomalies and have an impact on improved outcome.
  • It also provides important clues to the diagnosis of minor anomalies considered as markers of other CHD, like LSVC, assessment of thymus (absent / hypoplasia), ARSA, vascular rings. 

Dr Rajesh Kamble
Dr Alpana Joshi
Dr Previn Mistry