Doppler in Obstetrics by Nicolaides, Rizzo, Hecker & Ximenes
The 11-14 weeks scan by Nicolaides, Sebire, Snijiders & Ximenes
The 18-23 weeks scan by Pilu, Nicolaides, Ximenes & Jeanty
INTRODUCTION

Pulsed and color Doppler ultrasound improve the diagnostic accuracy of twodimensional gray-scale imaging in the prenatal detection of abnormalities of the heart and great arteries. The two methods are complementary to each other, with color Doppler being used for general assessment of flow in the region of interest and pulsed Doppler for targeted examination of flow in a vessel or across a valve1–10.

In pulsed Doppler ultrasound, the examiner positions a sample volume over the region of interest to obtain flow velocity waveforms as a function of time. This makes it possible to quantify blood flow as peak or time-averaged mean velocities, which allow the calculation of ratios (such as the E/A ratio) or blood volume (such as stroke volume or cardiac output) after measurement of vessel diameter. Color Doppler, which is technically easier to perform, allows a rapid assessment of the hemodynamic situation, but gives only descriptive or semi-quantitative information on blood flow. Color Doppler should be an integral part of the routine examination of a fetal heart because this helps to shorten the scanning time, but also provides improved reliability in diagnosing or excluding abnormalities.

EXAMINATION OF THE NORMAL HEART

Examination of the fetal heart using color Doppler is achieved through similar planes as gray-scale imaging. Several planes, including the abdominal view, four-chamber view, five-chamber view, the short-axis and the three-vessel view need to be assessed to achieve spatial information on different cardiac chambers and vessels as well as their  connections to each other1,2,4. The difference from two-dimensional scanning is that, with color Doppler, the angle of insonation should be as small as possible for optimal visualization of flow.

In the abdominal plane, the position of the aorta, inferior vena cava and the connection of the vein to the right atrium are examined. Pulsed Doppler sampling from the inferior vena cava, the ductus venosus or the hepatic veins can be achieved in longitudinal planes.

The four-chamber view allows the detection of many severe cardiac defects. Using color Doppler in an apical (Figure 1) or basal approach, the diastolic perfusion across the atrioventricular valves can be assessed; there is a characteristic separate perfusion of both inflow tracts during diastole (Figure 1). Using pulsed Doppler, there is a typical biphasic shape of the diastolic flow velocity waveform with an early peak diastolic velocity (E) and a second peak during atrial contraction (A-wave); E is smaller than A, and the E : A ratio increases during pregnancy toward 1, to be inversed after birth. In this plane, regurgitation across the atrioventricular valves, which is more frequent at the tricuspid valve, is easily detected during systole with color Doppler. Flow across the foramen ovale is visualized in a lateral approach of the four-chamber view. Color Doppler allows confirmation of the physiological right-to-left shunt and visualization of the pulmonary veins as they enter the left atrium.

Figure 1: Four-chamber view in real-time (left) and color Doppler. During diastole, flow is visualized entering from both the right and left atria (RA, LA) into the right and left ventricles (RV, LV) and the flows are separated by the interatrial and interventricular septum.

Figure 2: Five-chamber view in real-time (left) using color Doppler (right). The aorta, arising from the left ventricle, is seen and color shows the laminar flow across the aortic valve during systole. Compare with aortic stenosis (Figure 7) and overriding aorta (Figure 12).

Figure 3: Five-chamber view in real-time (left) using color Doppler (right). The pulmonar vein, arising from the right ventricle.

Figure 4: (a,b) Aortic Arch; (c) color doppler angio of the aortic arch
 
 
Figure 5: (a,b) Ductal Arch; (c) color doppler of the ductal arch
Figure 6: (a,b) Venous return (IVC & SVC); (c) color doppler angio of the venous return

he transducer is then tilted to obtain the five-chamber view and then the short-axis view. Using color Doppler, flow during systole is visualized (Figure 2 and Figure 3).

In these planes, the correct ventriculo–arterial connections (compare Figures 2 and 14), the non-aliased flow (compare Figures 2 and Figure 14) and the continuity of the interventricular septum with the aortic root are examined (compare Figures 2 and 12). With pulsed Doppler, a single peak flow velocity waveform for the aortic and pulmonary valves is demonstrated. The peak systolic velocity increases from 50 to 110 cm/s during the second half of pregnancy and is higher across the aortic than the pulmonary valve. Time to peak velocity in the aorta is longer than in the pulmonary trunk.

The three-vessel view enables assessment of the aortic arch and the ductus arteriosus. In the third trimester, an aliased flow is found within the ductus as a sign of the onset of constriction. When the fetal position is optimal, the aortic arch and ductus arteriosus can be seen in a longitudinal plane, allowing visualization of neck vessels.

Figure 7: Three-dimensional power Doppler ultrasound of the crossing of the great vessels in a 28-week fetus. AOA, aortic arch; DA, ductus arteriosus; LPA, left pulmonary artery; TP, pulmonary trunk.
Figure 8: Tricuspid atresia (*) and ventricular septal defect (VSD). Arrows show the direction of flow; due to the atresia of the tricuspid valve, blood entering the right atrium cannot enter directly into the right ventricle and it flows to the left atrium, left ventricle and across the VSD to the hypoplastic right ventricle (right).
EXAMINATION OF THE ABNORMAL HEART
Tricuspid atresia

In this condition, there is absence of the connection between the right atrium and the right ventricle. In the four-chamber view, the right ventricle is hypoplastic or absent and color Doppler demonstrates the absence of flow from the right atrium to the right ventricle (Figure 3). Blood from the right atrium flows across the foramen ovale to the left atrium and from there during diastole to the left ventricle. This unilateral perfusion across the left ventricular inflow tract is typical for this lesion. In the presence of an associated ventricular septal defect, a left-to-right shunt into the small right ventricular cavity is found. The postnatal prognosis depends on the anatomy of the great vessels. The ventriculo–arterial connection can be concordant or discordant, and the pulmonary valve can be patent, stenotic or atretic; color Doppler helps in the reliable differentiation between these conditions.

 
Figure 9: the right ventricle is hypoplastic or absent and color Doppler demonstrates the absence or minimum flow from the right atrium to the right ventricle.
Tricuspid dysplasia and Ebstein anomaly

In tricuspid dysplasia, the valve leaflets are correctly inserted but they are thickened. By contrast, the valve leaflets in Ebstein anomaly are inserted abnormally so that they are more apical in the right ventricle and their ability to close is reduced. In both conditions there is tricuspid regurgitation which is generally associated with dilatation of the right atrium and, in extreme forms, with gross cardiomegaly (Figure 4) 11,12. Color Doppler is used to confirm tricuspid regurgitation and spectral Doppler (Figure 5) is used to measure the pressure gradient and duration of the regurgitation. Since both anomalies are associated with an obstruction of the right ventricular outflow tract (pulmonary stenosis or atresia), it is mandatory to analyze the perfusion in the pulmonary trunk. In severe obstruction, retrograde flow within the ductus arteriosus is found (see Figure 6). This, however, does not prove pulmonary atresia because a patent but stenotic pulmonary valve, due to tricuspid regurgitation, can show the same features as an atresia and thus leads to a false-positive result12.

 
Figure 10: The characteristic finding is that of a massively enlarged right atrium, a small right ventricle, and a small pulmonary artery. Doppler can be used to demonstrate regurgitation in the right atrium
Pulmonary atresia and intact ventricular septum

This diagnosis includes a group of heart defects with an atretic pulmonary valve and an intact ventricular septum. The size and shape of the right ventricle show a wide range, from hypoplastic to normal sized or even dilated. The latter form is identical to tricuspid dysplasia with pulmonary atresia. In both former types, the right ventricle shows no contractility and the tricuspid valve movements are reduced. Color Doppler in the four-chamber view shows absence or reduced tricuspid flow and, during systole, there may be tricuspid valve regurgitation. In the three-vessel view or the short-axis view, there is absence of antegrade perfusion across the pulmonary valve and retrograde flow through the ductus arteriosus (Figure 6). The pulmonary trunk in these conditions is narrower than the ascending aorta, but is not severely hypoplastic because of retrograde perfusion through the ductus arteriosus. In some hearts with pulmonary atresia, communications between the hypoplastic right ventricle and the coronary arteries may be present and are detectable by color Doppler ultrasound13in mid-gestation. Their presence is associated with worse neonatal outcome.

 
Figure 11: Tricuspid valve dysplasia with severe tricuspid insufficiency and cardiomegaly. Retrograde flow from the right ventricle (RV) to the right atrium (RA) is seen in blue and turbulence is coded by green pixels.
 
Figure 12: Severe tricuspid regurgitation. Pulsed wave Doppler (left) is not useful due to the aliasing phenomenon and the maximal velocities that can be assessed are 180 cm/s (arrow). The continuous wave transducer allows assessment of very high velocities; in this case 420 cm/s
 
Figure 13: Hypoplastic right ventricle (arrow) in a fetus with pulmonary atresia and intact ventricular septum (a). Color doppler of the 4 chamber view with asymmetric flow between the left heart and right heart. Pulmonary valve atresia can be diagnosed using color Doppler by visualizing the great vessels – aorta (Ao) and pulmonary trunk (TP) – in the upper thorax and demonstrating the retrograde flow from the descending aorta across the ductus arteriosus toward the pulmonary valve.
Pulmonary stenosis
In the isolated form of this lesion, there is narrowing of the semilunar valves. In severe cases, a hypokinetic and hypertrophied right ventricle can be found but most cases are not detected prenatally. On two-dimensional imaging, the diagnosis is suspected by the presence of poststenotic dilatation of the pulmonary trunk and reduction of pulmonary valve excursion. With color Doppler, the diagnosis is easy and is based on the demonstration of turbulent flow across the pulmonary valve. In severe cases, a retrograde flow can be found through the ductus arteriosus. Doppler flow velocity waveforms using a continuous wave transducer enable the demonstration of high velocities (more than 2 m/s), which are typical of stenosis. These findings, either in color or in pulsed Doppler, are only typical of the isolated form and are not commonly found in conditions associated with a ventricular septal defect, such as tetralogy of Fallot or double outlet right ventricle. Fetal pulmonary stenosis can be associated in the third trimester with tricuspid insufficiency, leading in some cases to right atrial dilatation8.
Aortic stenosis

In general, the narrowing is found at the level of the aortic valve and a simple stenosis is rarely detected in the four-chamber view. However, a critical aortic stenosis is associated with a dilated and hypokinetic left ventricle with an echogenic endocardium, as a sign of endocardial fibroelastosis. Simple aortic stenosis can be detected only by using color Doppler (Figure 7). Antegrade turbulent flow (aliasing) is a characteristic finding in the five-chamber view (Figure 7). Pulsed Doppler analysis shows high velocities (more than 2 m/s) and a characteristic aliasing pattern. Continuous wave Doppler is therefore necessary to confirm the diagnosis (Figure 7). In critical aortic stenosis, there is antegrade turbulent flow across the aortic valve, but peak systolic velocities can vary from more than 2 m/s to values within the normal range, as an expression of left ventricular dysfunction9. Due to the high pressure in the left ventricle, both a mitral regurgitation and a left-to-right shunt at the level of the foramen ovale are found8. In severe left ventricular dysfunction, a retrograde flow is seen within the aortic arch.

Figure 14: Aortic stenosis with turbulent flow (green pixels), as seen in the five-chamber view (compare with normal findings in Figure 2). Continuous wave Doppler allows a quantification of the stenosis.
Hypoplastic left heart syndrome

In this condition, the aortic valve is generally atretic or severely stenotic and the left ventricle diminutive and non-contractile. The mitral valve is either atretic (Figure 8) or stenotic (Figure 9). Color Doppler demonstrates reduced or absent diastolic filling of the left ventricle 8. In the four-chamber view, there is unilateral perfusion of the right ventricle. Often, there is mild tricuspid regurgitation. Careful examination of the intra-atrial communication shows an abnormal left-to-right shunt. In hypoplastic left heart syndrome, there is retrograde perfusion of the neck vessels and coronary arteries which can also be used for the differential diagnosis14,15. Using color Doppler, it is then possible to confirm the diagnosis by demonstrating, in the three-vessel view, the retrograde perfusion in the hypoplastic aortic arch 14,15.

Figura 15: Hypoplastic left heart syndrome - the left ventricle diminutive, with color doppler there is an assimetric color perfusion of the chambers.
Ventricular septal defect

The defect can be either situated in the inlet, in the muscular part or, most commonly, in the perimembranous part of the ventricular septum. The defect can be suspected by two-dimensional ultrasound examination if it is larger than 3 mm. Color Doppler can help to identify small muscular septal defects (Figure 10). Although right and left ventricular pressures are quite equal prenatally, a bidirectional shunt across the defect is present. The best approach to examine a septal defect with color Doppler is the perpendicular insonation of the interventricular septum (Figure 10). In cases of an obstruction of an outflow tract, there is an unidirectional shunt to the contralateral side; in a ventricular septal defect with aortic stenosis, there is a left-to-right shunt8.

 
Figure 16: Hypoplastic left heart syndrome. The left ventricle (LV) is absent (?) due to mitral atresia and aortic atresia. Color Doppler shows the one-sided perfusion from the right atrium into the right ventricle (RA,RV). Compare with the normal four-chamber view in Figure 1 and with another hypoplastic left heart syndrome in Figure 9.
 
Figure 17: Hypoplastic left heart syndrome. In comparison with the fetus in Figure 8, this fetus shows a hypoplastic hypokinetic left ventricle. This is due to the combination of aortic atresia and patent but dysplastic mitral valve. Color Doppler shows similar features as in Figure 8, with one-sided perfusion across the right ventricular inflow tract.
 
Figure 18: Four-chamber view seen from the right side. With real-time scanning, the anatomy appears to be normal. The use of color Doppler demonstrates the presence of a muscular ventricular septal defect during the phase of a shunt (blue) between the right and left ventricles.
Atrioventricular septal defect
In this malformation, there is a combination of defects in the atrial and ventricular septum at the level of the atrioventricular connections. The septal valve leaflets are generally malformed and, in severe cases, they can be absent. In a complete atrioventricular septal defect, color Doppler produces a characteristic H-shape with biventricular diastolic flow across the right and the left inflow tracts and a communication at the level of the atrioventricular valves (Figure 11)6.During systole, the dysplastic valves are not able to close properly, leading to tricuspid and mitral regurgitation (Figure 12). If the regurgitation is severe, cardiac failure and non-immune hydrops develop16.
 
Figure 19: Four-chamber view in a fetus with Down syndrome demonstrating a complete atrioventricular septal defect (AV canal). The defect (*) can be recognized during diastole when the valves are patent but is better assessed using color Doppler, which demonstrates the interatrial and interventricular connection during diastole (H-shape). Compare with the normal findings in Figure 1.
Tetralogy of Fallot

This cardiac defect is defined by the association of a ventricular septal defect, an overriding aorta, an infundibular pulmonary stenosis and a secondary hypertrophy of the right ventricle. Prenatally, the first three signs are present and they can be diagnosed. Using two-dimensional ultrasound, the ventricular septal defect and overriding aorta can be seen in the five-chamber view. With color Doppler, the Y-shape of systolic blood flow from both ventricles into the overriding aorta can be visualized (Figure 13)6. However, an overriding vessel is not exclusive to tetralogy of Fallot, as it can be found in truncus arteriosus communis, in some forms of double outlet right ventricle or in pulmonary atresia with ventricular septal defect. It is, therefore, important to assess the anatomy and hemodynamics of the pulmonary trunk, when an overriding vessel is suspected.

Figure 20: overriding aorta can be seen in the five-chamber view. With color Doppler, the Y-shape of systolic blood flow from both ventricles into the overriding aorta can be visualized (a).
Figure 21: Five-chamber view in a fetus with tetralogy of Fallot (TOF), demonstrating the systolic perfusion from both the right and left ventricle (RV, LV) into the overriding aorta (Y-shape).
Double outlet right ventricle

This is a group of cardiac defects in which the aorta and pulmonary trunk originate from the right ventricle. The position of these vessels to each other is variable, but they usually have a parallel course. In most cases, the diagnosis is achieved using twodimensional ultrasound, but this is often facilitated by applying color Doppler (Figure 14). In hearts with double outlet right ventricle, obstructions of the pulmonary or aortic pathway can be present and are easily diagnosed by color Doppler. In patent atrioventricular valves, the left ventricle appears smaller than the right one and flow across the ventricular septal defect is found to be unidirectional from left to right.

In double outlet right ventricle (DORV) most of the aorta and pulmonary valve arise completely or almost completely from the right ventricle.The relation between the two vessels may vary, ranging from a Fallot-like to a TGA-like situation (the Taussig-Bing anomaly).
DORV is not a single malformation from a pathophysiological point of view. The term refers only to the position of the great vessels that is found in association with ventricular septal defects, tetralogy of Fallot, transposition, univentricular hearts. Pulmonary stenosis is very common in all types of DORV, but left outflow obstructions, from subaortic stenosis to coarctation and interruption of the aortic arch, can also be seen.
Figure 22: Double outlet right ventricle (DORV) with both the aorta (AO) and pulmonary trunk (TP) arising from the right ventricle. Color Doppler demonstrates blood flow from the right ventricle into both vessels and the flow is not turbulent because there is no stenosis.
Complete transposition of the great arteries

In this defect, the aorta arises from the right ventricle and the pulmonary trunk from the left ventricle. The diagnosis is suspected postnatally when the infant becomes cyanotic after closure of the ductus arteriosus and foramen ovale. Prenatally, the four-chamber view appears normal. The malformation is recognized when both arteries are visualized simultaneously and they appear to be parallel to each other (Figure 15); color Doppler is particularly helpful in demonstrating this sign. Color Doppler is also useful in demonstrating pulmonary stenosis and ventricular septal defect, which are occasionally found in transposition of the great arteries.

Figure 23a: (a) four chamber view with parallel outflow tracts from the base of the heart (b) left ventricule with the pulmonarythe left heart view demonstrating that the vessel connected to the left ventricle has a posterior course and bifurcates into the two pulmonary arteries (c) the vessel connected to the right ventricle has a long upward course and gives rise to the brachio-cephalic vessels.
Figure 23 :Transposition of the great arteries demonstrating the abnormal connection of the right ventricle (RV) with the aorta (AO) and the left ventricle (LV) with the pulmonary trunk (TP). Both great arteries show a parallel course.
Other heart defects

Color Doppler is useful in the diagnosis of a wide range of fetal heart defects, including abnormal connections of systemic or pulmonary veins, truncus arteriosus communis, anomalies of the aortic arch, and assessment of intracardiac hemodynamics in cardiomyopathies or in cardiac tumors.

Figure 24 : Rhabdomyoma (which represents excessive growth of cardiac muscle) a single or multiple echogenic masses impinging upon the cardiac cavities.
DIFFERENTIAL DIAGNOSIS OF TRICUSPID REGURGITATION
Regurgitation of the fetal atrioventricular valves is more common on the right side than on the left. Regurgitation of the tricuspid valve shows a wide range of severity, from harmless regurgitation of short duration15 to severe insufficiency lasting throughout the whole of systole (holosystolic), leading to huge dilatation of the right atrium (Figure 4). In some conditions, gross dilatation of the right atrium can be the first sign detected on real-time imaging11 and targeted color Doppler demonstrates severe insufficiency to be the underlying cause. On many occasions, however, tricuspid regurgitation is detected accidently when performing either a routine examination with color Doppler or during a targeted fetal echocardiographic scan in suspected fetal disease. Tricuspid regurgitation of short duration in early systole is observed in 3–5% of all healthy fetuses at mid-gestation17,18 and this is considered to be physiological. However, the detection of tricuspid regurgitation should stimulate a search for a possible underlying pathology (Table 1).

Figure 12: Regurgitation of the tricuspid valve. On the left, trivial regurgitation and, on the right, valve regurgitation in a fetus with an atrioventricular septal defect.
Table 1 Differential diagnosis of fetal tricuspid regurgitation
 
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Doppler in Obstetrics
Copyright © 2002 by Kypros Nicolaides, Giuseppe Rizzo, Kurt Hecker and Renato Ximenes
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