A Practical Approach to Fetal Heart Scanning Lindsey D. Allan

Evaluation o f the fetal heart can be readily incorporated into the obstetric ultrasound examination and need not add more than a few minutes to the examination. Correct analysis of the four-chamber view and both outflow tracts will exchule the majority o f serious congenital heart disease. Where cardiac malformations are identified during pregnancy, parents should be referred to a pediatric cardiologist with expertise and experience o f fetal heart scanning for further counseling. This allows for management o f the pregnancy to be tailored to the parents wishes and the type o f malformation found. Copyright �9 2000 by W.B. Saunders Company

C ardiac malformations are common, affect- ing about 8 of 1,000 pregnancies. However,

only about 3 of 1,000 are serious and readily detectable prenatally. It is important to detect serious forms of congenital heart disease in the fetus as they are associated with a significant morbidity and mortality in affected children. All forms of congenital heart disease nowadays can be treated, al though at varying levels of risk and quality of outcome. A clear understand- ing in early pregnancy of the prognosis for the child allows parents to make informed decisions concerning the management . In cont inuing pregnancies, the outcome for the affected child can be improved by optimizing perinatal man- agement. There are some categories of preg- nancy in which there is an increased risk of congenital hear t disease and these mothers should be referred to a fetal echocardiographer for detailed study. These include mothers with a family history of congenital hear t disease, mater- nal diabetes, exposure to cardiac teratogens in early pregnancy, and the detection of fetal ar- rhythmias or extracardiac abnormalities. This last group include those with extracardiac mal- formations, especially those with nuchal edema, or fetal hydrops. However, 90% of mothers who give birth to infants with congenital heart dis- ease have no high risk features noted in their pregnancy. The only hope these patients have of

From the Department of Pediatric Cardiology, New York Presbyterian Hospital, New Yark, NY. Address reprint requests to Lindsey D. Allan, MD, FRCP, FACC, Babies Hospital 2N, 3959 Broadway, New York, NY 10032; e-mail: la48@columbia.edu Copyright �9 2000 by W.B. Saunders Company O146-0005/00/2405-0002510. 00/0 doi: 10. 1053/sper. 2000.16551

congenital heart disease being detected is for something abnormal being recognised dur ing the obstetric scan. For this reason, the concept of "screening" the fetal heart in a simplified fashion during routine obstetric scanning was introduced. 1

Cardiac assessment during an anatomical ob- stetric survey is r ecommended to include at least a four-chamber view of the heart, 2 and this sin- gle view will detect about 60% of serious malfor- mations or abnormalities in about 2 of 1,000 pregnancies scanned. Despite the fact that al- most all pregnancies are scanned at some time during gestation and that the four-chamber view should be part of every ul trasound assessment, many forms of congenital heart disease recogni- sable in this view cont inue to be overlooked during obstetric eva lua t ion /Many authors have suggested a more detailed study be pe r fo rmed during obstetric evaluation to increase the de- tection of congenital heart disease by including identification of the great artery connections. 4 Accurate evaluation of the great arteries connec- tions will detect up to 90% of serious cardiac malformations.

P r a c t i c a l S c a n n i n g

For the perinatologist the following aspects of cardiac structure or function should be estab- lished: (1) The heart lies on the left side of the fetus, on the same side as the stomach; (2) The four-chamber view is normal; (3) The ventriculo- arterial connections are normal; and (4) The arch and duct are normal.

324 Seminars in Perinatology, Vol 24, No 5 (October), 2000: pp 324-330

A Practical Approach to Fetal Heart Scanning 3 2 5

The Heart Lies on the Left Side o f the Fetus (on the Same Side as the Stomach)

Normally, when the u l t rasound beam is swept cranially f rom a transverse section of the ab- d o m e n to the apex of the heart, it can be seen that both the s tomach and the hear t lie on the same side of the fetus (Fig 1). This will almost always mean that they are bo th on the left side but if they are discrepant, the side of each must be determined. Also, a l though rarely, they can both be normal but bo th lie on the r ight (situs inversus). The technique of Cordes et al 5 for de te rmin ing "sidedness" appears to be reliable. The first step is to or ientate the t ransducer in the long-axis of the fetus with the head to the r ight of the screen. The trans- ducer is then tu rned th rough 90 ~ in a clock- wise direction. If the spine is posterior, the left side of the fetus will be on the r ight o f the screen. Conversely, if the spine is anterior , the left side of the fetus wilt be on the left o f the screen.

The Four-Chamber View is Normal

A four -chamber view of the fetal hear t is ob- ta ined in a horizontal cross-section of the tho- rax jus t above the d iaphragm. A normal four- chambe r view excludes many forms of congeni ta l hear t disease. It must be evaluated

in a systematic fashion, which includes an as- sessment of hear t size, position, structure, and function.

Size. Normally, the heart occupies about one third of the thorax. If there is doubt about the heart size on a visual assessment, the area or the circumference of the heart can be measured and compared to normal values. 7

Position. Normally, the midl ine of the tho- rax passes th rough the left atrium, fo ramen ovale, the r ight atrium, and the anter ior cor- ner of the r ight ventricle, such that most of the hear t lies in the left chest (Fig 2). The inter- ventricular septum forms an angle of about 40 ~ with the midline, s An abnormal angle of the septum can indicate a cardiac malforma- tion or a space-occupying lesion within the chest. 9

Structure. To or ient in the four -chamber view, it is useful to relate the hear t to the spine. Opposi te the spine is the anter ior chest wall or s te rnum and below this is the r ight ventricle. Immedia te ly anter ior and to the left of the spine is the descending aorta and anter ior to that is the left atrium. The right atr ium and left ventricle can then be deduced.

In the four-chamber view, the following as- pects of structure should be seen:

Figure 1. (A) The stomach in the abdomen in the usual position. (B) Sweeping cranially, the four-chamber view in seen in the thorax just above the diaphragm, with the apex on the same side of the fetus as the stomach.

326 Lindsey D. Allan

Figure 2. The interventricular septum forms an an- gle of about 40 ~ to the midline of the thorax. The right ventricle lies below the sternum. The left atrium is anterior to the descending aorta, which is, in turn, anterior to the spine.

1. Two equally sized atria 2. Two equally sized ventricles 3. Two equally open ing atrioventricular valves,

no valvar regurgi tat ion 4. An intact "crux" of the heart, with differential

insertion, or "off-setting" of the atrioventric- ular valves.

5. The pu lmonary veins enter the back of the left a t r ium

6. The fo ramen ovale defect occupies the mid- dle third of the atrial septum

7. The ventricular sep tum is intact

In the last 10 weeks of pregnancy, there can be mild, or close to t e rm even fairly marked, dis- crepancy in the sizes of the ventricles with right hear t dilatation, even when the hear t is normal. However, pathological causes of right hear t di- latation should be excluded, such as coarctat ion of the aorta a~ or totally anomalous pu lmonary venous drainage, before attr ibuting any discrep- ancy to gestational age.

The a t r iovent r icu la r valves should be exam- ined by color flow m a p p i n g to ensure that the orifices are o f equal size and that bo th ventri- cles fill equally in diastole. In addi t ion, there should be no regurg i t a t ion f rom e i ther valve.

However , with some of the mos t r e cen t ultra- sound equ ipmen t , trivial t r icuspid regurgi ta- t ion can be seen in a no rma l hear t . Howeve r a cause for t r icuspid regurg i ta t ion shou ld be exc luded before deno t i ng this "physiological ," as it is no t as c o m m o n in the fetus as it is in pos tna ta l life. n

In the normal heart, the atrial sep tum meets the ventricular septum at the site of insert ion of the 2 atrioventricular valves, fo rming a "cross" at the crux or center of the hear t (Figs 1 and 2). Because the septal leaflet o f the tricuspid valve is inserted slightly lower in the ventricular sep tum than the septal leaflet of the mitral valve, this gives the appearance of "off-setting" or a cross that is not quite straight. This is an impor tan t normal finding that is lost in some cardiac mal- formations.

On examinat ion of the atrial septum, the fo- r amen ovale can be seen to occupy the middle third of the septum, al though the size of the fo ramen appears larger if it is imaged in apical projections. The flap valve is pushed open by a j e t of b lood f rom the ductus venosus via the inferior vena cava and lies in the cavity of the left a t r ium during most of the cardiac cycle. It has, however, a biphasic mot ion drifting toward the atrial sep tum towards the end of systole and closing for about 20% of the cardiac cycle dur- ing atrial contraction. 12 A right to left shunt th rough the fo ramen can be d o c u m e n t e d th roughou t most of the cardiac cycle, a l though a br ief j e t of left to right flow can occur dur ing atrial systole. The flap valve can somet imes be r edundan t and impinge on the left atrial wall in the normal fetus.

Finally, it is i m p o r t a n t to focus on the ven- t r icular s ep tum in o rde r to exc lude a ventric- u lar septal defect. The s ep tum should be imaged f rom the apex to the crux in the four- c h a m b e r view, thus imaging pa r t o f the mus- cular and inlet s ep tum and then the b e a m swept up to the aort ic outflow, which images mos t o f the rest o f the muscu la r s ep tum and the p e r i m e m b r a n o u s and out le t par ts o f the sep tum. It should be imaged in b o t h apical p ro jec t ion and lateral pro jec t ions (Fig 3). I f there is a real vent r icu lar septal defect , co lor flow will be seen to b reach the s ep tum in a lateral view when the sep tum is p e r p e n d i c u l a r to the beam. However , small defects, especial ly in the p e r i m e m b r a n o u s region or mul t ip le de-

A Practical Approach to Fetal Heart Scanning 327

Figure 3. There is equal color flow into both ventri- cles through the atrioventricular valves. Here the beam is almost at right angles to the septmn. No color is seen breaching the septum indicating it is intact.

fects in the muscular septum, can be over- looked particularly in early p regnancy when they may be below the resolut ion of the ultra- sound machine.

T h e Ventriculo-Arterial C o n n e c t i o n s are N o r m a l

The great artery connections can be assessed in a variety of different views, both transverse and longitudinal. The transverse views are usually easier to obtain and involve sweeping cranially from the four chamber view to show the aorta arising in the center of the heart just above the two atrioventricular valves (Fig 4). It is im- portant to note that the posterior leaflet of the aortic valve is in continuity with the anterior leaflet of the mitral valve and that the anterior wall of the aorta is continuous with the ventric- ular septum. Slight transducer angulation so that the beam cuts between the right shoulder and left hip often "opens out" the left ventricu- lar outflow tract optimally. The aorta sweeps out to the right of the thorax before turning left- wards to form the aortic arch. The pulmonary artery arises anterior and cranial to the aorta, is seen in a transverse section just above the aortic origin, and crossing over it. The pulmonary ar- tery, and its connect ion to the arterial duct, lie in an almost straight antero-posterior line, slightly to the left of the midline. In the trans- verse section (Fig 5), this view is known as the "three vessel view" as it shows the pulmonary artery in a long axis projection, with the aorta

Figure 4. (A) A diagramatic representation of the transducer beam is shown as it is swept though tile heart from the four chamber view successively to the aortic (AoV) and pulmonary valves. The most superior slice shows the aorta forming the arch just above the pulmonary artery and duct. 4oh, four chamber; 3V view, three vessel view. (B) Moving cranially from the four-chamber view allows the aortic origin from the left ventricle to be imaged. Angling slightly between the apex and the right shoulder helps to "open out" this view.

3 2 8 Lindsey D. Allan

Figure 5. Just above the aortic origin, the pulmonary connection to the duct is shown in the three vessel view. Normally the pulmonary artery is the largest, most anterior and leftwards of the three vessels, with the 'superior vena cava (SVC) the smallest, most pos- terior and rightwards. The ascending aorta lies be- tween these two vessels.

a n d s u p e r i o r vena cava ly ing to the r i gh t o f it. ~3 T h e p u l m o n a r y a r t e ry is the largest , m o s t an te- r i o r a n d lef twards o f the t h ree vessels with the s u p e r i o r vena cava the smallest , mos t p o s t e r i o r a n d r igh t wards. T h e a o r t a is in be tween b o t h in size a n d pos i t ion .

Figure 7. The transverse arch crossing the midline of the thorax in front of the spine and meet with the duct (D), which lies to the left and below the arch.

In the l o n g i t u d i n a l sect ions, the r i gh t vent r ic - u la r ou t f low t ract can be seen a r c h i n g over the lef t ven t r i c l e or, j u s t to the r i gh t o f this view, a r c h i n g over the a o r t a cu t in short-axis (Fig 6). W h a t e v e r views o f the g r e a t a r te r ies a re seen , the po in t s to be n o t e d inc lude : (1) T h e n o r m a l cross-over o f the g r e a t ar ter ies . (2) T h e n o r m a l size r e l a t i o n s h i p o f the 2 vessels. (3) N o r m a l

Figure 6. The arterial duct (D) in a long axis view of the fetus. The aorta (Ao) lies in the center of this scan plane.

Figure 8. The characteristic "hook" shape of the aor- tic arch with the head and neck vessels arising from the superior aspect of the arch.

A Practical Approach to Fetal Heart Scanning 329

unaliased color flow m app i ng across each arte- rial valve with no regurgitation. Normally, the pu lmonary artery is slightly larger than the aorta in size. I f there is a suggestion of discrepancy in the great artery sizes, they should be measured and compared to normal ranges. 6 Calculation of the A o / P A ratio is often useful, as some degree of outflow tract obstruction or an arch anomaly may be present where this is abnormal . 1~ The arterial valves must be in terrogated by ei ther pulsed or color Doppler , obta ined as close as possible to parallel to the direction of b lood flow. Color flow m app i ng is much quicker and easier. I f there is unaliased, non tu rbu len t flow across the arterial origin this indicates b lood flow at a normal velocity. However, if there is aliasing of the color signal, ei ther because of the dep th of the fetus or the t ransducer frequency, or because of an abnormal ly high velocity, pulsed Doppler must be used to obtain the exact velocity across the valve in question. The velocity of flow should not exceed 1 m / s e c across nor- mal arterial valves.

The Arch and Duct Are Normal

The arch and duct can be imaged in ei ther transverse or longitudinal views but all views are not essential to image in every normal fetus. The transverse aortic arch is seen immediately above the three vessel view as a curved vessel arising in the middle of the thorax and crossing the mid- line in f ront of the spine and trachea. This con- firms a normal left arch. By slight t ransducer angulat ion in the normal fetus, the aortic arch and duct can be imaged simultaneously. This allows the arch to be seen to the right o f the duct. The pu lmonary artery branches laterally into the right pu lmonary artery and duct. The aortic arch and duct should be similar in size and join just in f ront and to the left of the spine (Fig 7). In the long axis views, the duct can be seen just to the left of, and below, the aortic arch. The aortic arch forms a tight "hook" shape and gives off 3 cranial branch~es (Fig 8). The direction of flow in the arch and duct should bo th be entirely antegrade in any project ion and show no sites of turbulence. Measurements of vessel or chamber sizes are useful when they appea r abnormal but are not required routinely.

C o m m e n t s

The accuracy of all u l t rasound is dependen t on opera tor knowledge and exper ience and the im- age quality obtained. The image quality in turn is d e p e n d e n t on the combinat ion of opera tor skill and experience, machine resolution, the thickness of the materna l abdomen , fetal posi- tion and gestational age. By 18 weeks of gesta- tion, the four -chamber view and the two outflow tracts can be imaged transabdominally in t h e vast majority of patients by an exper ienced sonographer with m o d e r n equipment . In obese patients, or in late pregnancy, a limited study may have to be accepted after a range of trans- ducer frequencies and aids such as harmonic imaging have been tried. However, even where a study is op t imum in quality, there are confi- dence limits to fetal hear t scanning. Small ven- tricular septal defects will be overlooked and lesions such as aortic or pu lmonary stenosis, car- diomyopathies and cardiac tumors may evolve in later pregnancy, t5 In addition, the atrial septum and the arterial duct are always patent prenatally and persistent patency after birth, which are both forms of congenital hear t disease, albeit minor , cannot be predic ted f rom the fetal study. Also, the poore r the image quality, the wider the confidence limits. Al though the technique of fetal hear t scanning appears complex at first sight, b roken down into the above componen t s and in the hands of a motivated operator , a thorough assessment can usually be accom- plished in minutes. 16 In a fetus in an ideal posi- tion, sweeping f rom the a b d o m e n to the inlet of the thorax and down again by using color flow mapping, provides comprehensive informat ion providing p roo f of normality. I f the fetal posi- tion or imaging is difficult, as long as the essen- tials listed above are seen, major anomalies will rarely be overlooked.

R e f e r e n c e s

1. Allan LD, Crawford DC, Chita SK, et ah Prenatal screen- ing for congenital heart disease. Br MedJ 292:1717-1719, 1986

2. Ultrasonography in pregnancy: ACOG Technical Bulle- tin 187:1-8, 1993

3. Montana E, Khoury MJ, Cragan JD, et ah Trends and outcomes of prenatal diagnosis of congenital cardiac malformations by fetal echocardiography in a well de-

3 3 0 Lindsey D. Allan

fined birth population. Atlanta Georgia 1990-1994. J Am Coll Cardiol 28:1805-1809, 1996

4. Achiron R, Glaser J, Gelernter I, et al: Extended fetal echocardiographic examination for detecting malforma- tions in low risk pregnancies. B MJ 304:671-674, 1992

5. Cordes TM, O'Leary PW, SewardJB, et al: Distinguishing right from left: A standardized technique for fetal echo- cardiography. J Am Soc Echocardiogr 7:47-53, 1994

6. Allan LD: The normal fetal heart, in Allan LD, Horn- berger LK, Shartand GK (eds): Fetal Cardiology. Green- wich Medical Publishers. 2000 (in press)

7. Paladini D, Chita SK, Allan LD: Prenatal measurement of cardiothoracic ratio in evaluation of congenital heart disease. Arch Dis Child 65:20-23, 1990

8. Comstock CH: Normal fetal heart axis and position. Obstet Gynecol 70:255-259, 1987

9. Allan LD, Lockart S: Intrathoracic cardiac position in the fetus. Ultrasound Obstet Gynecol 3:93-96, 1993

10. Hornberger LK, Sahn DJ, Kleinman CS, et al: Antenatal diagnosis of coarctation of the aorta: a multicenter ex- perience. J Am Coll Cardiol 23:417-423, 1994

11. Gembruch U, Smrcek JM: The prevalence and clinical significance or tricuspid valve regurgitation in normally

grown fetuses and those with intrauterine growth retar- dation. Ultrasound Obstet Gynecol 9:374-382, 1997

12. Schmidt KG, Silverman NH, Rudolph AM: Assessment of flow events at the ductus venosus-inferior vena cavajunc- tion and at the foramen ovale in fetal sheep by use of multimodal ultrasound. Circulation 93:826-833, 1996

13. Yoo SJ, Lee YH, Kim ES, et al: Three-vessel view of the fetal upper mediastinum: An easy means of detecting abnormalities of the venu-icular outflow tracts and great arteries during obstetric screening. Ultrasound Obstet Gynecol 9:173-182, 1997

14. Allan LD, Chita SK, Anderson RH, et al: Coarctation of the aorta in prenatal life: An echocardiographic, ana- tomical, and functional study. Br Heart J 59:356-360, 1988

15. Yagel S, Weissman A, Rotstein Z, et al: Congenital heart defects. Natural course and in utero development. Cir- culation 96:550-555, 1997

16. Stumpflen I, Stumpflen A, Wimmer M, et al: Effect of detailed fetal echocardiography as part of routine pre- natal ultrasonographic screening on detection of con- genital heart disease. Lancet 348:854-857, 1996