Foetal Echocardiography

Foetal echocardiography is a detailed ultrasound examination of the foetal heart and great vessels. It is done by a trained operator (foetal medicine specialist, paediatric cardiologist or sonographer with cardiac training), usually at 18 to 22 weeks. A first trimester or early scan (12 to 14 weeks, transabdominal or transvaginal) is possible in high risk pregnancies.

A 3 to 7 MHz curvilinear probe is used for transabdominal imaging. The examination includes sequential segmental analysis: situs and cardiac position, the four chamber view, the left and right ventricular outflow tract views, the three vessel and three-vessel-trachea views, the aortic arch and ductal arch views, the bicaval view, and the venous connections.

B-mode, colour Doppler, pulsed wave Doppler, M-mode (rhythm and wall motion), spatiotemporal image correlation (STIC) and tissue Doppler or speckle tracking strain are used as needed.

Measurements include cardiac axis and size (cardiothoracic ratio), valve annulus diameters, ventricular dimensions, shortening fraction, Doppler of the atrioventricular and semilunar valves, ductus venosus, umbilical vein and arterial duct, and the mechanical PR interval (atrial to ventricular conduction time). The examination takes 30 to 60 minutes, with no radiation.

Congenital heart disease is the commonest group of congenital anomalies, with a birth prevalence of 6 to 9 in 1000 live births (about 25% are critical lesions needing intervention in the first year).

Up to 90% of foetuses with congenital heart disease are born to women with no known risk factor, so the routine anomaly scan (four chamber plus outflow tracts) is the main screening tool. Detection rates in routine practice are about 40 to 60% with the four chamber view alone and 70 to 90% in expert centres with outflow tract views.

Prenatal diagnosis reduces neonatal morbidity and mortality for duct dependent lesions (transposition of the great arteries, coarctation, hypoplastic left heart), by allowing delivery in a cardiac centre and early prostaglandin infusion. It also allows in utero therapy in selected conditions (foetal aortic valvuloplasty, atrial septostomy, antiarrhythmic therapy for tachycardia, steroids or immunoglobulin for autoimmune heart block).

Indications. Maternal: pre-existing diabetes (or gestational diabetes with early onset or poor control), phenylketonuria, autoimmune disease with anti-Ro (SSA) or anti-La (SSB) antibodies, congenital heart disease, teratogen exposure (lithium, retinoids, valproate, some anticonvulsants, alcohol), rubella and other infections, and assisted conception.

Family: previous child, parent or sibling with congenital heart disease, or a known genetic syndrome.

Foetal:

• Suspected cardiac anomaly on a routine scan (abnormal four chamber or outflow tract view, abnormal cardiac axis or position).
• Increased nuchal translucency (3.5 mm or more).
• Extracardiac anomaly (especially omphalocele, diaphragmatic hernia, and duodenal atresia).
• Chromosomal abnormality (trisomies 21, 18, 13, Turner syndrome, 22q11 deletion).
• Arrhythmia or hydrops.
• Monochorionic twins (twin-to-twin transfusion, which affects cardiac function).
• Increased risk of anomaly after cell free DNA testing, single umbilical artery with other findings, and abnormal ductus venosus flow.

Not for routine low risk pregnancy beyond the screening anomaly scan.

Contraindications and precautions. None. Output is kept as low as reasonably achievable (thermal index under 1.0, mechanical index under 1.0), especially for Doppler in the first trimester. The woman is placed semi-recumbent with a left tilt late in pregnancy. Counselling before the scan covers the limits of the study and the possibility of a serious diagnosis; counselling after an abnormal finding is done with the cardiology and foetal medicine team.

Preparation and technique. No preparation is needed.

A systematic sequence is followed:

• Abdominal situs: stomach on the left, liver on the right, aorta to the left of the spine, inferior vena cava to the right.
• Cardiac position: left, with the apex pointing left at about 45 degrees.
• Four chamber view: equal chambers, crux intact, two atrioventricular valves with the tricuspid valve slightly apical to the mitral, foramen ovale flap moving into the left atrium, intact interventricular septum, and moderator band in the right ventricle.
• Left ventricular outflow tract: aorta arising from the left ventricle, continuity of septum with the anterior aortic wall.
• Right ventricular outflow tract: pulmonary artery arising from the right ventricle and crossing the aorta at right angles.
• Three vessel and tracheal view: pulmonary artery, aorta and superior vena cava in a line of decreasing size, with the aortic and ductal arches forming a V shape to the left of the trachea.
• Aortic arch ("candy cane") and ductal arch ("hockey stick") views, and venous return.

Colour Doppler shows flow across valves, septa and arches. Heart rhythm is assessed by M-mode through the atrium and ventricle simultaneously or by Doppler of the mitral inflow and aortic outflow together. The study is repeated at 28 to 32 weeks for evolving lesions (coarctation, aortic stenosis progressing to hypoplastic left heart, cardiomyopathy, tumours).

Normal appearances. Foetal heart rate is 110 to 160 beats each minute (120 to 160 in mid-trimester), with a regular rhythm and a 1:1 atrioventricular relationship. The heart occupies about one third of the thoracic area, and the cardiothoracic circumference ratio is 0.45 to 0.5. The cardiac axis is 45 plus or minus 20 degrees to the left.

The two ventricles are of similar size, with a right ventricle slightly larger in late pregnancy; valve annulus diameters grow in proportion to gestation. Atrioventricular valves show a biphasic inflow pattern (E smaller than A, in foetal life), with normal velocities under 80 to 100 cm each second; the aortic and pulmonary peak systolic velocities are under 100 to 120 cm each second.

The ductus arteriosus shows forward systolic and diastolic flow with peak systolic velocity under 140 cm each second and pulsatility index over 1.9. The ductus venosus has forward flow with a positive a-wave. The mechanical PR interval is under 150 ms (mid-trimester, about 120 ms).

There is no pericardial effusion beyond a small physiological rim (under 2 mm), and no tricuspid regurgitation, other than trivial.

Interpretation.

• Abnormal four chamber view: hypoplastic left heart (small left ventricle, small mitral and aortic valves, retrograde flow in the aortic arch).
• Hypoplastic right heart (pulmonary atresia with intact septum, tricuspid atresia), atrioventricular septal defect (absent crux, common atrioventricular valve), large ventricular septal defect.
• Ebstein anomaly (apical displacement of the tricuspid valve, giant right atrium, cardiomegaly), univentricular heart.
• Cardiomyopathy (dilated, hypertrophic, thick or poorly contracting ventricles). Cardiac tumours (rhabdomyoma: echogenic, round, well defined mass in the ventricle wall or septum, often multiple and associated with tuberous sclerosis).
• Outflow tract anomalies: transposition of the great arteries (parallel great arteries, aorta from the right ventricle).
• Tetralogy of Fallot (large ventricular septal defect with overriding aorta and small pulmonary artery), double outlet right ventricle.
• Truncus arteriosus (single outlet over a ventricular septal defect).
• Pulmonary stenosis or atresia, aortic stenosis (dilated left ventricle with endocardial fibroelastosis, an evolving hypoplastic left heart).
• Coarctation of the aorta (disproportion of the ventricles and great vessels with right sided dominance, small isthmus, and arch hypoplasia, a difficult diagnosis).
• Abnormal three vessel view: right sided aortic arch, double aortic arch (vascular ring), aberrant subclavian artery, left superior vena cava, and discrepant vessel sizes; the three-vessel-trachea view detects arch anomalies.
• Tachyarrhythmia: supraventricular tachycardia (heart rate 220 to 300 beats each minute with 1:1 conduction, short ventriculoatrial interval or long, risk of hydrops).
• Atrial flutter (atrial rate 300 to 500 beats each minute with 2:1 block, ventricular rate 200 to 250). Ventricular tachycardia (rare, atrioventricular dissociation).
• Bradycardia: complete heart block (heart rate under 70 to 80 beats each minute, dissociation of atrial and ventricular activity; associated with anti-Ro and anti-La antibodies in a structurally normal heart, or with left atrial isomerism and heterotaxy in structural disease).
• Second degree block, blocked atrial bigeminy (atrial ectopics, rate 60 to 80, usually benign). Sinus bradycardia (long QT syndrome, hypoxia).
• First degree atrioventricular block in anti-Ro or anti-La positive pregnancies: mechanical PR interval over 150 ms (over the 95th centile for gestation), which may progress to complete block.
• Extrasystoles: premature atrial contractions are common and usually benign; follow up for tachycardia (1 to 2% risk) is done.
• Hydrops: ascites, pleural and pericardial effusion, and skin oedema, resulting from cardiac failure, arrhythmia, anaemia or structural disease.
• Functional abnormalities: tricuspid regurgitation (holosystolic jet over 80 cm each second suggests pathology; associated with right sided obstruction, volume overload, twin-to-twin transfusion, and aneuploidy), reduced shortening fraction, and abnormal ductus venosus (reversed a-wave), tissue Doppler for diastolic dysfunction.
• Ductal constriction: acute constriction of the ductus arteriosus (peak systolic velocity over 140 cm each second, pulsatility index under 1.9, right ventricular dilatation and dysfunction, tricuspid regurgitation) after maternal use of non-steroidal anti-inflammatory drugs or polyphenol rich foods in late pregnancy.
• Situs anomalies: heterotaxy (left or right isomerism), dextrocardia, mesocardia, and ectopia cordis; associated with complex disease and heart block.
• Extracardiac associations: increased nuchal translucency, cystic hygroma, diaphragmatic hernia (cardiac displacement), exomphalos, and chromosomal syndromes.
• Twin-to-twin transfusion syndrome: donor with small heart, recipient with cardiomegaly, tricuspid regurgitation, and ventricular hypertrophy with a risk of pulmonary stenosis.
• Maternal diabetes: septal hypertrophy (interventricular septal thickness over 5 mm or over the 95th centile) that may obstruct the left ventricular outflow, and congenital structural defects.

Critical values and findings needing immediate action.

• Duct dependent lesions (transposition, hypoplastic left heart, critical aortic stenosis, severe coarctation, pulmonary atresia): refer to a foetal cardiology centre, plan delivery in or near a paediatric cardiac centre, and prepare a prostaglandin infusion at birth.
• Sustained tachyarrhythmia with hydrops or ventricular dysfunction: urgent transplacental antiarrhythmic therapy (digoxin, flecainide, sotalol) in a specialist centre.
• Complete heart block with heart rate under 55 beats each minute, hydrops, or reduced ventricular function: urgent foetal medicine review (maternal dexamethasone, beta agonist, immunoglobulin, as the specialist judges) and delivery planning with neonatal pacing capability.
• Severe Ebstein anomaly, severe tricuspid regurgitation, or ductal constriction with hydrops: specialist review and early delivery decisions; stop the maternal medicine causing ductal constriction.
• Critical aortic stenosis with evolving hypoplastic left heart, and pulmonary atresia with intact septum: referral for evaluation of in utero intervention.
• Foetal heart failure with hydrops: urgent maternal and foetal assessment (infection, anaemia, arrhythmia, structural disease).
• Cardiac tumours with arrhythmia or outflow obstruction: planned delivery and neonatal cardiology.
• Abnormal heart with other severe anomalies or chromosomal abnormality: counselling about options, with offer of invasive testing and decisions on continuing pregnancy where permitted by law.

Limitations and artefacts.

• Detection depends on operator training, equipment, gestation, maternal obesity and foetal position. A normal study does not exclude all congenital heart disease.
• Some lesions develop in late pregnancy (aortic stenosis, coarctation, cardiomyopathy, cardiac tumours, pulmonary stenosis).
• Other lesions are not detectable in foetal life (atrial septal defect secundum, patent ductus arteriosus, small ventricular septal defects, mild valve stenosis, pulmonary vein anomalies), because the foramen ovale and ductus are open.
• Coarctation of the aorta is the lesion most often missed in prenatal diagnosis (detection 20 to 40%), and many cases appear only after the duct closes.
• Transposition of the great arteries is missed on the four chamber view alone, and needs outflow tract views.
• Anomalies can be falsely diagnosed: ventricular size disproportion in late pregnancy (physiological right dominance), small ventricular septal defects and false dropout of the septum, and left superior vena cava, which may be normal in isolation.
• Foetal arrhythmia can be misread: blocked atrial ectopics appear as bradycardia and can be mistaken for complete heart block; Doppler and M-mode confirm.
• Measurements require correction by gestational age or biometry, and Z-scores should be reported.
• Doppler velocities in foetal life are influenced by the angle, heart rate, and foetal breathing or movement.
• Early (first trimester) scans detect only major defects, and lesions can progress.
• Mid-trimester screening has high false positives for borderline findings (echogenic intracardiac focus, mild pulmonary or aortic size discrepancy), which cause anxiety.
• Prenatal diagnosis of a cardiac lesion does not predict neonatal outcome precisely; the grade of lesion may evolve.
• Postnatal confirmation by echocardiography is mandatory.

Special groups.

• Women with diabetes: strict glycaemic control before pregnancy reduces the risk; offer a detailed scan at 18 to 22 weeks and a follow up scan in the third trimester for septal hypertrophy.
• Women with anti-Ro or anti-La antibodies: weekly or twice weekly mechanical PR interval and heart rate monitoring from 16 to 26 weeks; hydroxychloroquine reduces recurrence of heart block in prior cases.
• Women taking teratogenic drugs: stop or switch drugs before conception if possible; early detailed scans.
• Previous child with congenital heart disease: recurrence risk of 2 to 5% (higher for some lesions, 10 to 15% when two first degree relatives are affected), so serial foetal echocardiography.
• Multiple pregnancy: monochorionic twins have a 9-fold increased risk of congenital heart disease; echocardiography for twin-to-twin transfusion.
• Conception by assisted reproduction: about 1.5 to 2 times increased risk, so offer a detailed scan.
• Women with obesity: reduced detection; transvaginal views in early pregnancy and repeat scans.
• Regions: routine four chamber and outflow screening is rarely available.
• Rheumatic disease, maternal infections (rubella, cytomegalovirus), unrecognised diabetes, and consanguinity raise risk.
• Late presentation means a postnatal diagnosis is the rule, and pulse oximetry screening of newborns helps to find critical lesions.
• Where foetal cardiology is unavailable, a basic cardiac screen (four chamber view, outflow tracts, three vessel view) by a trained sonographer is the best option, with referral for abnormal findings.

Follow up tests.

• Repeat foetal echocardiography at 28 to 32 weeks and serially for evolving lesions.
• Detailed anomaly scan for extracardiac abnormalities, nuchal translucency and cell free DNA screening.
• Amniocentesis or chorionic villus sampling with chromosomal microarray (including 22q11 deletion testing) and exome sequencing.
• Maternal anti-Ro and anti-La antibodies, maternal glucose tolerance test and HbA1c, infection serology (rubella, cytomegalovirus, parvovirus, toxoplasmosis).
• Foetal MR for brain anomalies.
• Postnatal echocardiography within 24 hours (and sooner for critical lesions), neonatal pulse oximetry.
• Genetic counselling, delivery planning with neonatal cardiology and cardiac surgery, and referral to foetal medicine and paediatric cardiology.