Fetal Cardiac Anatomy & Structural Heart Defects

Normal situs, cardiac position & axis

Systematic evaluation of fetal heart begins with situs, cardiac position, and cardiac axis before assessing detailed anatomy.

Situs & cardiac position

• Situs solitus:
  • Stomach on the left.
  • Liver predominantly on the right.
  • Aorta posterior and left of the spine; inferior vena cava (IVC) anterior and right.
• Cardiac position: Heart predominantly in the left chest; apex pointing left.
• Cardiac axis: Approximately 45° (±20°) to the left of the midline in the four-chamber view.

Abnormal situs or marked deviation of the cardiac axis raises suspicion for structural heart disease, lung lesions, diaphragmatic hernia, or body wall defects and should prompt a more detailed survey.

Standard cardiac screening views

A comprehensive fetal cardiac evaluation incorporates multiple views. Many structural defects are best recognized in specific planes.

Core screening views

• Four-chamber view (4CH):
  • Chamber size symmetry, AV valve offset, septal continuity.
  • Cardiothoracic area ratio and overall cardiac size.
• Left ventricular outflow tract (LVOT):
  • Continuity between interventricular septum and anterior wall of aorta.
• Right ventricular outflow tract (RVOT):
  • Origin of main pulmonary artery from right ventricle, branching pattern.
• Three-vessel view (3VV):
  • Relative size and position of pulmonary artery, aorta, and SVC.
• Three-vessel–trachea view (3VT):
  • Arch sidedness, “V” configuration of ductus arteriosus and aortic arch wrapping the trachea.
• Aortic arch view and ductal arch view for arch morphology and flow.

View-based detection cues

Atrioventricular & septal defects

Atrioventricular septal defect (AVSD / AV canal)

Atrioventricular septal defect (AVSD) refers to a spectrum of defects involving the atrial septum, ventricular septum, and AV valves, due to abnormal development of endocardial cushions.

AVSD may be balanced (symmetric ventricular size) or unbalanced with dominance of one ventricle. The degree of imbalance has implications for postnatal surgical strategy and long-term circulation (biventricular vs single-ventricle pathways).

Ventricular septal defect (VSD)

VSDs are common and range from small, isolated defects to large components of complex malformations.

• Types include perimembranous, muscular, inlet, and outlet VSDs.
• On ultrasound, they appear as an interruption in the bright echogenic line of the septum, best seen in 4CH and outflow tract views.
• Color Doppler can confirm shunting when acoustic windows and size permit.

Isolated small VSDs may be difficult to detect prenatally and may close spontaneously after birth; large defects or VSDs associated with other anomalies warrant more detailed survey and counseling.

Conotruncal lesions

Tetralogy of Fallot (TOF)

Tetralogy of Fallot (TOF) is characterized by ventricular septal defect, overriding aorta, right ventricular outflow obstruction, and right ventricular hypertrophy (the latter may be more evident postnatally).

Assessment of pulmonary valve and branch pulmonary arteries helps estimate severity of outflow obstruction. Arch sidedness and aortic arch anomalies are also relevant in TOF and related conotruncal lesions.

Transposition of the great arteries (TGA)

D-transposition of the great arteries is characterized by ventriculoarterial discordance: the aorta arises from the right ventricle and the pulmonary artery from the left ventricle.

Truncus arteriosus & DORV (brief)

• Truncus arteriosus:
  • Single arterial trunk overriding a large VSD, supplying systemic, pulmonary, and coronary circulations.
  • Absent separate pulmonary valve and main pulmonary artery; pulmonary arteries arise directly from the trunk.
  • Strong association with 22q11.2 deletion.
• Double-outlet right ventricle (DORV):
  • Both great arteries arise predominantly from the right ventricle.
  • VSD relationship to great arteries and associated outflow obstruction patterns vary.

Conotruncal lesions often share overlapping features. Detailed fetal echocardiography, including Doppler and arch views, is essential to refine the diagnosis and assist with perinatal planning.

Left-sided obstructive lesions

Hypoplastic left heart syndrome (HLHS)

HLHS encompasses a spectrum of underdevelopment of left-sided structures, typically including the left ventricle, mitral valve, aortic valve, and ascending aorta.

HLHS is a ductal-dependent lesion postnatally. From a prenatal imaging standpoint, documenting atrial septal patency, arch anatomy, and the presence of extracardiac anomalies is important for counseling and perinatal planning.

Aortic stenosis

• Thickened, doming aortic valve with increased Doppler velocities across the valve.
• Variable degrees of left ventricular dilation or dysfunction, depending on severity and timing.
• In severe, early-onset cases, evolution to HLHS phenotype may occur.

Coarctation of the aorta

Coarctation of the aorta is a narrowing of the aortic arch, most often near the insertion of the ductus arteriosus. Prenatal diagnosis is challenging and relies on indirect findings.

Coarctation may not be definitively diagnosable prenatally. Suspicion is often based on ventricular and great vessel asymmetry and arch appearance; postnatal echocardiography is required for confirmation.

Right-sided obstructive lesions

Pulmonary stenosis & pulmonary atresia

• Pulmonary stenosis:
  • Thickened pulmonary valve; increased systolic velocities across RVOT.
  • Post-stenotic dilatation of the main pulmonary artery in some cases.
• Pulmonary atresia:
  • Absent forward flow across the pulmonary valve.
  • Dependent on ductus arteriosus for pulmonary blood flow.
  • Right ventricle may be hypoplastic (with intact septum) or hypertrophied (with large VSD in complex forms).

Ebstein anomaly & tricuspid dysplasia

• Ebstein anomaly:
  • Apical displacement of the septal leaflet of the tricuspid valve.
  • “Atrialization” of a portion of the right ventricle.
  • Massive right atrial enlargement; variable tricuspid regurgitation.
• Tricuspid dysplasia:
  • Thickened, dysplastic tricuspid valve with regurgitation and right-sided enlargement.

Right-sided obstructive lesions often present with cardiomegaly, abnormal Doppler velocities, and in severe cases hydrops. Documenting ductal flow and assessing for rhythm abnormalities are important components of the evaluation.

Venous return anomalies & systemic venous variants

Total / partial anomalous pulmonary venous return (TAPVR / PAPVR)

• TAPVR:
  • All pulmonary veins drain to a systemic vein or right atrium instead of the left atrium.
  • Prenatal detection is challenging; clues include absent direct pulmonary vein connections to the left atrium and abnormal venous confluence behind the left atrium.
• PAPVR:
  • One or more pulmonary veins drain abnormally, others connect normally.

Systemic venous anomalies

• Persistent left superior vena cava (LSVC):
  • Often seen as an additional vessel on the left of the pulmonary artery in 3VV/3VT.
  • Typically drains to the coronary sinus; may be isolated or associated with other CHD.
• Interrupted IVC with azygos continuation:
  • Absent intrahepatic IVC; azygos vein posterior to aorta appears prominent in transverse views.
  • Strongly associated with heterotaxy and other complex cardiac anomalies.

Careful evaluation of pulmonary and systemic venous connections is especially important in fetuses with heterotaxy, abnormal situs, or complex structural heart disease.

Cardiomegaly, pericardial effusion & hydrops

Cardiomegaly, pericardial effusion, and hydrops can occur with structural heart disease, rhythm disturbances, anemia, infection, twin complications, and other causes.

Cardiomegaly

• Qualitative impression of an enlarged heart occupying more than one-third of thoracic area in 4CH.
• Can be due to volume overload, pressure overload, or primary cardiomyopathy.
• May accompany AV valve regurgitation, high-output states, or cardiac tumors.

Pericardial effusion

• Anechoic fluid surrounding the heart in the pericardial space.
• Small effusions may be transient; larger effusions raise concern for tamponade physiology and hydrops.
• Causes include structural heart disease, infection, anemia, chromosomal anomalies, or idiopathic effusions.

Hydrops & high-output states

• Hydrops fetalis includes at least two of: ascites, skin edema, pleural effusion, pericardial effusion.
• Cardiac causes include severe AV valve regurgitation, arteriovenous malformations, severe anemia, and advanced CHD.

When cardiomegaly, effusion, or hydrops are present, an integrated evaluation of structure, function, rhythm, Doppler, and extracardiac findings is needed to define etiology and guide counseling.

Rhythm abnormalities (diagnostic overview)

Fetal rhythm disorders may be intermittent or sustained and can affect cardiac performance, growth, and the risk of hydrops. Ultrasound assessment includes M-mode and Doppler techniques to compare atrial and ventricular activity.

Irregular rhythms & ectopy

• Premature atrial contractions (PACs):
  • Most common cause of irregular rhythm.
  • Usually benign and transient; may be conducted or blocked.

Tachyarrhythmias

• Supraventricular tachycardia (SVT) with atrial rates >180–200 bpm and 1:1 AV conduction in many cases.
• Atrial flutter with very rapid atrial rates and variable ventricular response.

Bradyarrhythmias

• Sinus bradycardia, often transient (e.g., fetal sleep, maternal medications).
• Complete heart block with AV dissociation: atrial rate normal, ventricular rate low.

This module focuses on recognition of rhythm patterns on ultrasound. Decisions regarding medical therapy, delivery timing, and postnatal management require multidisciplinary input and are not covered here.
SEE Pharmacological Management of Cardiac Arrhythmias in the Fetal and Neonatal Periods Batra AS ,et al

References

Selected classic and frequently cited references for fetal cardiac screening and structural congenital heart disease are listed in abbreviated form. Contemporary guidelines and detailed fetal echocardiography texts should also be consulted.

1. Allan LD, et al. Fetal echocardiography: standard views and screening for congenital heart disease. Foundational description of screening planes.
2. Yagel S, et al. Fetal cardiac scanning: comprehensive approach to standard and extended views.
3. Hornberger LK, et al. Prenatal diagnosis of left-sided obstructive lesions, including HLHS and coarctation.
4. Pasquini L, et al. Detection and classification of conotruncal anomalies on fetal echocardiography.
5. Donofrio MT, et al. Diagnosis and classification of fetal arrhythmias.
6. American Institute of Ultrasound in Medicine (AIUM). Practice parameter for the performance of detailed obstetric ultrasound and fetal echocardiography.
7. International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) guidelines on fetal cardiac screening and fetal echocardiography.