2.4 Normal Fetal Cardiac Spatial Relationships

Fetal Cardiac Position and Axis

Before undertaking a detailed interrogation of intracardiac anatomy, the fetal echocardiographer must firmly establish the heart's position within the fetal chest, its overall size, and its orientation. These fundamental spatial relationships are assessed primarily from a true transverse cross-section of the fetal thorax at the level of the four-chamber view. Errors in determining position and axis are common pitfalls that can lead to misdiagnosis of complex lesions.

Cardiac Position

The position of the heart refers to its location within the thoracic cavity relative to the midline. It is defined by where the bulk of the cardiac mass and the cardiac apex are situated.

• Levocardia (Normal): The heart is located predominantly in the left hemithorax, with the apex pointing clearly to the left. This is the normal, expected finding in a healthy fetus.
• Dextrocardia: The heart is located predominantly in the right hemithorax, with the apex pointing to the right. Dextrocardia can be primary (an intrinsic embryological anomaly associated with situs inversus or heterotaxy) or secondary (where the heart is mechanically pushed or pulled to the right by an extracardiac mass, such as a congenital diaphragmatic hernia, a CPAM, or a hypoplastic right lung).
• Mesocardia: The heart is located centrally in the midline of the thorax, with the apex pointing directly anteriorly. Mesocardia is abnormal and is frequently associated with heterotaxy syndromes or other chromosomal anomalies.

Cardiac Axis (Levocardia)

The cardiac axis is a specific quantitative measurement of the heart's rotation within the chest. It is the angle formed by two intersecting lines: a line drawn directly through the interventricular septum, and an anteroposterior line perfectly bisecting the chest (drawn from the fetal spine to the sternum).

• Normal Axis: The normal fetal cardiac axis is 45 degrees ± 20 degrees to the left of the midline.
• Abnormal Axis: An axis outside this range is a major red flag. An extreme leftward axis (e.g., 75-90 degrees) is highly suspicious for a structural heart defect, most notably Ebstein anomaly, Tetralogy of Fallot, or coarctation of the aorta. A rightward axis is seen in primary dextrocardia or when the heart is displaced by a left-sided diaphragmatic hernia.

Cardiothoracic Ratio

The overall size of the fetal heart is evaluated by comparing its area or circumference to that of the entire thorax, yielding the cardiothoracic ratio. This should be measured in a true transverse four-chamber view during diastole.

• Normal Cardiothoracic Ratio: The area of the fetal heart should occupy approximately one-third (33%) of the total area of the chest (an area ratio of ~ 0.33). The circumference ratio is typically around 0.45 to 0.50 and remains relatively constant throughout the second and third trimesters.
• Cardiomegaly: An enlarged heart (area > 1/3 of the chest) is a critical finding. It can indicate volume overload (e.g., from severe tricuspid regurgitation, an AV fistula, sacrococcygeal teratoma, or fetal anemia), intrinsic myocardial dysfunction (cardiomyopathy), or a severe structural defect like Ebstein anomaly, where the right atrium becomes massively dilated.

Fetal Situs and Segmental Arrangement

Situs refers to the position and arrangement of the asymmetrical visceral organs (the stomach, liver, and spleen) and the atria of the heart along the left-right axis. Determining situs is the mandatory first step in the sequential segmental analysis of the fetal heart.

1. Situs Solitus (Normal)

In situs solitus, the organs and atria are in their normal, standard anatomical positions. This is the expected arrangement in a healthy fetus.

• Stomach: Located on the left side of the abdomen.
• Liver (major lobe): Located on the right side.
• Heart (Apex): Located in the left chest (levocardia).
• Great Vessels: The descending aorta is positioned to the left and slightly anterior to the spine. The inferior vena cava (IVC) is to the right of the spine and slightly anterior to the aorta.
• Atria: The morphological right atrium is on the right, and the morphological left atrium is on the left.

2. Situs Inversus

Situs inversus is a complete, perfect mirror-image reversal of normal anatomy.

• The stomach is on the right, the bulk of the liver is on the left, and the heart is in the right chest (dextrocardia) with the apex pointing right. The aorta and IVC are also reversed.
• Interestingly, if the reversal is complete (situs inversus totalis), the internal cardiac connections are usually concordant, and the incidence of congenital heart defects is relatively low (roughly 3-5%), though this is slightly higher than in the general population.

3. Situs Ambiguus (Heterotaxy Syndromes)

Situs ambiguus, commonly known as heterotaxy, occurs when there is a disorganized or abnormal arrangement of organs that is neither standard solitus nor complete inversus. This implies a catastrophic failure to establish normal left-right asymmetry early in embryogenesis. Heterotaxy is an extremely high-risk finding; it is associated with very complex and severe congenital heart disease in up to 90% of cases.

Heterotaxy is broadly classified into two categories based on atrial isomerism (the pathological condition of having two morphologically similar atria):

Protocol Highlight: Determining situs requires a continuous transverse sweep from the fetal stomach up into the chest. If the stomach and the fetal heart apex are not on the same side, or if the aorta and IVC are abnormally positioned relative to the spine, the sonographer must meticulously evaluate the fetus for heterotaxy and refer the patient for advanced counseling due to the high likelihood of complex cardiac anomalies.