5.1 2D Measurements & Biometry

2D Measurements & Biometry in Fetal Echocardiography

Introduction to Fetal Cardiac Biometry

Fetal cardiac biometry involves the quantitative assessment of the heart's size, its internal chambers, and the great vessels. In the realm of fetal echocardiography, precise 2D measurements are fundamental to diagnosing congenital heart defects (CHDs), monitoring fetal growth, and assessing the hemodynamic impact of various fetal conditions. Establishing normal reference ranges for these measurements is critical, as deviations can serve as early indicators of structural anomalies or functional compromise. Measurements are typically obtained from specific, standardized sonographic planes, ensuring reproducibility and accuracy across different examinations and operators.

The Cardiothoracic Ratio (CTR)

One of the most foundational assessments in fetal echocardiography is the evaluation of overall cardiac size relative to the fetal chest, known as the cardiothoracic ratio (CTR). This ratio can be measured using either circumference or area.

• Circumference Ratio: The normal cardiothoracic circumference ratio is typically less than 0.50 (specifically, around 0.45 to 0.50) throughout gestation. It is calculated by dividing the cardiac circumference by the thoracic circumference at the level of the four-chamber view.
• Area Ratio: The cardiothoracic area ratio is normally approximately 0.33 (or one-third) of the total chest area. This is obtained by tracing the epicardial border of the heart and dividing it by the tracing of the inner margin of the ribs.

Cardiomegaly is diagnosed when these ratios are exceeded. An enlarged heart can be indicative of numerous pathological states, including Ebstein anomaly, tricuspid valve dysplasia, severe structural defects, fetal anemia, arteriovenous malformations, or early signs of hydrops fetalis. Conversely, a significantly reduced cardiothoracic ratio might suggest pulmonary hypoplasia or oligohydramnios leading to thoracic compression.

Internal Cardiac Chamber Dimensions

Precise measurement of the internal cardiac chambers is essential for evaluating the symmetry and proportional growth of the fetal heart. In a normal developing fetus, the right and left sides of the heart are roughly equal in size, though the right side may become slightly larger in the third trimester due to its dominance in fetal circulation.

Ventricular Proportions

The right ventricle (RV) and left ventricle (LV) are measured at the level of the atrioventricular valves in the four-chamber view, typically at end-diastole. The standard RV/LV width ratio is approximately 1.0. A ratio significantly greater than 1.0 may indicate right ventricular volume overload (e.g., from tricuspid regurgitation) or a hypoplastic left heart. A ratio less than 1.0 could suggest left ventricular enlargement or right heart hypoplasia.

Wall Thickness

Ventricular wall thickness is measured to assess for myocardial hypertrophy, which can occur in conditions such as maternal diabetes, outflow tract obstructions, or hypertrophic cardiomyopathy. The interventricular septum (IVS) and the free walls of both ventricles should be measured at end-diastole. The thickness normally increases with gestational age but should generally not exceed 3-4 mm in the second trimester and 5-6 mm near term. Measurements exceeding these normative values warrant further investigation for hypertrophic processes.

Atrial Dimensions

The left atrium (LA) and right atrium (RA) are also evaluated in the four-chamber view. The interatrial septum, featuring the foramen ovale, normally bows into the left atrium. The atria should appear symmetrical, with the RA often appearing slightly larger due to the entry of the inferior vena cava and superior vena cava. Disproportionate enlargement of the atria can point to atrioventricular valve regurgitation or restrictive foramen ovale.

Great Vessel Diameters

The evaluation of the great vessels—the main pulmonary artery (PA) and the ascending aorta (Ao)—is crucial for identifying conotruncal anomalies. Measurements are typically taken in the short-axis or three-vessel view (3VV) at the level of the valves or just distal to them during systole (maximal diameter).

Pulmonary Artery to Aorta Ratio (PA/Ao)

In the normal fetal heart, the pulmonary artery is slightly larger than the aorta. The normal PA/Ao ratio is approximately 1.1 to 1.2.

• Decreased Ratio (< 1.0): Suggests a diminutive pulmonary artery, which may be seen in tetralogy of Fallot, pulmonary atresia, or other right heart obstructive lesions.
• Increased Ratio (> 1.5): Indicates a dilated pulmonary artery or a hypoplastic aorta. A dilated PA can occur with absent pulmonary valve syndrome, while a hypoplastic aorta is a hallmark of left heart obstructive lesions like hypoplastic left heart syndrome (HLHS) or aortic stenosis.

Table of Expected 2D Measurements

Aortic and Ductal Arch Measurements

The aortic arch and the ductus arteriosus (ductal arch) are vital pathways in fetal circulation. Their measurements are pivotal for diagnosing conditions like coarctation of the aorta or premature constriction of the ductus arteriosus.

Aortic Arch

The aortic arch is visualized in the sagittal plane, displaying a "candy cane" appearance. Measurements are taken at several key points: the ascending aorta, the transverse arch (between the brachiocephalic and left common carotid arteries, and between the left common carotid and left subclavian arteries), and the aortic isthmus (the segment just before the insertion of the ductus arteriosus).

The isthmus is typically the narrowest portion of the arch. A disproportionately narrow isthmus or a reversed flow pattern in this region is a strong indicator of coarctation of the aorta. Establishing normal z-scores based on gestational age is critical for accurate diagnosis, as the absolute dimensions are very small and highly dependent on fetal size.

Ductal Arch

The ductal arch, visualized as a "hockey stick" in the sagittal plane, connects the main pulmonary artery to the descending aorta. It is typically wider than the aortic arch. Measurement of the ductus arteriosus is important, especially when maternal use of NSAIDs (like indomethacin) is reported, as these medications can cause premature constriction. A narrowed ductal diameter, coupled with increased flow velocities on Doppler, signifies this dangerous complication, which can lead to right heart failure and hydrops. The ductal arch diameter normally grows proportionally with gestational age, maintaining its role as a low-resistance shunt bypassing the fetal lungs.