Transthoracic echocardiography (TTE) is an essential tool in pediatric cardiology due to its wide availability and safety. The last guidelines on this topic were published in 2006, and advancements in the technique have allowed innovations such as three-dimensional transthoracic echocardiography (3D TTE) and strain imaging (deformation techniques) to redefine the imaging paradigm in pediatric cardiology departments. These new guidelines provide details on both conventional techniques and more recent advancements. 

It is worth noting that, unlike adult patients, children present unique and critical considerations in this type of echocardiography (e.g., small children with high heart rates or older patients who have often undergone interventions and may present with poor acoustic windows). 

For beginners to TTE, these guides highlight echocardiographic planes and the overall essence from preparation to performance of TTE. However, basic concepts of TTE such as the difference between pulsed wave (PD) and continuous wave (CW) Doppler, for example, are taken for granted (PD: allows the measurement of velocity at a specific point and only measures up to a velocity limit, e.g. the aortic valve. CW: measures velocity along a column, e.g. left ventricular outflow tract or LVOT, aortic valve and ascending aorta; it will give us the total velocity without limit, but without specifying where there is a greater degree of flow acceleration and, therefore, stenosis). For experienced echocardiographers, they highlight misconceptions that are rooted in the world of TTE, which we assume to be true in clinical practice and which we will detail in this commentary. 

Indications 

TTE should be performed in every child with suspected heart disease, congenital (CHD) or acquired heart conditions, genetic or systemic diseases with potential cardiac involvement, and those with a family history of cardiovascular disease. Examples of initial indications for TTE include abnormal findings on fetal echocardiography, signs suggestive of heart disease, or children undergoing treatments with potential cardiac toxicity, such as oncological therapies. 

On the other hand, follow-up TTE is indicated for all congenital and acquired heart diseases, regardless of treatment status, as well as for pulmonary hypertension and monitoring of cardiotoxic treatments. 

Technical Aspects 

All pediatric echocardiography laboratories must be equipped with transducers of various frequencies. Low frequencies are recommended for older children to minimize ultrasound attenuation caused by body mass, while high frequencies are essential for younger children to provide greater precision. Equipment must support various functionalities of TTE. 

The standards for image optimization should include patient preparation, image acquisition, and storage to allow for the review of previous studies, offline quantification, and anonymized analyses. Every pediatric TTE must include a comprehensive evaluation of anatomy, ventricular function, and cardiovascular physiology. This is achieved through a combination of two-dimensional imaging, color Doppler, pulsed (PW) and continuous Doppler (CW), tissue Doppler imaging (TDI), 3D TTE, and strain evaluation. 

Linear measurements must always be performed along the ultrasound beam’s axis, as axial resolution is superior to lateral resolution. The distance between the transducer and the structure being measured should be minimized. Doppler studies (PW and CW) must always be performed in planes parallel to blood flow. Prior to using PW or CW Doppler, color Doppler interrogation should be performed with an appropriately sized color box (neither too large nor too small), and velocity ranges adjusted to maintain a frame rate equal to or above 20 Hz. 

Simultaneous use of two-dimensional imaging and color Doppler (dual mode) significantly reduces the frame rate and should only be employed when assessing low-velocity structures such as pulmonary veins and coronary arteries. 

Standard Echocardiographic Views and Anatomical Orientation 

The updated guidelines provide a detailed description of standard echocardiographic views, including anatomical structures and techniques for obtaining them. In pediatric cardiology, images must be anatomically oriented, ensuring that anterior or superior structures are displayed at the top of the monitor and right-sided structures are shown on the left. In apical and subcostal views, the apex of the image should be displayed at the bottom for consistency. 

A systematic cardiac segmental analysis should be performed in every echocardiogram, beginning with subcostal or parasternal long-axis views. Particular emphasis is placed on subcostal, suprasternal, and right parasternal views, which hold greater importance in pediatric imaging compared to adult echocardiography. 

Segmental Anatomical and Functional Analysis Protocol 

1. Pulmonary and Systemic Veins: 

TTE is the primary imaging technique for evaluating venous connections, abnormal drainage, size, and the presence of obstructions (e.g., turbulence on color Doppler, loss of normal phasic variation, or increased velocities). The assessment of systemic and pulmonary venous drainage must be included in all initial pediatric TTEs using two-dimensional imaging, color Doppler, and spectral Doppler. 

The superior vena cava (SVC), inferior vena cava (IVC), hepatic veins, and the right atrium-to-coronary sinus connection should be evaluated using subcostal, parasternal, and apical posteriorly inclined views. In adolescents and adults, right parasternal views enable clear imaging of the SVC and IVC. 

In children, no studies have established a reliable correlation between IVC size and right atrial pressure. Contrast echocardiography with agitated saline is useful in diagnosing systemic venous anomalies and obstructions. Suspected IVC interruption should be evaluated in subcostal views. Suprasternal views allow visualization of the innominate vein’s drainage into the SVC and identification of retroaortic innominate veins, left SVCs, or dual SVC systems. 

Pulmonary veins must be assessed in suprasternal short-axis views, where the typical “crab” image shows the right inferior pulmonary vein connecting to the left atrium. However, this view does not exclude anomalous drainage of the right pulmonary vein into the SVC; subcostal and right parasternal views are preferable for identifying such drainage. Apical posteriorly inclined views can show pulmonary drainage but do not differentiate between upper and lower veins or between left and right. 

Total anomalous pulmonary venous connection (TAPVC) should be suspected in subcostal views if a significant right-to-left shunt is visible through the foramen ovale, combined with a small left atrium. Partial anomalous pulmonary venous connection (PAPVC) may not associate with right ventricular dilation, requiring exclusion of connections such as the right superior pulmonary vein draining into the SVC, the right inferior pulmonary vein draining into the IVC, the left superior pulmonary vein draining into the innominate vein, or the left inferior pulmonary vein draining into the coronary sinus. 

2. Atria and Atrial Septum (AS): 

TTE must evaluate atrial size, morphology, and venous and atrioventricular (AV) valve connections. The atrial septum should be examined in all initial pediatric TTEs. 

Atrial size is analyzed using apical four-chamber or subcostal long-axis views, with subcostal short-axis and parasternal views as supplementary options. The 2010 guidelines suggest measuring atrial dimensions (major and minor axes) in apical four-chamber views. While M-mode measurements of left atrial (LA) diameter relative to the aortic root were previously used to assess ductus arteriosus impact, they correlate poorly with LA volume and are not included in neonatal TTE guidelines. 

Standard practice includes calculating LA volume using LA area and length measurements obtained in apical views during end-systole before mitral valve opening. These measurements are crucial for assessing diastolic function and are especially relevant in cases of mitral valve dysfunction, left-sided volume overload, hypertrophy, or diastolic dysfunction. Normal LA volume values are established for children, and LA strain can be used to analyze left ventricular (LV) diastolic function. 3D TTE has been utilized for measuring LA volume and strain in healthy pediatric populations. 

Suspected secundum or sinus venosus atrial septal defects (ASD) are indicated by right atrial (RA) or right ventricular (RV) dilation. Subcostal and right parasternal views provide optimal ASD evaluation due to the ultrasound beam’s perpendicular orientation. Color Doppler confirms defects, and spectral Doppler assesses shunt direction. Defect size should be measured in orthogonal planes, including superior, inferior, anterior, and posterior edges. 3D TTE is particularly helpful for these measurements. 

3. Atrioventricular Valves (AV): 

AV valve assessment in TTE should include annular size, leaflet anatomy, papillary muscles, and chordae tendineae. Standard measurements involve the annular diameter during diastole, from inner edge to inner edge, at the highest leaflet insertion points in apical and parasternal views. Routine area measurements are not performed in children due to the scarcity of normal reference values and limited validation. 

Anatomical and functional AV valve analyses require multiple planes and perspectives. For stenosis, color Doppler evaluates the time integral to calculate mean gradients for quantitative analysis. High heart rates in children may confound mean gradient assessment, as do factors like nonparallel Doppler angles, AV valve regurgitation, or congenital heart defects that increase AV valve flow (e.g., atrial or ventricular septal defects). For mitral stenosis, TTE must measure pulmonary pressures. 

In cases of valvular regurgitation, pediatric-specific considerations such as multiple jets and undefined severity grades necessitate qualitative evaluation via color Doppler or indirect severity indicators. Preferred parameters include atrial and ventricular dilation or the presence of reversed flow in pulmonary or systemic veins. Effective regurgitant orifice and regurgitation fraction are minimally validated in children, though vena contracta measurements, typically adopted from adult values, are gaining traction in pediatric labs. 

4. Right Ventricle (RV):

The RV is challenging to evaluate through TTE due to its trabeculated structure, complex geometry, and retrosternal position. Comprehensive assessment requires imaging in subcostal, apical, parasternal, and modified planes such as the RV-focused three-chamber apical view (obtained medially on the chest) and the right anterior oblique subcostal view (achieved by counterclockwise rotation of the transducer from the subcostal long-axis view, visualizing the inflow and outflow tracts simultaneously). 

Morphological abnormalities in congenital heart disease (CHD) affect RV dimensions and function, with significant interindividual variability. Therefore, a complete RV analysis must include qualitative assessment and multiple parameters to evaluate abnormal hemodynamic conditions influencing RV measurements (e.g., estimating pulmonary artery systolic pressure [PASP] via tricuspid regurgitation velocities, assuming right atrial pressure inaccurately). 

RV size assessment through TTE shows weak correlation with linear two-dimensional parameters, moderate correlation with two-dimensional area measurements, and strong correlation with 3D TTE-derived volumes compared to MRI. Linear dimensions (basal, mid, and longitudinal diameters) are obtained in apical views during end-diastole, while area measurements are taken in apical views during end-systole (frame prior to tricuspid valve opening) and end-diastole. 3D TTE is increasingly used for volume calculations, providing valuable insights despite limitations in children, such as volume underestimation and limited data on normative values and validity in geometrically altered RVs. 

Functional assessment parameters like tricuspid annular plane systolic excursion (TAPSE) and fractional area change (FAC) offer insight into systolic performance. TAPSE evaluates longitudinal displacement of the tricuspid valve annulus in apical four-chamber views as a measure of RV systolic function. Its precision improves with color Doppler, particularly in pressure overload scenarios, though it lacks representation of apical function and circumferential/radial shortening. FAC considers systolic and diastolic areas, providing information on radial and longitudinal function. 

Additional functional measurements, including RV volumes, ejection fraction (EF), and strain analysis via 3D TTE, correlate better with MRI data and are particularly useful in conditions like pulmonary hypertension or tetralogy of Fallot. RV strain is measured in apical four-chamber views, showing high reproducibility when the same platform is used for longitudinal analysis. 

5. Left Ventricle (LV) and Interventricular Septum (IVS):

The LV and IVS are evaluated using apical, parasternal, and subcostal views. Serial measurements of LV size, global/regional systolic function, and diastolic function are critical in pediatric TTE. Linear measurements of the LV are taken during end-diastole and end-systole. 

While adult guidelines recommend measuring LV dimensions in parasternal long-axis views, pediatric guidelines suggest parasternal short-axis views for improved accuracy. LV diameter is used as a size proxy, though linear one-dimensional measurements are representative only when LV geometry is circular. 

Wall thickness measurements during diastole/systole should avoid including RV trabeculations, particularly in asymmetric hypertrophy cases. Dimensional and functional assessments are primarily performed in two-dimensional mode due to the reduced accuracy and reproducibility of M-mode. 

Volume measurements via the biplane Simpson method in two- and four-chamber apical views are standard practice, including for children. 3D TTE provides superior correlation and reproducibility with MRI compared to M-mode and two-dimensional approaches, enhancing systolic/diastolic functional assessments. Global LV evaluation combines M-mode, two-dimensional, 3D TTE, and strain analysis. In cases with altered LV geometry or segmental wall motion abnormalities, strain and 3D TTE volumes are preferred. 

The IVS must be evaluated in all initial pediatric TTEs. It should present a circular contour in subcostal and parasternal short-axis views. Doppler and color Doppler are employed to identify septal defects, determine shunt direction, and assess the defect’s size, location, and surrounding structures. PW and CW Doppler quantify shunt flow and defect restriction. 

6. Right Ventricular Outflow Tract (RVOT) and Pulmonary Valve (PV):

The RVOT is a complex muscular structure best visualized in subcostal, anteriorly inclined apical, and parasternal long- and short-axis views. These align with the Doppler beam to optimize flow evaluation. In tetralogy of Fallot, subcostal and modified right anterior oblique views are particularly useful for visualizing the RVOT, tricuspid valve, and conal septum. 

RVOT dimensions are measured from inner edge to inner edge during end-diastole. Proximal RVOT measurements (from free anterior wall to aortic annulus) are obtained in parasternal short-axis views, while distal measurements (immediately before the PV) are taken in parasternal long-axis views. 

The PV is evaluated in the same views as the RVOT, with morphology best assessed in parasternal views. The valvular annulus is measured from inner edge to inner edge during maximum systolic opening in parasternal long-axis views (avoiding short-axis views, as this is a common but incorrect practice). 

Doppler techniques are essential for assessing PV obstruction and regurgitation. PW Doppler is applied above and below the valve to evaluate dynamic stenosis or multilayered obstructions. Peak velocities for stenosis or regurgitation are measured using color Doppler, with detailed annotation of the acquisition plane in reports to mitigate errors related to RVOT and PV Doppler measurements. Morphology and size should guide interpretation in cases where: 

• • Large shunts equalize pulmonary and systemic pressures. 

• • Pulmonary hypertension is physiologically present in neonates. 

• • Severe tricuspid regurgitation or low RV output alters gradient estimations. 

7. Left Ventricular Outflow Tract (LVOT) and Aortic Valve (AV):

The LVOT represents the area below the aortic valve, bounded by the interventricular septum (IVS) and the anterior mitral valve leaflet. Unlike the RVOT, the LVOT lacks a muscular subaortic cone, with fibrous continuity between the AV and mitral valve. 

The LVOT is best measured in parasternal long-axis views during mid-systole, 3–10 mm below the aortic annulus. The AV is assessed in systole, with measurements taken from inner edge to inner edge at the point of maximum opening in parasternal long-axis views. Morphological evaluation is performed in parasternal short-axis views to visualize all three aortic leaflets simultaneously. 3D TTE can be employed for detailed morphological analysis and frontal plane imaging of the valve. 

Doppler interrogation of the LVOT and AV should utilize subcostal long-axis, apical three-chamber, right parasternal, and suprasternal views. Color Doppler is used to detect areas of aliasing, followed by PW Doppler to exclude stenosis at the LVOT, AV, or supravalvular levels. Color Doppler also measures peak gradients across the LVOT. 

In adults, aortic regurgitation is quantified using pressure half-time and ascending flow slope evaluation; however, these techniques lack validation in children. Pediatric assessment relies on qualitative jet evaluation via color Doppler and indirect parameters such as diastolic flow reversal in the aorta and LV dilation. Doppler evaluation for stenosis includes peak and mean gradients obtained from multiple planes. Since gradients in apical views are often lower than those in right parasternal views, the report should specify the acquisition plane. 

8. Arteries and Branches:

TTE must evaluate the size, morphology, and flow of both pulmonary arteries. Proximal pulmonary artery (PA) and branch measurements are performed in parasternal short-axis views during mid-systole at maximum expansion, from inner edge to inner edge. Mild tilts in the plane may reveal branch origins, with asymmetry potentially indicating pulmonary sling. 

Right PA evaluation is optimal in parasternal short-axis views, while parasternal long-axis and high left parasternal views are superior for the left PA. Color Doppler is applied to detect stenosis or patent ductus arteriosus (PDA) with diastolic flow in the PA. Doppler interrogation should be conducted in parasternal short-axis views or modified anterior apical views when assessing the proximal PA. 

Coronary arteries, due to their small size and superficial location, require high-frequency transducers with reduced ultrasound sector width and optimized frame rates. The size, origin, and proximal course must be analyzed. Parasternal short-axis views assess the proximal left coronary trunk, circumflex artery (Cx), left anterior descending artery (LAD), and the proximal right coronary artery (RCA). Parasternal long-axis views reveal the anterior RCA origin and the Cx in the left atrioventricular (AV) groove. Clockwise rotation and posterior tilting of this view display the posterior descending artery in the interventricular groove. Apical posteriorly tilted views show the distal RCA over the right AV groove, while anterior tilting reveals the left coronary artery and its bifurcation into the LAD and Cx. 

Artery size should be measured at maximum expansion, recording z-scores and serial measurements for longitudinal evaluation. Color Doppler is critical for diagnosing anomalous origins, with two-dimensional and Doppler modes required to confirm anomalies. Reversed flow suggests origin from the PA or coronary ostial atresia. 

Serial aortic measurements are essential for a wide range of conditions. Parasternal long-axis views are used for proximal aortic segments, including the annulus, sinuses of Valsalva, sinotubular junction, and ascending aorta at the level of the right pulmonary artery. In children, aortic dimensions are measured in mid-systole at maximum expansion from inner edge to inner edge, unlike adult guidelines that recommend diastolic measurements from anterior edge to anterior edge. 

For the aortic arch, suprasternal long-axis views allow analysis of the proximal transverse arch, distal transverse arch, descending thoracic aorta, and the aortic isthmus. Measurements can be obtained at each of these levels. Suprasternal short-axis views using two-dimensional and color Doppler assess arch laterality and brachiocephalic trunk bifurcation. Increased distance between the second and third branches, narrowing of the isthmus, and posterior notching with turbulent flow raise suspicion of aortic coarctation. 

Doppler interrogation of the arch is mandatory to assess shunt direction and degree of restriction in cases of PDA. Ascending aortic Doppler evaluation is conducted from suprasternal long-axis, apical three-chamber, right parasternal, or subcostal views, particularly when subvalvular or valvular stenosis is suspected. In small children, exclusive use of three-chamber views may underestimate gradients. Sequential PW and CW Doppler should be used to detect obstructions, while abdominal aorta flow is assessed in subcostal short-axis views. 

COMMENTARY: 

TTE reaffirms that, in many aspects, children are not just small adults. Beyond anatomical differences, cardiovascular physiology also introduces unique challenges and complexities. One of the main limitations in pediatric echocardiography lies in quantifying the severity of valvular disease. This limitation arises due to the following factors: 

• Peak instantaneous gradients differ from peak-to-peak gradients. 

• Physiological states such as fever can lead to gradient variability. 

• The pressure recovery phenomenon causes discrepancies between gradients measured by TTE and catheterization, with this effect being more pronounced in younger children compared to adults and in mild stenosis compared to severe cases. 

• Left ventricular dysfunction may be associated with lower gradients, even in severe stenosis. 

• Gradients measured in neonates may be lower than those observed in older children and adults. 

As such, assessing severity must incorporate valve morphology and z-scores. While valve area estimation through planimetry or continuity equation can be useful, particularly with 3D TTE, significant variation due to minimal plane changes, limited temporal resolution, and random measurement error have prevented its standardization for quantifying aortic stenosis severity in children. 

As a general rule, the report generated from any echocardiographic study must use terminology that is universally understood within the center and include a summary, z-scores, and normality values. All findings should be described in a structured format with segmental analysis. Doppler gradient acquisition planes and z-scores used for measurement must be explicitly noted to ensure accurate temporal comparison. It is important to remember that using different z-scores across studies may obscure a patient’s progression. Reports should be archived for a reasonable period following the study, and findings must be communicated promptly to referring physicians. 

REFERENCE: 

Lopez L, Saurers DL, Barker PCA, Cohen MS, Colan SD, Dwyer J, Forsha D, et al. Guidelines for Performing a Comprehensive Pediatric Transthoracic Echocardiogram: Recommendations From the American Society of Echocardiography. J Am Soc Echocardiogr. 2024 Feb;37(2):119-170. doi: 10.1016/j.echo.2023.11.015.