1. Introduction
The knowledge of fetal cardiac function assessment as well as fetal echocardiography itself has advanced dramatically in recent decades [1–5]. With its different modalities, fetal echocardiography has become indispensable for cardiac function assessment [3,6]. With the implementation of fetal echocardiography in clinical routine, two foci arose: the detection of structural congenital cardiac defects (CHD) and subsequently the assessment of fetal cardiac morphometry and function [2,7–10]. Fetal echocardiography is crucial for indirect evaluation of fetal ventricular size and functional parameters since the fetal heart cannot be measured directly in utero [11]. A functional cardiac assessment provides early detection of subclinical cardiac dysfunction and detects intrauterine functional cardiac changes and might help to improve and predict perinatal outcomes, including prediction of subsequent cardiovascular risk based on fetal programming [8,12,13]. To record complex fetal myocardial function with its morphometry, encompassing longitudinal, radial, and circumferential deformation (strain), two-dimensional (2D) speckle-tracking echocardiography (STE) provides unique information of cardiac function using myocardial deformation imaging by tracking the endocardial border [2,10,14–16]. Thus, STE allows both, the evaluation and quantification of global and regional, more precisely, segmental biventricular morphometry and function [2,5,6,16–18]. Its variety of quantifiable parameters for comprehensive analysis enables the simultaneous measurements of ventricular as well as atrial size, shape and contractility within a few minutes [14,16,18–24].
The end-diastolic diameter (ED) or the sphericity index (SI) characterizes parameters for fetal cardiac morphometry (ventricular size and shape), and endocardial global longitudinal strain (EndoGLS) or transverse fractional shortening (FS) for fetal cardiac function (ventricular contractility) [2].
Measurement of the ED (syn. 24-segment transverse widths/segment length) of all 24 segments allows determination of the width and change in size of the ventricles and objective assessment of right-to-left disproportion, which is often associated with cardiovascular malformations [21,23].
The SI (syn. 24-segment SI) is calculated by the ratio between the end-diastolic mid-basal apical length divided by each of 24 transverse lengths (SI = end-diastolic length/end-diastolic transverse length), enables assessment of the ventricular shape and is useful to detect abnormal cardiac function resulting from remodeling of the ventricles. [21,22].
The endoGLS, a commonly used deformation parameter [10,14,17], is calculated as the difference in the length of the endocardium from the base of the lateral wall across the apex to the base of the septal wall at end-diastole and end-systole (endoGLS = [(end-systolic endocardial length – end-diastolic endocardial length)/end-diastolic endocardial length] x 100) [19]. In the small fetal heart, measurement of longitudinal global strain appears to be the most accurate and sensitive parameter to visualize pathologies [25], even though others state that the diagnostic and prognostic value of fetal endoGLS has remained uncertain [10]. Because global strain parameters, such as the endoGLS, are less sensitive to local noise and their measurement is more practical, it is considered to be more robust compared to the segmental ones [25,26]. There is still limited data available on the normal ranges of endoGLS during pregnancy [2]. A significant correlation between the endoGLS and the gestational age (GA) is controversially debated in the current literature. Both, an increase and a decrease in endoGLS as well as constant values have been reported previously with advancing GA [2,3,10,14,19,25,27–29]. These conflicting data regarding normal reference ranges and changes of endoGLS complicate the interpretation of current results [14]. Both, abnormal increase and decrease of endoGLS values may indicate cardiac dysfunction, depending on the pathophysiology [12].
The FS (syn. 24-segment transverse FS) is calculated as the difference in the transverse length of each of the 24 segments at end-diastole and end-systole (FS = [(end-diastolic length – end-systolic length)/end-diastolic length] x 100) provides a comprehensive method for assessing ventricular contractility [19–21].
2D-STE has become widely used to assess fetal cardiac function to detect cardiac dysfunction in numerous pathological intrauterine conditions by estimating early fetal cardiac adaptive changes in several pregnancy related complications, including CHD like coarctation of the aorta, aortic or pulmonal stenosis or hypoplastic left ventricle [19–23,30–34], fetal growth restriction (FGR) [17,35], gestational diabetes [36–39], twin-to-twin transfusion syndrome (TTTS) [40,41], pre-eclampsia [42], intrahepatic cholestasis of pregnancy (ICP) [43], fetal non-compaction cardiomyopathy (NCCM) [44,45] or autoimmune diseases [13].
Ultrasound examination is highly examiner-dependent – especially in complex cardiac anatomy – and acquisition and quantification of fetal cardiac functional parameters with accurate prenatal diagnosis by fetal echocardiography depends on the skill and experience of the operator [25,46,47]. The applicability of speckle-tracking is limited when imaging conditions are suboptimal [40]. Therefore, extensive training and education of future experts is of enormous significance [48–51]. Applications based on artificial intelligence (AI), on whose algorithms prenatal diagnostics will rely on increasingly, are transforming the way clinicians use ultrasound [50,52–65].
Recently, we demonstrated the benefit of an automated tool for less experienced operators to assess another cardiac functional parameter, the right ventricular modified myocardial performance index (RV-Mod-MPI) in normal pregnancies [47].
The purpose of this study was to demonstrate the feasibility of whether less experienced operators can handle and might benefit from an automated tool of 2D-STE for offline analysis of diverse parameters for cardiac function in the clinical routine.