2D MPE is a novel imaging approach for the assessment of regional and global right ventricular function in patients with repaired ToF. From one apical acoustic window, visualisation and quantification of four different RV walls (lateral, anterior and two inferior walls) can be performed using electronic plane rotation. Moreover, this technique is easily applicable in daily clinical practice when image quality is reasonable, has a short-learning curve and additional acquisition time. Quantification of RV function using 2D MPE was highly feasible and functional values correlated positively with 3D RVEF measurement. Multi-plane functional values in ToF patients are reduced compared to those of healthy individuals in all RV walls. Furthermore, there are evident RV regional differences present, namely higher values in the lateral and inferior walls compared to those of the anterior and inferior CV walls. As a result, multi-wall averaged values were reflective of lower global RV wall function when compared to the standard lateral wall value.
Right ventricular echocardiographic assessment in repaired Tetralogy of Fallot
All multi-plane functional parameters were significantly reduced in ToF patients compared to matched healthy controls. It is reported that RV systolic function as assessed using conventional echocardiographic parameters such as TAPSE measurement is impaired in >75% of cases (10). Furthermore, the use of speckle tracking also reveals the prevalence of abnormalities in RV myocardial deformation (11, 12). Of the three parameters assessed in this study, RV-LS is reported to have the strongest correlation with CMR-derived RVEF when compared to TAPSE or RV S’ measurements (13). 74% of ToF patients included in this study had echocardiographic evidence of either pressure overload, volume overload or a combination of both. It is widely accepted that long term overload of the RV leads to progressive chamber dilatation and dysfunction, and provides one of the main risk factors for adverse cardiovascular events (4). Notably, RV global longitudinal strain has been shown to be significantly lower in chronic pressure overload compared to volume overload (14, 15). However, volume overload from chronic PR leads to pronounced apical broadening and thus eccentric bulging of the RV free wall (16, 17). This change in RV geometry towards a rounded apical curvature can result in increased wall stress and reduced apical deformation (18). For this reason, RV free wall strain progressively decreases from base to apex in ToF (11, 17, 18). Despite using a single segment RV-LS measurement, this notion accounts for a lower average value of the entire RV wall in all multi-plane views when compared to normal RV geometry.
Detailed RV functional assessment using 2D MPE was more feasible than by 3D echo in ToF patients. In addition to superior spatial and temporal resolution (19), each 2D MPE view can be optimised by the sonographer to improve RV wall endocardial border visualisation, whether by altering probe position, angulation or instrumentation. This can also be used to overcome difficulties posed by severely dilated RV’s which 3D pyramidal datasets may fail to fully accommodate. Due to retro-sternal orientation, the RV anterior wall and RV apex are particularly affected by echo artefacts and dropout and these regions can become more difficult to track reliably with 3D.
New insights from multi-plane RV function assessment
The basis of this study was the work carried out in our centre by McGhie et al. (2017) (6), demonstrating feasibility of this four-view multi-plane RV assessment model and proposing normal values in a healthy population. The authors reported higher mean TAPSE, RV-S’ and RV-LS values in the lateral and anterior walls compared to the inferior and inferior CV walls, albeit still within normal reported limits (8, 19, 20). Regional differences were also observed in the present study, with the highest values of all functional parameters observed in the lateral and inferior walls. Functional values of the anterior wall were notably reduced. Since this RV wall is most proximal to the RVOT, the presence of a trans-annular patch from primary surgical reconstruction may explain reduced function in this region. This same multi-plane model has been applied in a cohort of children with repaired ToF (21), where conversely the lowest longitudinal strain values were reported in the two RV inferior walls. The inferior wall also correlated strongest with cardiac magnetic resonance (CMR) derived RVEF. In the present study, all MPE functional values correlated positively with 3D RVEF, with the exception of inferior CV wall RV-LS. LV-RV interdependence by virtue of the interlacing fibers of the interventricular septum (IVS) may account for these regional differences in RV wall function (5). Given that the inferior CV wall lies most adjacent to the IVS, reduced or altered septal contraction could plausibly impact the deformation of this wall more than the lateral wall. Therefore, RV-LS measurement here may not be reflective of global RV function, hence the weak correlation with RVEF. Further insights of interdependence include the notion that a reduction in LV twist alters right ventricular mechanics in ToF patients (22, 23). It has also previously been demonstrated that reduced RV strain, more than dilatation was a predictor of abnormal LV torsion, highlighting the interdependence of these deformational parameters (12).
Limitations
This is a single centre study with many inherently associated limitations including a relatively small study group with smaller sub-groups. Due to the retrospective nature of the study, an insufficient number of patients underwent cardiac magnetic resonance imaging (cMRI) at the same time as echocardiography therefore the two imaging modalities were not able to be compared. It has however previously been demonstrated that 3D echo-derived RVEF correlates strongly with the gold-standard cMRI-derived measurement (24). The general aim of the study was to determine the feasibility of multi-plane RV echocardiographic assessment and quantification in ToF patients and furthermore to report any regional differences in the RV walls. Interpretation of RV septal function should also incorporate indices of RV synchronicity in addition to LV deformation. Whilst this would be of particular interest in ToF patients, it was felt to be beyond the scope and aims of the present study and remains for future research to investigate. Finally, owing to its retro-sternal position, adequate visualisation of the entire anterior wall of the RVOT was only possible in a small group of the study population and was thus omitted. This region of the RV remains difficult to assess quantitatively with echocardiography.
Future perspectives
The next step is to enrol patients in a multi-centre study with other specialist congenital heart disease centres to further assess this novel echocardiographic method in addition to comparison with CMR. Multi-plane RV assessment pre- and post-pulmonary valve intervention would also be of added value. This method could be extended to a broader range of other congenital pathologies involving volume or pressure overload of the right ventricle in addition to primary or secondary pulmonary hypertension. Furthermore, segmental assessment of deformation may provide more insights into the synchronicity of RV wall contraction. Multi-plane imaging of the RV is a novel method and all echocardiograms were performed by an experienced sonographer with expertise in congenital heart disease. Whilst we believe that multi-plane assessment of the right ventricle involves a short learning curve of around 20 echocardiograms, significant experience in echocardiography with attention to detail is essential to good image quality. This method enables the echocardiographer to assess RV function in greater detail and could be utilised in centres without the availability of RV 3D analysis software.