This study aimed to analyze discrepancies between echo and CMR measurements of the LV in pediatric patients with AR/MR. Our study found that pediatric patients with AR/MR had a greater discrepancy in LV size interpretation by Z-score when compared to non-AR/MR patients. This discrepancy persisted when short axis measurements were incorporated to derive LVEDV by bullet method.

Given the significant range of patient size in pediatric cardiology, Z-scores are often used to serially interpret measurements to determine the development of ventricular enlargement and dysfunction [17]. Provided an adequately representative normative dataset and a patient with a normal body surface area, Z-scores can provide LV size interpretation representative of a pathological process such as AR/MR by identifying those outside the “normal” range of standard deviation [12,13,14, 17]. If a patient is at an extreme end of body size the Z-scores may under or overestimate the measurement [17]. At the same time, the interchanging use of normative datasets depending on imaging modality may lead to discrepancies. Limitations still persist even when applying the same normative equations on both echo and CMR measurements as demonstrated in Fig. 5; even though Z-scores are more concordant, there is still a consistent negative bias such that echo-based measurements would produce lower Z-scores compared to CMR measurements of the same patient.

The noted discrepancies of LV interpretation are likely the result of non-uniform changes in LV geometry that may be less sensitive to echo-based measurements. In AR, the LV has been shown to have a greater curvature of the anterobasal/anterolateral/inferoapical regions and lower curvature in the anteroapical region resulting in a round shape [18]. In MR, LV enlargement also develops into a spherical shape leading to worsening remodeling [19]. These factors likely contribute to the differences in interpretation by LVIDd or LVEDV by bullet which rely on geometrical assumptions that the LV is conical or bullet shaped. This dimensional limitation can also be seen in functional assessments in the LV, as noted in a study by Clark et al. where there was only 64.4% agreement between echo and CMR regarding LVEF [15].

In our study the overall diagnostic performance of echo in identifying moderate LV enlargement was worse for AR/MR pediatrics patients compared to controls. These discrepancies between echo and CMR may have significant ramifications to the long-term management of AR/MR as LV measurements often play a crucial aspect in determining intervention. In adult cohort studies such as Malahfji et al., indexed LVEDV and LVESV by CMR were independently associated with the composite adverse outcome of development of symptoms, LV dysfunction or death and volumetric measurements demonstrated favorable performance over echo-based LV diameters in guiding aortic valve replacement therapy [20]. However, as evidenced by a survey conducted by Boyett Anderson and Hokasen, there is significant practice variation amongst pediatric cardiology providers in terms of follow-up and intervention of children with AR [21]. In the pediatric population, children are likely followed serially from a young age by linear echo measurements and may then be found to have drastic re-interpretation of LV size after an evaluation by CMR leading to a potential significant change in the provider’s approach. The picture is further complicated by the lack of standard cut off thresholds for intervention based on LV size. Future research is needed to determine standards regarding LV size in relation to intervention on AR/MR and such work must include both echo and CMR due to the clear discrepancy between the two methods.

While CMR remains an accurate standard for absolute LV measurements, it does not have the robust normative studies in the same scope as the PHN dataset. Olivieri et al. and Buechel et al. encompass two single center studies with approximately 150 normal controls, compared to the multicenter nature of PHN encompassing 3000 studies [12,13,14]. This study practically demonstrates how differences in the regression model between the datasets contribute to the discrepancies in interpretation. Even if there are attempts to apply the same regression equation between imaging modalities there is consistently a negative bias in echo-based measurements, likely from the aforementioned limitations in geometrical assumptions. Thus, for pediatric cardiologists there needs to be continued attention to the source and derivation of Z-scores, given its emphasis in managing pediatric heart disease.

The specific limitations within our study include the following: Only the LVEDV measurements by bullet method were all measured by a single observer, whereas the LVEDV measurements by CMR were directly taken from clinical reports, potentially risking an effect from interobserver variability. This effect is likely minimal as the three readers responsible for CMR reports during the study period have previously demonstrated excellent agreement for LVEDV (average intraclass correlation coefficient of 0.994 for LVEDV) [13]. Secondly, was the reliance on single center normative data sets for CMR Z-scores. We also did not exclude patients with abnormally low or high body surface area such as small infants which may have potentially skewed the Z-score derivation.

Further research is needed to investigate three dimensional measurements that account for the increasingly spherical shape of the LV in patients with AR/MR in order to improve concordance in interpretation of LV enlargement between echo and CMR. As suggested by Clark et al., the concordance between echo and CMR may improve if three-dimensional echo was measurements are used [15]. Additional work also needs to be done in the development of large, multicenter normative datasets for CMR in order to improve Z-score calculations. Continued comparisons in the performance of different Z-scores, both for echo and CMR is also important.