Comparison of matching by body volume or gestational age for calculation of observed to expected total lung volume in fetuses with isolated congenital diaphragmatic hernia

Abstract

Objective To determine the bias induced by matching fetuses according to gestational age (GA) or fetal body volume (FBV) when calculating the observed to expected total fetal lung volume (o/e TFLV) in cases of isolated congenital diaphragmatic hernia (CDH). Methods This was a single-center, retrospective study on archived magnetic resonance (MR) images of fetuses with isolated CDH over a 10-year period. We retrieved the TFLV,GA and o/e TFLVGA, and delineated FBV to obtain TFLVFBV in each case. We evaluated the relationship between o/e TFLVFBV and o/e TFLVGA by Bland-Altman analysis. All outliers were manually identified, and their specific clinical features were retrieved. Results Records of a total of 377 MR examinations of 225 fetuses were identified and included in the analysis. The mean (±SD) time spent on FBV measurement was 16.12 ± 4.95 min.On reproducibility analysis of FBV measurement (n = 10), the intraobserver intraclass correlation coefficient (ICC) was 0.998 and the interobserver ICC was 0.999. FBV was highly correlated with GA (R2 =0.899; P<0.0001). There was good agreement between o/e TFLVGA and o/e TFLVFBV, with a mean difference of -1.10% and 95% limits of agreement of -8.58 to 6.39. There were no outliers in fetuses that had an o/e TFLV<25%. Discrepancies induced by different methods were more likely in women with a body mass index ≥25 kg/m2 (+16.5%), fetuses with an estimated fetal weight (EFW)≤10th centile (+21.3%) or an EFW >90th centile (+14.7%). Conclusions Discrepancies in matching by FBV and GA when calculating o/e TFLV are more likely in fetuses with an abnormal EFW or in fetuses carried by overweight women. The clinical relevance of using FBV rather than GA for calculation of the o/e TFLV might be limited, as there was no discrepancy between the two methods in fetuses with small lungs (<25%), which is the group of most interest for lung volume assessment.