We thank the correspondents for their insightful letter highlighting the theoretical disadvantages of (1) image acquisition of the inferior vena cava (IVC) in its long axis and (2) normalization of vessel diameters to body surface area (BSA) rather than to the square root of BSA.
To the first point, we acknowledge the technical pitfalls that can result in erroneous measurement of IVC diameter when displayed in the long axis. However, we chose this method in our investigation because it is the standard endorsed by the American Society of Echocardiography and the European Association of Echocardiography (now European Association of Cardiovascular Imaging) by which IVC measurements are made for the purpose of right atrial pressure estimation in adults and in children. We believe that sufficient care can (and must) be taken to record the IVC through its midline while displaying it longitudinally. In our judgment, the disadvantages of the method can be overcome, and the introduction of nonstandard methodology would have diminished comparability with foundational work in this field. Moreover, inconsistency in measurement methodology would result in inferior data quality.
To the second point, we note that pediatric echocardiographers have long appreciated the nonlinearity of the relation between dimensions of cardiovascular structures and BSA. To account for variation in cardiac dimensions due to subject size, we and others have applied a variety of corrections, mostly involving correlations to exponential powers of BSA, including the square root suggested by Vonder et al . To evaluate the impact of correcting for the square root of BSA versus BSA itself, we generated a model to predict IVC diameter from BSA 0.5 for comparison with the original model used to predict IVC diameter from BSA:
Model 1 : IVC diameter = 5.056 + 6.541 BSA, adjusted r 2 = 0.517 Model 2 : IVC diameter = – 2.077 + 13.92 BSA 0.5 , adjusted r 2 = 0.534
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