We read with great interest the study by Kutty et al ., who reported normal values for superior and inferior vena cava dimensions, systemic venous filling fractions, inferior vena cava “collapsibility index,” and right atrial dimensions to aid in the quantitative assessment of right-sided cardiac structures in children. Systematic issues in the study design compel us to comment on their report.
Systemic venous diameters and right atrial area and volumes were measured using two-dimensional echocardiography. However, the investigators’ choice of views may have substantially limited the accuracy of diametric, cross-sectional areal and volumetric measurements of vessels and atria. In their study, the maximal superior and inferior vena cava diameters were measured in the long axis of the vessel at end-expiration. But the long axis does not guarantee or illustrate that the maximal diameter of a vessel is actually measured accurately: the cross-sectional echo plane could well inadvertently intersect the vessel obliquely or not intersect the midline of the vessel. The short-axis view of a vessel, fundamentally, is a better view for measuring cross-sectional dimensions. Then again, an oblique cross-sectional plane can inadvertently be mistaken for a right angle plane if the vessel is not round ( Figure 1 ), as can easily be the case when central venous pressure is low. Essentially, depending on filling status and venous pressure, a venous vessel is not fully round, resulting in varying maximal diameters that can be measured in one vessel at the same level. In fact, the roundness of the vessel can vary during the breathing cycle. Hence, the investigators indicate in their report a relationship between bodily posture, vessel diameter, and the yet unknown intravascular (venous) pressure. To be honest, a false maximal diameter may well be measured with a long-axis view without the operator’s noticing ( Figure 2 ).
