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It has recently been reported in critically ill patients that a linear relationship exists between cardiac index (CI) measured with thermodilution and mean aortic blood flow velocity (MAFV). This hypothesis can be validated mathematically only if the aortic area (AA index) indexed to body surface area (BSA) remains constant and if the relationship between aortic diameter (ØAo) and BSA is nonlinear. However, several other equations have described the relationship between BSA and, respectively, ØAo and aortic area (AA) in children. The aim of this study was to determine if the relationships calculated between BSA and aortic size in children (without left ventricular outflow tract abnormality) could validate the hypothesis that MAFV and CI are well linked linearly, leading to its use to determine CI.Two hundred and thirty-two measurements performed in 126 children and infants were retrospectively analysed. ØAo was measured in the long axis view at the annulus using two-dimensional mode echocardiography with a 5-MHz transducer. Various linear and nonlinear relationships between BSA and, respectively, ØAo, ØAoindex, AA and AAindex were determined based on a nonlinear regression method with a model as follows: y = a(xc) = b. The comparisons between regressions were conducted based on the estimation error.The relationships between ØAo and BSA appeared nonlinear and was well described by: ØAo=2.96(BSA1/4) − 1.31 with a non-zero y-intercept and ØAo=1.64(BSA1/2) with a zero y-intercept. In contrast, the relationships between AA and BSA were linear. The AAindex was not linked to BSA and can be considered as constant. The coefficient a of the equation appeared similar to those obtained mathematically with the relationship previously described between MAFV and CI.The hypothesis that CI can be extrapolated to the measurement of MAFV appears valid as regards the relationships calculated between aortic size and BSA in children without left ventricular outflow tract abnormality.