Sex-specific prediction equations for VmaxFRC in infancy -: A multicenter collaborative study

Measurements of maximal flow at functional residual capacity (V(over dot)max(FRC)) from partial forced expiratory maneuvers remain the most popular method for assessing small airway function in infants and young children. However, the lack of appropriate reference data that are both applicable outside the centers that developed them and reflect the normal variability between healthy subjects has limited interpretation of V(over dot)max(FRC) results in both clinical practice and research. To address this problem, we collated V(over dot)max(FRC) data from 459 healthy infants (226 boys) tested on 654 occasions during the first 20 months of life from three collaborating centers. Multiple linear regression analysis indicated that sex, age, and length were important predictors of V(over dot)max(FRC), which was, on average, 20% higher in girls than in boys during the first 9 months of life. (V(over dot)max(FRC))(0.5) (ml (.) second(-1)) = 4.22 + 0.00210 x length(2) (cm) for boys (RSD = 3.01; R-2 = 0.48), and -1.23 + 0.242 x length for girls (RSD = 2.72; R-2 = 0.49). Alternative models incorporating both age and length z scores are also described. Failure to use sex-specific prediction equations for V(over dot)max(FRC) may preclude detection of clinically significant changes in girls and lead to false reports of diminished airway function in boys. Appropriate use of scores, which indicate a "normal" range (z scores of 0 +/- 2) for V(over dot)max(FRC), during infancy should also improve interpretation of both clinical and research studies.