Istituto di Fisiologia Clinica     
Pistelli F., Bottai M., Viegi G., Di Pede F., Carrozzi L., Baldacci S., Pedreschi M., Giuntini C. Smooth reference equations for slow vital capacity and flow-volume curve indexes. In: American Journal of Respiratory and Critical Care Medicine, vol. 161 (03) pp. 899 - 905. American Journal of Respiratory and Critical Care Medicine, 2000.
In 1986 our research group derived reference values for slow vital capacity (VC) and flow-volume curves from "normal" subjects participating in the baseline survey of the Po River Delta epidemiological study (northern Italy) (1). The mathematical technique proposed by Knudson and colleagues (2) was applied to the distribution of VC, Forced Vital Capacity (FVC), and FEV 1 , separately in males and females. Stratified linear regressions were fitted on the three age intervals corresponding to the growth, plateau, and decline phases of lung function. The same age intervals were used for all spirometric indexes. This approach introduces conflicting estimates at the points of transition between equations that fit different lung function phases (3, 4). Further, using the same age intervals for flows and volumes may cause an inadequate fitting of lung function data (4). Modeling the complex dependence of lung function on age by fitting continuous nonlinear functions might represent an alternative approach (4-6). However, the functional form usually required involves the estimation of a large number of parameters, which may yield poor precision. On the other hand, an overly simplified functional form would lead to biased estimates and incorrect inferences. Piecewise polynomial regressions may provide enough flexibility yet keep the number of parameters relatively small. In particular, natural cubic splines provide curves that are continuous over the entire age span up to the second derivative, yielding smooth predictions that are linear in the extreme age intervals (7). The aim of this study was to derive reference values for VC and flow-volume curve indexes from the "normal" subjects who participated in one or both cross-sectional surveys of the Po river delta study by applying natural cubic splines. The advantages of newly proposed reference equations (cubic splines models) over the old ones (stratified linear regressions), in modeling lung function data, have also been assessed by comparing the present with the previously published reference values.
URL: http://ajrccm.atsjournals.org/cgi/reprint/161/3/899?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&author1=giuntini&searchid=1&FIRSTINDEX=0&volume=161&resourcetype=HWCIT
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