Comparison of measures of forced expiration

Abstract

Theoretical relationships among a number of parameters were derived for idealized timed vital capacity (TVC) and maximal expiratory flow-volume (MEFV) curves to determine a minimal set of independent parameters. Normal pediatric subjects and those with cystic fibrosis were studied to verify these relationships experimentally. The average flow over the middle half (FEF25–75%) of the forced vital capacity (FVC) and flows at various exhaled percentages of the FVC (FEF50%, FEF75%), as well as moments of the TVC and MEFV curves were computed. From the TVC moments, a mean transit time (MTT) and an index of dispersion (ID) were also calculated. The minimum information needed to detect pulmonary mechanical changes associated with obstructive lung disease requires at least two reproducible measures: one related to the mean slope (e.g., FEF25–75%/FVC or MTT) and the other to the shape (e.g;, ID) of the effort-independent portion of the MEFV curve.