Comparison of measures of forced expiration.

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. 