Forced expiratory time (FET) is a spirometrically derived variable thought to reflect lung function, but its physiological basis remains poorly understood. We developed a mathematical theory of FET assuming a linear forced expiratory flow-volume profile that terminates when expiratory flow falls below a defined detection threshold. FET is predicted to correlate negatively with both FEV1 and FVC if variations in the rate of lung emptying (relative to normal) among individuals in a population exceed variations in the amount of lung emptying. We retrospectively determined FET pre- and postmethacholine challenge in 1,241 patients (818 had normal lung function, 137 were obstructed, and 229 were restricted) and examined its relationships to spirometric and demographic variables in both hyperresponsive and normoresponsive individuals. Mean FET was 9.6 ± 2.2 s in the normal group, 12.3 ± 3.0 s in those with obstruction, and 8.8 ± 1.9 s in those with restriction. FET was inversely related to FEV1/FVC in all groups, negatively related to FEV1 in the obstructed patients, and positively related to FVC in both the normal and restricted patients. There was no relationship with methacholine responsiveness. Overall, our theory of the relationship between FET to the spirometric indices is supported by these findings and potentially explains how FET is affected by sex, age, smoking status, and possibly body mass index.NEW & NOTEWORTHY Forced expiratory time (FET) has long been felt to reflect important physiological information about lung function but exactly how has never been clear. Here, we use a model analysis to assess the contributions of airway narrowing versus airway closure to FET in a population of individuals and find support for the theory that FET correlates positively with FEV1 if the amounts of lung emptying over a forced expiration vary from predicted values more than variations in the rates of lung emptying, whereas the correlation is negative in the opposite case.
Keywords: airway closure; airway narrowing; forced expiratory time; methacholine challenge; spirometry.