Spirometric and anthropometric determinants of forced expiratory time in a general population

Forced expiratory time (FET) has gained new interest in the joint recommendation of the American Thoracic Society (ATS) and the European Respiratory Society (ERS) for the assessment of spirometry. Interpretation of FET is, however, difficult as limited information is available on spirometric and anthropometric determinants of FET in populations.

We studied a non-selected population sample including healthy non-smokers with a structured interview and flow-volume spirometry. Regular medication, if any, was continued. Spirometry of 603 individuals (248 men, 355 women) fulfilled predefined quality criteria based on modified ATS recommendations. FET from the flow-volume curve with the largest sum of forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC) was used in analyses.

The mean FET in the population sample was 10.7 (95% CI 10.4-11.1) s and in healthy non-smokers 9.8 (9.2-10.4) s. Men had on average longer FET (11.3 s versus 10.3 s), but the gender difference was not significant when FEV1/FVC was used as a covariate. FEV1/FVC (r = -0.613, P<0.01) and maximal mid-expiratory flow (MMEF) correlated negatively and age (r = 0.279, P<0.01), body mass index (BMI) and smoking pack-years positively with FET.

The findings indicate that mean FET is around 11 s in a non-selected adult population and around 10 s in healthy non-smokers. FET increases slightly with age and BMI, suggesting age- and weight-related changes in pulmonary mechanics. The negative correlation of FET with FEV1/FVC and MMEF even in healthy non-smokers indicates that airflow limitation, either pathological or physiological, tends to prolong FET.