Forced midexpiratory time: reference values and the effect of cigarette smoking.

BACKGROUND

Forced midexpiratory time (FET25-75%), the time required to exhale from 25 to 75% of forced vital capacity (FVC), has been advocated as a relatively volume-independent measure of obstruction. Previous estimates of normal FET25-75% have not systematically studied the effect of age, height, sex, and smoking history.

METHODS

We analyzed flow-volume loops from 369 normal lifetime nonsmokers and smokers, a random sample of the population of Michigan. Linear models including age and/or height were considered, with and without logarithmic transformation.

RESULTS

The best models used age, or age and height, and no transformation. Age and height contributed much less to the variability of FET25-75% than to spirometric flows previously investigated. The largest r2 was found in the male smokers, with the largest contribution from age, suggesting a smoking effect.

CONCLUSIONS

As age was the major contributor to the value of FET25-75% and that contribution was small, we suggest the use of simplified age-stratified reference values derived from our data.