The Flow-Ratio Index: An Approach for Measuring the Influence of Age and Cigarette Smoking on Maximum Expiratory Flow-Volume Curve Configuration

Abstract

A forced expiratory flow ratio, derived from the average slope of the maximum expiratory flow-volume (MEFV) curve over a specified volume interval, was examined in healthy asymptomatic cigarette smokers and nonsmokers. This index was developed to have the properties that it would be (1) simple to calculate, (2) less effort dependent than indices that incorporate peak flow, and (3) free of influence from the configurational detail and noise frequently occurring at higher lung volumes on MEFV curves. Forced expired vital capacity maneuvers were performed by participants in a worksite health promotion program. Data from asymptomatic individuals with normal pulmonary function were analyzed for 49 cigarette smokers and 52 nonsmokers; 25 individuals had MEFV curves collected twice over a one-year interval. The ratio of flow derived from an average MEFV slope to instantaneous flow was calculated over the lower half of the vital capacity. Flow ratios were expressed as a percentage of the instantaneous flow at 75 percent of the expired vital capacity (FR75). This ratio was reproducible from year 1 to year 2 (r = 0.86, p<0.0001). Furthermore, the FR75 was well correlated with age among cigarette smokers and nonsmokers (r = 0.68 and 0.63 respectively). The slope of the least squares regression equation relating FR75 to age was significantly greater among smokers than nonsmokers (2.90 percent per year vs 1.73 percent per year, p<0.025). While there was a significant interactive influence of age and total pack-years on the FR75 (F = 2.91, p=0.02), this index did not differ systematically by gender. We conclude that the FR25 is a more sensitive index of altered lung function in cigarette smokers than are results of conventional pulmonary function.