Sex, Lungs, And Forced Expiratory Flows

Abstract

Women display lower maximal, forced expiratory flows (FEFs) across the vital capacity range compared with height- and age-matched men. This sex-difference on FEFs is believed to increase the prevalence of ventilatory constraint in women compared with men at a given ventilation during exercise. It remains unclear whether this sex-difference in FEFs is evident when absolute lung size is taken into account. Purpose: To investigate the influences of sex, TLC and residual volume on FEFs at rest, and during exercise in healthy adults. METHODS: Forty-seven untrained adults (24 women; 29 ± 6 yrs) with normal pulmonary function (>90% pred.) participated in this study. Subjects completed 9 min of constant-load cycling at 75% of the peak work rate achieved during an incremental cycling test. TLC and RV were obtained at rest and at the end of exercise using He-dilution, and inspiratory and forced vital capacity manoeuvres. FEFs were obtained at 25, 50 and 75% of vital capacity (FEF25%, FEF50% and FEF75%, respectively). Multivariate analyses of covariance (MANCOVA) were used to examine the effect of sex on FEFs after controlling for RV and TLC. RESULTS: Height, TLC and RV were larger for men than women (P < 0.05), as expected. The TLC to height ratio was greater in men than women (45 ± 9 ml·cm-1 v 35 ± 5 ml·cm-1, P < 0.05). FEFs increased during exercise for both sexes (P < 0.05), suggesting bronchodilation. The MANCOVA revealed a main-effect of sex on FEFs at rest and during exercise, where FEFs were systematically higher for men than women (P < 0.05). Importantly, however, the main-effect of sex on FEFs disappeared when TLC and RV were entered into the model as covariates. CONCLUSIONS: The differences in FEFs of our subjects could not be explained on the basis of sex alone. Instead, our analyses suggests that FEFs appear similar between young men and women if one controls for static lung volumes - a result that was also observed in the presence of exercise-induced bronchodilation. Thus, the higher prevalence of ventilatory constraint during exercise in women compared with height-matched men is likely the result of women having a smaller TLC relative to their height, as opposed to a sexual dimorphism in airway size/caliber for a given lung size (dysanapsis). Supported by NIH Grant HL71478