Smaller lungs in women affect exercise hyperpnea

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Abstract
We subjected 29 healthy young women (age: 27 ± 1 yr) with a wide range of fitness levels [maximal oxygen uptake (V˙o2 max): 57 ± 6 ml ⋅ kg−1 ⋅ min−1; 35–70 ml ⋅ kg−1 ⋅ min−1] to a progressive treadmill running test. Our subjects had significantly smaller lung volumes and lower maximal expiratory flow rates, irrespective of fitness level, compared with predicted values for age- and height-matched men. The higher maximal workload in highly fit (V˙o2 max > 57 ml ⋅ kg−1 ⋅ min−1,n = 14) vs. less-fit (V˙o2 max < 56 ml ⋅ kg−1 ⋅ min−1,n = 15) women caused a higher maximal ventilation (V˙e) with increased tidal volume (Vt) and breathing frequency (fb) at comparable maximal Vt/vital capacity (VC). More expiratory flow limitation (EFL; 22 ± 4% of Vt) was also observed during heavy exercise in highly fit vs. less-fit women, causing higher end-expiratory and end-inspiratory lung volumes and greater usage of their maximum available ventilatory reserves. HeO2 (79% He-21% O2) vs. room air exercise trials were compared (with screens added to equalize external apparatus resistance). HeO2 increased maximal expiratory flow rates (20–38%) throughout the range of VC, which significantly reduced EFL during heavy exercise. When EFL was reduced with HeO2, Vt, fb, andV˙e (+16 ± 2 l/min) were significantly increased during maximal exercise. However, in the absence of EFL (during room air exercise), HeO2 had no effect onV˙e. We conclude that smaller lung volumes and maximal flow rates for women in general, and especially highly fit women, caused increased prevalence of EFL during heavy exercise, a relative hyperinflation, an increased reliance on fb, and a greater encroachment on the ventilatory “reserve.” Consequently, Vt andV˙e are mechanically constrained during maximal exercise in many fit women because the demand for high expiratory flow rates encroaches on the airways’ maximum flow-volume envelope.

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