The Metabolic Cost Of Breathing During Exercise: Effects Of Age And Sex

During exercise there is an increase in ventilation and a corresponding increase in the uptake of O2 by the respiratory muscles. Both sex-based structural differences in the respiratory system and age-associated declines in pulmonary function likely increase the O2 cost of breathing (VO2RM) during exercise. PURPOSE: We sought to assess the effects of age and sex on VORM during exercise in healthy younger and older, males and females. METHODS: Forty healthy participants (10 young males 23 ± 3 y; 10 young females 23 ± 3 y; 10 older males 63 ± 3 y, 10 older females 63 ± 6 y) mimicked their exercise breathing patterns (voluntary hyperpnea) in the absence of exercise. We had participants match the important mechanical components of exercise hyperpnea: minute ventilation, tidal volume, breathing frequency, duty cycle, operating lung volume and the work of breathing at 45, 60, 75 and 100% of maximal O2 uptake. To determine VO2RM, we calculated a net oxygen cost by subtracting whole-body resting VO2. RESULTS: Young females had a higher VO2RM for a given absolute ventilation compared to the young males at ventilations >75 L/min (P = 0.031). Similarly, older females had a higher VO2RM for a given absolute ventilation compared to the older males at ventilations >70 L/min (P = 0.023). At maximal exercise, VO2RM represented a greater fraction of whole-body VO2 in young females, 12.8 ± 3.9%, compared to young males, 10.7 ± 3.0% (P = 0.011), while VO2RM represented a similar fraction of whole-body VO2 in older females 13.5 ± 2.3% and 13.2 ± 3.3% in older males. Furthermore, both younger and older males had better respiratory muscle efficiency than their female counterparts at maximal exercise (Female: 5.6% ± 1.0% vs 4.8% ± 1.3%, P = 0.011; Males: 7.0% ± 1.8% vs 6.1% ± 2.5%, P = 0.015). Similarly, younger participants were more efficient than older participants (6.5% ± 1.5% vs. 5.5 ± 2.0%; P = 0.001). CONCLUSION: The normal age-related changes in respiratory structure and function, in addition to sex-based differences in airway anatomy, have a significant effect on the efficiency and cost incurred to breathe during exercise. The greater cost in older individuals, especially older women, may indicate a greater fraction of cardiac output is directed to the respiratory muscles and may influence blood flow distribution during exercise. British Columbia Lung Association and the Natural Science and Engineering Research Council of Canada