Mechanical constraints on exercise hyperpnea in a fit aging population

We studied 12 physically fit (V̇O₂max = 44 ml·kg^-1·min^-1) older subjects (age = 63 to 77 yr) who showed the usual age-related declines in lung function (i.e., reduced maximal expiratory flow rates, vital capacity and increased functional residual capacity, closing capacity, and residual volume). We measured the optimal transpulmonary pressures for maximal expiratory airflow and the capacity of the muscles of inspiration for developing pleural pressure (taking into account the effects of lung volume and flow rate). Within these mechanical constraints to ventilation we plotted tidal pleural pressure-volume loops for mild through maximal exercise according to a measured end-expiratory lung volume (EELV). We found EELV to decrease a mean of 0.26 ± 0.09 L and maximal effective pleural pressures to be reached in nine subjects near EELV with only light to moderate exercise intensities, whereas peak inspiratory pressure was only 45% of the capacity for pressure generation. With progressive increases in exercise intensity, EELV increased, and pleural pressures encroached to a greater extent on the maximal effective pressures; however, they remained effective in the majority of subjects. During maximal exercise EELV was 0.13 ± 0.10 L greater than resting values, 20% of the VT reached maximal effective pressures, and 83% of the capacity for inspiratory pressure was achieved. Three subjects significantly surpassed their maximal effective expiratory pressures, and four subjects achieved 95 to 100% of the capacity for inspiratory pressure generation. These subjects also showed no further increase in ventilation while breathing 0.02 to 0.05 Fl(CO₂) at maximal exercise. We conclude that substantial mechanical limits to expiratory flow and to inspiratory pressure development are commonly incurred in the older, highly fit subject during heavy exercise; however, these limitations only vary rarely lead to an inadequate ventilatory response.