Flow limitation and regulation of functional residual capacity during exercise in a physically active aging population

In 29 older (69 ± 1 yr), physically active subjects (V̇O₂max = 44 ± 2 ml·kg^-1·min^-1), we determined the effect of an age-related decline in elastic lung recoil (i.e., V̇max₅₀ = 65% of 30-yr-old adults) 30-yr-adults) on the ventilatory response to progressive exercise. More specifically, we assessed if expiratory airflow limits were achieved and how this may modulate the regulation of end-expiratory lung volume (EELV). We found that with only mild to moderate (50 to 75% V̇O₂max) exercise, the mean EELV was reduced 0.38 ± 0.07 L, and that expiratory flow limitation was present over 25 ± 4% of the VT. In 11 subjects during this intensity of exercise, EELV was within their closing capacity. As exercise intensity progressed, VT plateaued at 58 ± 2% of the vital capacity, and increased expiratory air flow rates were achieved by significantly increasing the EELV back to near resting levels, thereby moving a portion of the expiratory tidal flow-volume envelope away from the constraints of the effort independent portion of the maximal flow-volume curve. During heavy exercise, end-inspiratory lung volume (EILV) approached 90% of TLC. To achieve greater expiratory flow with maximal exercise, EELV remained similar to the previous intensity, and a significantly greater portion of the tidal expiratory flow-volume envelope (> 40% of the VT) became flow-limited. Despite this significant expiratory limitation, a rise in EELV, and an EILV approaching TLC, TI/Ttot remained constant throughout exercise, and the ventilatory response for the metabolic demand (V̇A/V̇CO₂) was appropriate. When subjects were divided into two groups based on V̇O₂max, we found that the least fit subjects (n = 15, V̇O₂max = 150% predicted) had a reduced capacity for expiratory flow generation relative to the fittest subjects (n = 14, V̇O₂max = 210% pred) and therefore were more hyperinflated during exercise and incurred greater expiratory flow limitation for a given V̇E. We conclude that during exercise a major consequence of the loss of elastic recoil with age may be the sacrifice of an optimal length for inspiratory muscles and an increased cost of breathing in the face of high ventilatory demands. In addition, the loss of elastic recoil with aging appears to parallel the loss in V̇O₂max with age.