Ventilation and respiratory mechanics during exercise in younger subjects breathing CO₂ or HeO₂

To determine if ventilation (V̇E) during maximal exercise would be increased as much by 3% CO₂ loading as by resistive unloading of the airways, we studied seven subjects (39 ± 5 years; mean ± S.D.) during graded cycle ergometry to exhaustion while breathing: (1) room air (RA); (2) 3% CO₂, 21% O₂, and 76% N₂; or (3) 79% He and 21% O₂) V̇E and respiratory mechanics were measured during each 1-min increment (20 or 30 W) in work rate. V̇E during maximal exercise was increased 21 ± 17% when breathing 3% CO₂ and 23 ± 16% when breathing HeO₂ (P < 0.01). Further, the ventilatory response to exercise above ventilatory threshold (VTh) was increased (P < 0.05) when breathing HeO₂ (0.89 ± 0.26 L/min/W) as compared with breathing RA (0.65 ± 0.12). When breathing HeO₂, end-expiratory lung volume (%total lung capacity, TLC) was lower during maximal exercise (46 ± 7) when compared with RA (53 ± 6, P < 0.01). In conclusion, V̇E during maximal exercise can be augmented equally by 3% CO₂ loading as by resistive unloading of the airways in younger subjects. This suggests that in younger subjects with normal lung function there are minimal mechanical ventilatory constraints on V̇E during maximal exercise.