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

To determine if ventilation ((V) over dotE) 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: (I) room air (RA); (2) 3% CO2, 21% O-2, and 76% N-2; or (3) 79% He and 21% O-2). (V) over dotE and respiratory mechanics were measured during each I-min increment (20 or 30 W) in work rate. (V) over dotE 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, (V) over dotE 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 (V) over dotE during maximal exercise. (C) 1997 Elsevier Science B.V.