Ventilatory response to exercise in subjects breathing CO2 or HeO2.

To investigate the effects of mechanical ventilatory limitation on the ventilatory response to exercise, eight older subjects with normal lung function were studied. Each subject performed graded cycle ergometry to exhaustion once while breathing room air; once while breathing 3% CO2-21% O2-balance N2; and once while breathing HeO2 (79% He and 21% O2). Minute ventilation (VE) and respiratory mechanics were measured continuously during each 1-min increment in work rate (10 or 20 W). Data were analyzed at rest, at ventilatory threshold (VTh), and at maximal exercise. When the subjects were breathing 3% CO2, there was an increase (P < 0.001) in VE at rest and at VTh but not during maximal exercise. When the subjects were breathing HeO2, VE was increased (P < 0.05) only during maximal exercise (24 +/- 11%). The ventilatory response to exercise below VTh was greater only when the subjects were breathing 3% CO2 (P < 0.05). Above VTh, the ventilatory response when the subjects were breathing HeO2 was greater than when breathing 3% CO2 (P < 0.01). Flow limitation, as percent of tidal volume, during maximal exercise was greater (P < 0.01) when the subjects were breathing CO2 (22 +/- 12%) than when breathing room air (12 +/- 9%) or when breathing HeO2 (10 +/- 7%) (n = 7). End-expiratory lung volume during maximal exercise was lower when the subjects were breathing HeO2 than when breathing room air or when breathing CO2 (P < 0.01). These data indicate that older subjects have little reserve for accommodating an increase in ventilatory demand and suggest that mechanical ventilatory constraints influence both the magnitude of VE during maximal exercise and the regulation of VE and respiratory mechanics during heavy-to-maximal exercise.