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 lu ng function were studied. Each subject performed graded cycle ergometr y to exhaustion once while breathing room air; once while breathing 3% CO2-21% O-2-balance N-2; and once while breathing HeO2 (79% He and 21 % O-2). Minute ventilation (VE) and respiratory mechanics were measure d continuously during each 1-min increment in work rate (10 or 20 W). Data were analyzed at rest, at ventilatory threshold (VTh), and at max imal exercise. When the subjects were breathing 3% CO2, there was an i ncrease (P < 0.001) in VE at rest and at VTh but not during maximal ex ercise. When the subjects were breathing HeO2, VE was increased (P < 0 .05) only during maximal exercise (24 +/- 11%). The ventilatory respon se to exercise below VTh was greater only when the subjects were breat hing 3% CO2 (P < 0.05). Above VTh, the ventilatory response when the s ubjects were breathing HeO2 was greater than when breathing 3% CO2 (P < 0.01). Flow limitation, as percent of tidal volume, during maximal e xercise was greater (P < 0.01) when the subjects were breathing CO2 (2 2 +/- 12%) than when breathing room air (12 +/- 9%) or when breathing HeO2 (10 +/- 7%) (n = 7). End-expiratory lung volume during maximal ex ercise was lower when the subjects were breathing HeO2 than when breat hing room air or when breathing CO2 (P < 0.01). These data indicate th at older subjects have little reserve for accommodating an increase in ventilatory demand and suggest that mechanical ventilatory constraint s influence both the magnitude of VE during maximal exercise and the r egulation of VE and respiratory mechanics during heavy-to-maximal exer cise.