Kinetics of excess CO2 output during and after intensive exercise

In order to clarify the kinetics of excess CO2 output during and after intensive exercise, six male subjects were each instructed to perform 40-, 60- and 80-s cycle ergometer exercises (282 +/- 9 W, 90 rpm). Ventilation and gas exchange parameters were recorded breath-by-breath, and lactate concentration (La) was repeatedly measured with blood samples from a finger tip. The increase in La from the resting value to peak value and the duration of exercise showed a significant linear relationship (r = 0.91, p<0.01) passing through zero, indicating that lactic acid was produced at a constant rate in working muscles from the beginning of exercise. However, in contrast to this increase in La, excess V.CO2, defined as the difference between V.CO2 and V.O2, showed a temporary negative value after the start of exercise. Subsequently, excess V.CO2 became positive, reaching a peak at 60 s post-exercise, and then decreased down to zero at about 9 min after the end of the 80-s exercise. End-tidal CO2 rose above the pre-exercise level during exercise and at about 3 min post-exercise, and thereafter remained below the pre-exercise level. Excess CO2, calculated by the sum of excess V. CO2 from the start of exercise to the 10th min after the end of exercise, was significantly COrrelated with the increase in La from resting to 10 min post-exercise (r = 0.88, p<0.01). These results suggest that although excessive CO2 output (excess CO2) in response to intensive exercise is related to the increase in lactic acid, the time course of excessive CO2 output (excess V.CO2) is delayed, relative to the production of lactic acid, and is affected by hyperventilation.