The influence of active gas exchange on the slope of phase III at rest and after exercise.

Because exercise increases the slope of phase III of the single-breath nitrogen (SB-N2) test in normal persons, we hypothesized that this increase might be due to increased O2 consumption. During the prolonged expiration implicit in the SB-N2 test, the amount of O2 absorbed is greater than the amount of CO2 eliminated. The decrease of gas in volume and the proportional increase in N2 concentration that result might, therefore, increase the slope of phase III, even at rest, but much more after exercise. To test this hypothesis, we had 10 normal volunteers perform SB-N2 tests and reversed SB-N2 tests (SB-R) before and after graded bicycle ergometer exercises. The SB-R test yielded a negative slope for phase III; however, an increasing N2 concentration during prolonged expiration tended to flatten this slope. At rest, the positive slope of phase III for the SB-N, test was steeper than the negative slope of phase III in the SB-R test by a mean +/- SD of 0.23 +/- 0.05% N2/L. After exercise, the slope of phase III of the SB-N2 test increased, while that of the SB-R showed no change. These data suggested that active gas exchange, rather than mechanical factors, is responsible for the increased slope of phase III after exercise. The data also raised the possibility that the slope of phase III at rest is partly the result of gas exchange.