Effect of exercise intensity on the changes in alveolar slopes of carbon dioxide and oxygen expiratory profiles in humans

The slope of the expired alveolar partial pressure of carbon dioxide profile increases during exercise. Its relationship to metabolic rate, however, remains to be determined at high exercise intensities. We therefore determined the slope of alveolar partial pressures of carbon dioxide and oxygen ((P)over dot(A)CO(2), (P)over dot(A)O(2), respectively) during incremental cycle ergometer exercise (an increment of 40 W each minute) to exhaustion in 11 normal subjects. The (P)over dot(A)CO(2) and (P)over dot(A)O(2) increased as linear functions of carbon dioxide production and oxygen uptake ((V)over dotO(2)), respectively, up to the estimated lactate threshold (theta (-)(La)). At Higher intensities (P)over dot(A)CO(2), increased disproportionately but (P)over dot(A)O(2) continued to increase at the same rate in 7 subjects but increased more rapidly in the remainder. The rate of change in (P)over dot(A)CO(2) per unit rate of change in PO, averaged 3.15 (SD 1.05) (mmHg.s(-1)). (l.min(-1))(-1) while the rate of change in (P)over dot(A)O(2) per unit rate of change in (V)over dotO(2) averaged -3.53 (SD 0.79) (mmHg.s(-1)).(l-min(-1))(-1) over this range. The more rapid increase in (P)over dotCO(2) above theta (-)(La) was consistent with an accelerated CO, exchange, whereas the more rapid rate of change in (P)over dot(A)O(2) in 3 of the subjects may have reflected the development of an increased distribution of the ventilation perfusion ratio in addition to the effects of hyperventilation.