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 profil e increases during exercise. Its relationship to metabolic rate, however, r emains to be determined at high exercise intensities. We therefore determin ed 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 cyc le ergometer exercise (an increment of 40 W each minute) to exhaustion in 1 1 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)ove r dotO(2)), respectively, up to the estimated lactate threshold (theta (-)( La)). At Higher intensities (P)over dot(A)CO(2), increased disproportionate ly but (P)over dot(A)O(2) continued to increase at the same rate in 7 subje cts but increased more rapidly in the remainder. The rate of change in (P)o ver dot(A)CO(2) per unit rate of change in PO, averaged 3.15 (SD 1.05) (mmH g.s(-1)). (l.min(-1))(-1) while the rate of change in (P)over dot(A)O(2) pe r 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 dot CO(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 sub jects may have reflected the development of an increased distribution of th e ventilation perfusion ratio in addition to the effects of hyperventilatio n.