KINETICS OF CO UPTAKE AND DIFFUSING-CAPACITY IN TRANSITION FROM REST TO STEADY-STATE EXERCISE

In the transition from rest to steady-state exercise, O2 uptake from the lungs (VO2) depends on the product of pulmonary blood flow and pulmonary arteriovenous O2 content difference. The kinetics of pulmonary blood flow are believed to be somewhat faster than changes in pulmonary arteriovenous O2 content difference. We hypothesized that during CO breathing, the kinetics of CO uptake (VCO) and diffusing capacity for CO (DL(CO)) should be faster than VO2 because changes in pulmonary arteriovenous CO content difference should be relatively small. Six subjects went abruptly from rest to constant exercise (inspired CO fraction = 0.0005) at 40, 60, and 80% of their peak VO2, measured with an incremental test (VO2peak). At all exercise levels, DL(CO) and VCO rose faster than VO2 (P < 0.001), and DL(CO) rose faster than VCO (P < 0.001). For example, at 40% VO2peak, the time constant (tau) for DL(CO) in phase 2 was 19 +/- 5 (SD), 24 +/- 5 s for VCO, and 33 +/- 5 s for VO2. Both VCO and DL(CO) increased with exercise intensity but to a lesser degree than VO2 at all exercise intensities (P < 0.001). In addition, no significant rise in DL(CO) was observed between 60 and 80% VO2peak. We conclude that the kinetics of VCO and DL(CO) are faster than VO2, suggesting that VCO and DL(CO) kinetics reflect, to a greater extent, changes in pulmonary blood flow and thus recruitment of alveolar-capillary surface area. However, other factors, such as the time course of ventilation, may also be involved. In addition, we conclude that the relative increase in DL(CO) with exercise intensity is attenuated at high levels of exercise in most subjects, suggesting that capillary surface area recruitment may approach a saturation level before maximum exercise.