Kinetics of mixed venous CO2 pressure in constant-load exercise.

We have studied the kinetics of mixed venous C02 pressure (Pi-,co2) at constant work rates of 300, 450 and 600 kpmlmin (Yano, 1986). The PVco, increased at the onset of exercise, and reached a steady state at 300 and 450 kpmlmin. However, at 600 kpmlmin, the PVco2 indicated a relatively high steady state in comparison with those at other work rates, and decreased after 5 min of exercise. This overshoot may be related to an early increase of lactate at the onset of exercise, since this phenomenon was only observed at the highest exercise intensity of the study. However, lactate was not measured in thjs study. The purpose of the present study is, therefore, to examine if PVco2 overshoots because of lactate increase during exercise.