Cardiovascular and oxygen uptake kinetics during sequential heavy cycling exercises

The purpose of the present study was to assess the relationship between the rapidity of increased oxygen uptake (V̇O₂) and increased cardiac output (CO) during heavy exercise. Six subjects performed repeated bouts on a cycle ergometer above the ventilatory threshold (∼80% of peak V̇O₂) separated by 10-min recovery cycling at 35% peak V̇O₂. V̇O₂ was determined breath-by-breath and CO was determined continuously by impedance cardiography. CO and V̇O ₂ values were significantly higher during the 2-min period preceding the second bout. The overall responses for V̇O₂ and CO were significantly related and were faster during the second bout. Prior heavy exercise resulted in a significant increase in the amplitude of the fast component of V̇O₂, with no change in the time constant and a decrease in the slow component. Under these circumstances, the amplitude of the fast component was more sensitive to prior heavy exercise than was the associated time constant.