On the method of fitting cardiac output kinetics in severe exercise.

The kinetic responses of oxygen uptake (VO₂) and cardiac output (Q) describe the rate at which these physiological variables approach the required steady state value with work rate transitions. In this issue of the Journal, Adami and colleagues examined the kinetic responses during the transition to severe intensity exercise (metabolic demands exceeding maximal VO₂). Two methods were described for fitting VO₂ kinetics: one was an exponential model that referenced the time course of VO₂ relative to an apparent plateau while the second examined the rate of change with respect to the value predicted to be 120% of maximal VO₂. The rate of change of the primary adaptive component described by the time constant (tau2) was considerably slower when referenced to the predicted VO₂ (62.5 s) than when fit by the exponential model (20.3 s). For the description of Q kinetics Adami and colleagues fitted only the exponential model. We investigated the impact of fitting the kinetics of Q relative to a predicted value for this severe work rate as was done for VO₂. The time course for Q was reconstructed from their group mean fitting parameters then referenced to values for the required Q based on the literature. The estimate for the time constant (tau2) exceeded the value determined from the exponential model in which the curve fit was referenced to an apparent plateau by more than sixfold (86.4 s vs. 13.5 s). This outcome suggests that future investigations should explore further the dynamic interactions of metabolic regulatory factors and the limitations of the O₂ supply system when describing the system kinetics.