Coupling of ${\dot{\text{V}}}_{{\text{E}}}$ and ${\dot{\text{V}}\text{CO}}_{2}$ kinetics: insights from multiple exercise transitions below the estimated lactate threshold

Abstract

During a step-change in exercise power output (PO), ventilation (${\dot{\text{V}}}_{{\text{E}}}$) increases with a similar time course to the rate of carbon dioxide delivery to the lungs (${\dot{\text{V}}\text{CO}}_{2}$). To test the strength of this coupling, we compared ${\dot{\text{V}}}_{{\text{E}}}$ and ${\dot{\text{V}}\text{CO}}_{2}$ kinetics from ten independent exercise transitions performed within the moderate-intensity domain. Thirteen males completed 3–5 repetitions of ∆40 W step transitions initiated from 20, 40, 60, 80, 100, and 120 W on a cycle ergometer. Preceding the ∆40 W step transitions from 60, 80, 100, and 120 W was a 6 min bout of 20 W cycling from which the transitions of variable ∆PO were examined. Gas exchange (${\dot{\text{V}}\text{CO}}_{2}$ and oxygen uptake, ${\dot{\text{V}}\text{O}}_{2}$) and ${\dot{\text{V}}}_{{\text{E}}}$ were measured by mass spectrometry and volume turbine. The kinetics of the responses were characterized by the time constant (τ) and amplitude (Δ${\dot{\text{V}}}_{{\text{E}}}$/Δ${\dot{\text{V}}\text{CO}}_{2}$). Overall, ${\dot{\text{V}}\text{CO}}_{2}$ kinetics were consistently slower than ${\dot{\text{V}}\text{O}}_{2}$ kinetics (by ~ 45%) and τ${\dot{\text{V}}\text{CO}}_{2}$ rose progressively with increasing baseline PO and with heightened ∆PO from a common baseline. Compared to τ${\dot{\text{V}}\text{CO}}_{2}$, τ${\dot{\text{V}}}_{{\text{E}}}$ was on average slightly greater (by ~ 4 s). Repeated-measures analysis of variance revealed that there was no interaction between τ${\dot{\text{V}}\text{CO}}_{2}$ and τ${\dot{\text{V}}}_{{\text{E}}}$ in either the variable baseline (p = 0.49) and constant baseline (p = 0.56) conditions indicating that each changed in unison. Additionally, for Δ${\dot{\text{V}}}_{{\text{E}}}$/Δ${\dot{\text{V}}\text{CO}}_{2}$, there was no effect of either variable baseline PO (p = 0.05) or increasing ΔPO (p = 0.16). These data provide further evidence that, within the moderate-intensity domain, both the temporal- and amplitude-based characteristics of V̇_E kinetics are inextricably linked to those of ${\dot{\text{V}}\text{CO}}_{2}$.