Model for the behaviour of compartmental CO₂ stores during incremental exercise

Abstract

The respiratory exchange ratio (RER) is a valid method for determining fat and carbohydrate oxidation during exercise when the exchange of respiratory gas is in a state of steady flux between the tissue and fluid compartments and the alveoli. However, under incremental intensity or heavy exercise conditions, the movement of electrolytes, fluids, and CO₂ between body-fluid compartments is accentuated, leading to increased hydrogen-ion concentration ([H⁺]), decreased bicarbonate-ion concentration ([HCO₃⁻]) and CO₂ stores, and the excretion of additional CO₂ at the alveoli (i.e. H⁺+HCO₃⁻ → CO₂+H₂O) elevating the CO₂ minute volume. This non-respiratory CO₂ excretion can invalidate use of the RER for determination of fat and carbohydrate oxidation. Direct measurement of the labile CO₂ store and non-respiratory CO₂ excretion during exercise is difficult. Therefore, physicochemical models were derived to illustrate the likely behaviour of compartmental CO₂ stores during 8 W·min⁻¹ incremental cycling exercise to formulate correction factors to the RER for the non-respiratory CO₂ component. From these models, a polynomial regression equation was derived to describe the change in the total labile CO₂ store volume during incremental exercise from the relationship with blood HCO₃⁻ content: CO₂ volume (ml) = -17x²+464x+650, where x is the arterialised blood standard HCO₃⁻ concentration (mmol·l⁻¹), relative to resting conditions. Non-respiratory CO₂ excretion (ml·min⁻¹) was then determined from the rate of change in CO₂ volume. The modelling method could allow for straightforward calculation of the non-respiratory CO₂ excretion rate for future validation work.