title: Model for the behaviour of compartmental CO(2) 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 CO(2) between body-fluid compartments is accentuated, leading to increased hydrogen-ion concentration ([H(+)]), decreased bicarbonate-ion concentration ([HCO(3) (-)]) and CO(2) stores, and the excretion of additional CO(2) at the alveoli (i.e. H(+)+HCO(3) (-) --> CO(2)+H(2)O) elevating the CO(2) minute volume. This non-respiratory CO(2) excretion can invalidate use of the RER for determination of fat and carbohydrate oxidation. Direct measurement of the labile CO(2) store and non-respiratory CO(2) excretion during exercise is difficult. Therefore, physicochemical models were derived to illustrate the likely behaviour of compartmental CO(2) stores during 8 W.min(-1) incremental cycling exercise to formulate correction factors to the RER for the non-respiratory CO(2) component. From these models, a polynomial regression equation was derived to describe the change in the total labile CO(2) store volume during incremental exercise from the relationship with blood HCO(3) (-) content: CO(2) volume (ml) = -17x(2)+464x+650, where x is the arterialised blood standard HCO(3) (-) concentration (mmol.l(-1)), relative to resting conditions. Non-respiratory CO(2) excretion (ml.min(-1)) was then determined from the rate of change in CO(2) volume. The modelling method could allow for straightforward calculation of the non-respiratory CO(2) excretion rate for future validation work.
authors.affiliation: Sport and Exercise Sciences, Institute of Food, Nutrition, and Human Health, Massey University, Private Bag 756, Wellington, New Zealand. D.S.Rowlands@massey.ac.nz
authors.forename: David S
authors.initials: DS
authors.lastname: Rowlands
mesh_terms: D001944:Breath Tests; D050260:Carbohydrate Metabolism; D002245:Carbon Dioxide / Q000032:analysis* / Q000378:metabolism*; D015444:Exercise / Q000502:physiology*; D005223:Fats / Q000378:metabolism; D006801:Humans; D008962:Models, Theoretical; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000502:physiology*; D011659:Pulmonary Gas Exchange; D012119:Respiration
chemical_list: D005223:Fats; D002245:Carbon Dioxide