Measuring V̇O₂ in hypoxic and hyperoxic conditions using dynamic gas mixing with a flow-through indirect calorimeter

Measurements of gas exchange while breathing gases of different O₂ concentrations are useful in respiratory and exercise physiology. High bias flows required in flow-through indirect calorimetry systems for large animals like exercising horses necessitate the use of inconveniently large reservoirs of mixed gases for making such measurements and can limit the amount of equilibration time that is adequate for steady-state measurements. We obviated the need to use a pre-mixed reservoir of gas in a semi-open flow-through indirect calorimeter by dynamically mixing gases and verified the theoretical accuracy and utility of making such measurements using the mass-balance N₂-dilution method. We evaluated the accuracy of the technique at different inspired oxygen fractions by measuring exercising oxygen consumption (V̇O₂) at two fully aerobic submaximal exercise intensities in Thoroughbred horses. Horses exercised at 24% and 50% maximum oxygen consumption (V̇O₂ max) of each horse while breathing different O₂ concentrations (19.5%, 21% and 25% O₂). The N₂-dilution technique was used to calculate V̇O₂. Repeated-measures ANOVA was used to tested for differences in V̇O₂ between different inspired O₂ concentrations. The specific V̇O₂ of the horses trotting at 24%V̇O₂max and cantering at 50%V̇O₂max were not significantly different among the three different inspired oxygen fractions. These findings demonstrate that reliable measurements of V̇O₂ can be obtained at various inspired oxygen fractions using dynamic gas mixing and the N₂-dilution technique to calibrate semi-open-circuit gas flow systems.