Measuring V̇O(2) in hypoxic and hyperoxic conditions using dynamic gas mixing with a flow-through indirect calorimeter

Measurements of gas exchange while breathing gases of different O(2) 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 N(2)-dilution method. We evaluated the accuracy of the technique at different inspired oxygen fractions by measuring exercising oxygen consumption (V̇O(2)) at two fully aerobic submaximal exercise intensities in Thoroughbred horses. Horses exercised at 24% and 50% maximum oxygen consumption (V̇O(2) max) of each horse while breathing different O(2) concentrations (19.5%, 21% and 25% O(2)). The N(2)-dilution technique was used to calculate V̇O(2). Repeated-measures ANOVA was used to tested for differences in V̇O(2) between different inspired O(2) concentrations. The specific V̇O(2) of the horses trotting at 24%V̇O(2)max and cantering at 50%V̇O(2)max were not significantly different among the three different inspired oxygen fractions. These findings demonstrate that reliable measurements of V̇O(2) can be obtained at various inspired oxygen fractions using dynamic gas mixing and the N(2)-dilution technique to calibrate semi-open-circuit gas flow systems.