Model for the behaviour of compartmental CO(2) stores during incremental exercise

Area of Science:

• Exercise Physiology
• Biophysics
• Metabolic Biochemistry

Background:

• The respiratory exchange ratio (RER) is commonly used to assess fat and carbohydrate oxidation during exercise.
• However, RER accuracy is compromised during high-intensity or incremental exercise due to physiological changes affecting CO2 dynamics.
• Specifically, increased hydrogen ion and decreased bicarbonate ion concentrations lead to non-respiratory CO2 excretion, invalidating standard RER calculations.

Purpose of the Study:

• To develop physicochemical models to estimate CO2 stores and non-respiratory CO2 excretion during incremental exercise.
• To derive correction factors for the RER to account for non-respiratory CO2 output.
• To provide a method for more accurate metabolic substrate utilization assessment during strenuous physical activity.

Main Methods:

• Physicochemical models were created to simulate CO2 compartmental behavior during 8 W/min incremental cycling exercise.
• A polynomial regression equation was established relating labile CO2 store volume to blood bicarbonate concentration.
• Non-respiratory CO2 excretion was calculated from the rate of change in the modeled CO2 volume.

Main Results:

• A polynomial regression equation (CO2 volume (ml) = -17x(2)+464x+650, where x is blood HCO3- concentration) was derived to model CO2 store changes.
• The models illustrate the behavior of compartmental CO2 stores under incremental exercise conditions.
• The method allows for the determination of non-respiratory CO2 excretion rates from changes in CO2 volume.

Conclusions:

• Physicochemical modeling offers a potential method to correct RER for non-respiratory CO2 excretion during incremental exercise.
• This approach can improve the accuracy of determining fat and carbohydrate oxidation rates.
• Further validation is needed, but the modeling method shows promise for practical application in exercise science.