Oxygen dissociation curve inflection point during incremental exercise: a trigger for the Bohr effect

Area of Science:

• Exercise Physiology
• Biochemistry
• Cardiopulmonary Function

Background:

• The gas exchange threshold (GET) is a key indicator during cardiopulmonary exercise testing.
• A previous hypothesis linked the oxygen dissociation curve (ODC) inflection point to the GET.
• Rightward ODC shifts were observed at the GET, but the precise initiation point was unclear.

Purpose of the Study:

• To investigate the mechanistic link between the ODC inflection point and the GET.
• To analyze historical femoral venous blood gas data using a modern biochemical model.
• To determine if the ODC inflection point precedes and aligns with the GET.

Main Methods:

• Utilized a validated mechanistic biochemical model for O2, CO2, and proton binding to hemoglobin (Hb).
• Analyzed femoral venous blood gas data from 1994 cardiopulmonary exercise testing.
• Constructed both in vitro ODCs and the in vivo ODC (dynamic blood chemistry curve).

Main Results:

• The model showed CO2 and HbNH3+ binding equilibrium at the in vitro ODC inflection point (36% O2Hb saturation).
• This equilibrium initiated the Bohr shift, steepening the in vivo ODC for improved oxygen unloading.
• The in vivo ODC inflection point closely matched the measured GET.

Conclusions:

• The GET is mechanistically linked to the in vivo ODC inflection point.
• The ODC inflection point, influenced by CO2 and proton binding, is crucial for understanding ODC shifts during exercise.
• Determining the ODC inflection point's alignment with these factors is physiologically relevant.