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

## TL;DR

This study shows that the inflection point of the oxygen dissociation curve during exercise is linked to the gas exchange threshold and is influenced by hemoglobin's binding with CO2 and protons.

## Contribution

The study reveals that the Bohr effect is triggered when oxygen saturation crosses the in vitro ODC inflection point.

## Key findings

- The in vivo ODC inflection point aligns with the gas exchange threshold during exercise.
- At the ODC inflection point, CO2 binding to hemoglobin equals HbNH3+ binding, triggering the Bohr effect.
- The model supports a mechanistic link between the gas exchange threshold and the in vivo ODC inflection point.

## Abstract

We previously hypothesized that the inflection point of the oxygen dissociation curve (ODC) is linked to the gas exchange threshold (GET) during cardiopulmonary exercise testing. This hypothesis was supported by femoral venous blood gas data sampled during constant exercise below and above the GET, which showed that the ODC shifts rightward at the GET. What had gone unnoticed since these original observations in 1994 was that this rightward shift begins slightly earlier, precisely when the oxygen saturation crosses the ODC inflection point. To investigate this phenomenon, we analyzed the 1994 femoral venous blood gas data obtained during cardiopulmonary exercise testing using a modern validated mechanistic biochemical model of oxygen (O2), carbon dioxide (CO2), and proton binding to hemoglobin (Hb). We constructed the ODC for each data point, as well as the in vivo ODC—a composite curve reflecting changes in dynamic blood chemistry during exercise—to assess its alignment with the GET. The model revealed that, at the in vitro ODC inflection point (36% O2Hb saturation), the amounts of CO2 bound to Hb equalized with HbNH3+ eventually predominating. This equilibrium apparently triggered the Bohr shift, steepening the in vivo ODC to improve O2 unloading to the tissues. Shortly afterwards, the in vivo ODC reached its inflection point, matching the measured GET. Our findings support that the GET is mechanistically linked to the in vivo ODC inflection point. These results highlight the physiological relevance of determining the ODC inflection point and its alignment with HbNH3+ and CO2 binding as critical factors in understanding ODC shifts during cardiopulmonary exercise testing.