Figure 4. A threshold shift scenario.

The HVR (as defined in Fig. 2), is shown at sea level (A) and high altitude (B). The only difference between the ventilatory responses to CO₂ at sea level and those at altitude at a given isoxic tension (hyperoxic or hypoxic) is a decrease in threshold (leftward shift); the metabolic hyperbola is assumed to be unchanged. If the choice of isocapnia is made relative to the resting P_CO2, then the altitude HVR compared to the HVR at sea level is artefactually increased as the inset box shows in a side-by-side comparison.