Equivalent Air Altitude and the Alveolar Gas Equation

Area of Science:

• Physiology
• Aerospace Medicine
• Environmental Health

Background:

• Normobaric hypoxia (NH) is frequently employed as a substitute for hypobaric hypoxia (HH) in training and research settings.
• NH aims to replicate the inspired oxygen partial pressure (P(I)o₂) of altitude at site pressure by adjusting the fraction of inspired oxygen (FIo₂).
• This method creates an 'equivalent air altitude' by maintaining a specific P(I)o₂.

Purpose of the Study:

• To investigate the physiological equivalence between normobaric hypoxia (NH) and hypobaric hypoxia (HH).
• To determine if identical signs and symptoms of hypoxia occur under both NH and HH conditions.
• To analyze the differences in alveolar gas composition under equivalent hypoxic conditions.

Main Methods:

• The study involves analyzing the alveolar air equation under conditions of NH and HH.
• Comparison of coupled alveolar oxygen (PAo₂) and carbon dioxide partial pressures (PAco₂) for both types of hypoxia.
• Evaluation of the impact of nitrogen dilution and respiratory exchange effects on gas composition.

Main Results:

• Alveolar oxygen (PAo₂) and carbon dioxide (PAco₂) partial pressures are not identical for NH and HH, even when P(I)o₂ is equivalent.
• Differences in alveolar gas composition are attributed to a nitrogen dilution effect and the respiratory exchange effect.
• Physiological responses to NH cannot be considered identical to HH solely based on equivalent hypoxic P(I)o₂.

Conclusions:

• Normobaric hypoxia (NH) is not a perfect physiological surrogate for hypobaric hypoxia (HH).
• The distinct alveolar gas compositions under equivalent P(I)o₂ necessitate caution when extrapolating NH findings to HH.
• Further research is needed to understand the full implications of these differences for training and research.