Equivalent Air Altitude and the Alveolar Gas Equation

It is expedient to use normobaric hypoxia (NH) as a surrogate for hypobaric hypoxia (HH) for training and research. The approach matches inspired oxygen partial pressure (P(I)o₂) at the desired altitude to that at site pressure (PB) by reducing the inspired fraction of oxygen (FIo₂) to <0.21 using the equation: PIo₂= (PB - 47) × FIo₂, where 47 mmHg is the vapor pressure of water at 37°C. The investigator then has at site pressure the equivalent PIo2 as at altitude, i.e., the NH exposure is at an "equivalent air altitude." Some accepted as fact identical signs and symptoms of hypoxia for both conditions. However, those that derived the alveolar air equation showed that the coupled alveolar oxygen (PAo₂) and carbon dioxide partial pressures (PAco₂) for NH and HH are not identical when PIo₂is equivalent. They attribute the difference in alveolar gas composition under equivalent PIo₂to a nitrogen dilution effect or, more generally, to the respiratory exchange effect. Those that use NH as a convenient surrogate for HH must concede that physiological responses to NH cannot be identical to the responses to HH given only equivalent hypoxic PIo₂.