Alveolar Gas Equation

The alveolar gas equation is used to calculate alveolar oxygen partial pressure, as it is impossible to collect gases directly from the alveoli. This equation provides a close estimate of P_AO₂ inside the alveoli. The variables in the equation can affect the P_AO₂ inside the alveoli in various physiological and pathophysiological states.

Alveolar Gas Equation

1. P_AO₂ = [(P_atm − P_H2O) FiO₂] − (P_aCO₂/RQ)

where P_atm is the atmospheric pressure (760 mm Hg at sea level), P_H2O is the partial pressure of water (approximately 45 mm Hg), FiO₂ is the fraction of inspired oxygen, P_aCO₂ is the partial pressure of carbon dioxide in alveoli (around 40-45 mmHg under normal physiological conditions), and RQ is the respiratory quotient. The value of the RQ can vary depending on the type of diet and metabolic state. RQ is different for carbohydrates, fats, and proteins; the average value is around 0.82 for the human diet. Indirect calorimetry can provide better measurements of RQ by measuring the VO₂ (oxygen uptake) and VCO₂ (carbon dioxide production).

RQ = amount of CO₂ produced/amount of oxygen consumed

At sea level, the alveolar P_AO₂ is:

1. P_AO₂ = [(760 − 47) 0.21] − (40/0.8) = 99.7 mm Hg.

The 3 major variables of the equation are the atmospheric pressure, amount of inspired oxygen, and carbon dioxide levels. Each variable has an important clinical significance and can help explain physiological and pathophysiological states.