Alveolar Gas Equation

Abstract

The alveolar gas equation is used to calculate alveolar oxygen partial pressure as it is not possible to collect gases directly from the alveoli. The equation is helpful in calculating and closely estimating the PaO2 inside the alveoli. The variables in the equation can affect the PaO2 inside the alveoli in different physiological and pathophysiological states.Alveolar Gas EquationPatm is the atmospheric pressure (at sea level 760 mm Hg), PH2O is partial pressure of water (approximately 45 mm Hg). FiO2 is the fraction of inspired oxygen. PaCO2 is partial pressure of carbon dioxide in alveoli (in normal physiological conditions around 40 to 45 mmHg). RQ is the respiratory quotient. The value of the RQ can vary depending upon the type of diet and metabolic state. RQ is different for carbohydrates, fats, and proteins (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 consumedAt sea level, the alveolar PAO2 is: The 3 major variables of the equation are the atmospheric pressure, amount of inspired oxygen, and levels of carbon dioxide. Each has an important clinical significance and can help explain different physiological and pathophysiological states.