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Introduction

• 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 the different physiological and pathophysiological state.
• The value of the RQ can vary depending upon 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).

PAO₂ = (Patm - PH₂O) FiO₂ - PaCO₂/RQ

At sea level, the alveolar PAO₂ is: PaO₂ = (760 - 47) 0.21 - 40/0.8 = 99.7 mm Hg.

A-a gradient = PAO₂ - PaO₂

• PAO₂ is the partial pressure of Alveolar O₂
• PaO₂ is the partial pressure of O₂ in the arterial blood
• 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 (usually 40 to 45 mmHg).
• RQ is the respiratory quotient.

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.