Alveolar Gas Equation

I previously wrote about the oxygen delivery equation mathematically describing a vital concept in physiology. Now I want to discuss another critically important formula in acute care medicine – the alveolar gas equation.

where PAO2 is the oxygen tension in the alveoli, FiO2 is the fractional inspired concentration of oxygen, PB is the barometric pressure, PH2O is the partial pressure of water vapor, PaCO2 is the arterial carbon dioxide tension, and RQ is the respiratory quotient

The alveolar gas equation (first line) can be simplified to the second line by assuming the atmospheric pressure is 760 mmHg (sea level), the partial pressure of water vapor at 37 degrees Celsius is 47 mmHg, and the respiratory quotient represents a balanced utilization of metabolic substrates (RQ ~ 0.8).

The most important relationship shown by this equation is that as arterial carbon dioxide (PaCO2) increases, the alveolar tension of oxygen (PAO2) must decrease. This implies that worsening hypercarbia will ultimately lead to hypoxemia.

Additionally, decreasing the inspired oxygen concentration (FiO2) or increasing the altitude (decreased barometric pressure, PB) will also reduce the alveolar tension of oxygen (PAO2) independent of the arterial carbon dioxide (PaCO2).

Overall, the alveolar gas equation gives us an idea of how effectively oxygen is transferred into the blood. By drawing an arterial blood gas, we know the arterial partial pressures for carbon dioxide (PaCO2) and oxygen (PaO2). The alveolar oxygen tension (PAO2) represents the theoretical maximum value for PaO2 if 100% of alveolar oxygen was transferred across the alveolar membrane.

The lower limit is a factor of diffusion capacity across the alveolar membrane, the oxygen content gradient between mixed venous blood and the pulmonary artery blood, and primarily a factor of ventilation-perfusion (V/Q) mismatch. The greater than calculated PAO2 – PaO2 difference, the more these factors are contributing to a patient’s oxygen exchange.

Understanding the alveolar gas equation is important to know if a patient’s oxygenation and ventilation make sense based on measured lab values, and how we can go about optimizing variables to improve these parameters. 🙂

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