M04.07.011 Factors Affecting Alveolar PO₂

The alveolar partial pressure of oxygen (PAO₂) is a crucial factor in understanding gas exchange in the lungs. The alveolar air equation helps in calculating PAO₂ and is vital for diagnosing and managing conditions that cause hypoxemia. Several factors influence PAO₂, including atmospheric pressure, inspired oxygen concentration, and alveolar carbon dioxide pressure.

Alveolar Air Equation

The alveolar air equation is expressed as:

• Patm: Atmospheric pressure (typically 760 mm Hg at sea level).
• FiO₂: Fractional concentration of inspired oxygen (0.21 in room air).
• PaCO₂: Alveolar pressure of carbon dioxide.
• RQ: Respiratory quotient, typically 0.8.

This equation helps in the differential diagnosis of hypoxemia by evaluating the alveolar-arterial (A–a) gradient of oxygen.

Factors Affecting PAO₂

1. Atmospheric Pressure (Patm)
   • Increased Patm (e.g., hyperbaric chamber) raises PAO₂.
   • Decreased Patm (e.g., high altitude) lowers PAO₂.
2. Fractional Concentration of Oxygen (FiO₂)
   • Increased FiO₂ raises PAO₂.
   • Room air typically has FiO₂ of 0.21.
3. Alveolar Pressure of Carbon Dioxide (PaCO₂)
   • Increased PaCO₂ lowers PAO₂.
   • Decreased PaCO₂ raises PAO₂.
   • PaCO₂ is often used interchangeably with arterial CO₂ pressure in calculations.

Effect of PACO₂ on PAO₂

Hyperventilation and Hypoventilation

• Hyperventilation (e.g., PaCO₂ = 20 mm Hg):htmlCopy codePAO₂ = PiO₂ - PaCO₂
• Example: 150 - 20 = 130 mm Hg
• Hypoventilation (e.g., PaCO₂ = 80 mm Hg):htmlCopy codePAO₂ = PiO₂ - PaCO₂
• Example: 150 - 80 = 70 mm Hg

Alveolar–Blood Gas Transfer: Fick’s Law of Diffusion

Factors Affecting Diffusion Rate

Structural Factors:

• Surface Area (A):
  • Decreases in conditions like emphysema.
  • Increases during exercise.
• Membrane Thickness (T):
  • Increases in diseases like pulmonary fibrosis.
  • Thicker membranes reduce the rate of gas diffusion.

Gas-Specific Factors:

• Diffusion Constant (D):
  • Primarily dependent on gas solubility.
  • CO₂ diffuses faster than O₂ due to greater solubility.
• Partial Pressure Gradient (P₁ – P₂):
  • Larger gradients increase diffusion rates.
  • O₂ Gradient: Typically 100 – 40 = 60 mm Hg.
  • CO₂ Gradient: Typically 47 – 40 = 7 mm Hg.

Points to Remember

• The alveolar air equation is essential for assessing factors affecting PAO₂.
• Increased atmospheric pressure or inspired oxygen concentration raises PAO₂, while increased PaCO₂ lowers it.
• The diffusion of gases across alveolar membranes is influenced by structural lung changes and the specific properties of the gases.
• CO₂, despite having a smaller gradient, diffuses faster due to higher solubility than O₂.

Bibliography

• West, J. B. (2012). Respiratory Physiology: The Essentials. Lippincott Williams & Wilkins.
• Nunn, J. F. (2010). Nunn’s Applied Respiratory Physiology. Elsevier Health Sciences.
• Levitzky, M. G. (2007). Pulmonary Physiology. McGraw-Hill Medical.