(A = alveolar, a = arterial)

To investigate causes of !Hypoxemic/Type I RF

A-a O₂ Gradient

Normal value:

• ≤ 5-10 mmHg (in young adult), some sorces says 10-15 mmHg
• ≤ (age/4) + 4 (for age 40-60)
• Low value essentially means:
  • Low diffusion
  • V/Q mismatch

Calculation:

• A-a Gradient = PAO2 - PaO2
• PaO2 = measured by ABG
• PAO2 = calculated by the alveolar gas equation
  • PAO2 = PIO₂ - (PaCO2/RQ)
  • PAO2 = FiO₂ (Patm - PH2O) - (PaCO2/RQ)
  • PAO2 = 150 - PaCO2 * 1.25
  • Where PIO₂ = 150 in room air normal altitude

Clinical Pearl:

• VS DLCO (75% - 125%) :
  • Low DLCO is one of the reasons for a low A-a gradient