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ABG Interpretation

A-a Gradient



  • The alveolar-arterial (A-a) gradient is a comparison of the partial pressure of O₂ in the alveoli and in arterial blood.
      • Normal Range

      • 5 - 25
    • Calculating the A-a Gradient

    • The A-a gradient is calculated as the alveolar partial pressure of oxygen (PAO₂) minus the arterial partial pressure of oxygen (PaO₂):
  • A-a gradient = PAO₂ - PaO₂
  • The alveolar value is calculated, based on the patient's inspired FiO₂ and the PCO₂ from their blood gas result, while the arterial value is the PaO₂ from the patient's blood gas result.
    • Calculating the Alveolar Pressure of Oxygen

    • To calculate the A-a gradient, first calculate the alveolar partial pressure of oxygen (PAO₂), using the alveolar gas equation:
  • PAO₂ = ( FiO₂ × ( Patmos - PH₂O ) ) -
    • FiO₂: the fraction of inspired oxygen that the patient is inspiring - 0.21 on room air
    • Patmos: atmospheric pressure - 760mmHg at sea level
    • PH₂O: the water vapour pressure - 47mmHg at 37°
    • PCO₂:  the partial pressure of carbon dioxide, from the patient's ABG result
    • R: the respiratory quotient - 0.8
  • This can be simplified further, assuming that the patient is normothermic and at sea level:
  • PAO₂ = ( FiO₂ × ( 760 - 47 ) ) -
  • To produce the following formula:
  • PAO₂ = ( FiO₂ × 713 ) -

Elevated A-a Gradient

  • An elevated A-a gradient indicates that the partial pressure of O₂ is higher in the alveoli than in arterial blood, indicating a V/Q mismatch.
    • Causes of Elevated A-a Gradient

    • Dead space ventilation - pneumonia, asthma, COPD, pulmonary embolismVentilation without perfusion
    • Left to right shunt - pulmonary oedema, ARDS, pneumoniaPerfusion without ventilation
    • Alveolar hypoventilation - pulmonary fibrosis, interstitial lung disease
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