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Test Findings

Elevated A-a Gradient



The alveolar-arterial (A-a) gradient is a comparison of the partial pressure of Oâ‚‚ 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 (PAOâ‚‚) minus the arterial partial pressure of oxygen (PaOâ‚‚):
A-a gradient = PAOâ‚‚ - PaOâ‚‚
The alveolar value is calculated, based on the patient's inspired FiOâ‚‚ and the PCOâ‚‚ from their blood gas result, while the arterial value is the PaOâ‚‚ 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 (PAOâ‚‚), using the alveolar gas equation:
PAOâ‚‚ = ( FiOâ‚‚ Ã— ( Patmos - PHâ‚‚O ) ) -
  • FiOâ‚‚: the fraction of inspired oxygen that the patient is inspiring - 0.21 on room air
  • Patmos: atmospheric pressure - 760mmHg at sea level
  • PHâ‚‚O: the water vapour pressure - 47mmHg at 37°
  • PCOâ‚‚:  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:
PAOâ‚‚ = ( FiOâ‚‚ × ( 760 - 47 ) ) -
To produce the following formula:
PAOâ‚‚ = ( FiOâ‚‚ × 713 ) -


An elevated A-a gradient indicates that the partial pressure of Oâ‚‚ 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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