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A-a Gradient or Alveolar-arterial PO2 difference – measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen. The A-a Gradient can help determine the cause of hypoxia.

  • Assesment of oxygenation
  • valid only on room air
  • increase with age

Causes of increased (A-a) difference:

  • COPD
  • Atelectasis
  • Pneumonia
  • Pulmonary edema
  • Asthma
  • Pneumothorax
  • ARDS
  • Interstitial Lung disease

Increase in all cases of hypoxemia, except hypoventilation and high altitude.

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