I. See Also

II. Indications

III. Physiology

  1. Efficiency of gas exchange between alveolus and artery

IV. Calculation at sea level

  1. A-a Gradient = PaO2 - FIO2 x (760 - 47) - (PaCO2/0.8)
  2. FIO2 on room air = 0.21

V. Interpretation: Calculating a normal A-a Gradient

  1. A-a Gradient = (Age/4) + 4
  2. Young person at sea level
    1. A-a increases 5 to 7 mmHg for every 10% increase FIO2
    2. Room Air: 10 to 20 mmHg
    3. 100% oxygen: 60 to 70 mmHg
  3. Increased age affects A-a Gradient (at sea level)
    1. Age 20 years: 4 to 17 mmHg
    2. Age 40 years: 10 to 24 mmHg
    3. Age 60 years: 17 to 31 mmHg
    4. Age 80 years: 25 to 38 mmHg

VI. Interpretation: Hypoxemia causes differentiated by A-a Gradient

  1. Increased A-a Gradient
    1. V/Q Mismatch
      1. Congestive Heart Failure
      2. Adult Respiratory Distress Syndrome (ARDS)
      3. Pneumonia
      4. Atelectasis
    2. Pulmonary shunt
      1. Pulmonary Embolism
      2. Patent Foramen Ovale
      3. Atrial Septal Defect
    3. Alveolar hypoventilation
      1. Interstitial Lung Disease
  2. Normal A-a Gradient
    1. Hypoventilation
      1. Neuromuscular disorders
      2. Central nervous system disorder
    2. Low inspired FIO2 (e.g. high altitude)

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