Nephrology Book

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A-a Gradient

Aka: A-a Gradient, Alveolar-Arterial Oxygen Tension Difference, (A-a) PO2 difference, (A-a) DO2
  1. See Also
    1. Arterial Blood Gas
  2. Indications
    1. Hypoxemia
  3. Physiology
    1. Efficiency of gas exchange between alveolus and artery
  4. Calculation (at sea level)
    1. A-a Gradient = PaO2 - FIO2 x (760 - 47) - (PaCO2/0.8)
    2. FIO2 on room air = 0.21
  5. 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
  6. Interpretation: Hypoxemia causes differentiated by A-a Gradient
    1. Increased A-a Gradient
      1. Right to Left Intrapulmonary Shunt (due to fluid filled alveoli)
        1. Congestive Heart Failure
        2. Adult Respiratory Distress Syndrome (ARDS)
        3. Lobar Pneumonia
      2. V/Q Mismatch (due to lung dead space)
        1. Pulmonary Embolism
        2. Atelectasis
        3. Pneumonia
        4. Obstructive Lung Disease (e.g. Asthma, COPD)
        5. Pneumothorax
      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)
  7. References
    1. Davies (1986) Acute Respiratory Failure, Cyberlog, Cardinal Health Systems, p. 22-3

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