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  • Defined as PaO2 <60 mm Hg
    • Amount of oxygen available is a function of the arterial oxygen content and blood flow
  • Occurs in states of low CO, low hemoglobin, or low SaO2 (arterial oxygen saturation)
  • Relative Hypoxemia
    • PaO2 is < expected for given level of inhaled O2 (e.g. PaO2 of 100 on FiO2 100%)
    • Can calculate amount of relative hypoxemia by A-a gradient
      • P(A-a)O2 = 145 – PaCO2 – PaO2 (normal is <10 in young, healthy patients)


  • Hypoventilation
    • Always associated with increased PaCO2
    • Normal A-a gradient
  • Right-to-left shunt
    • Occurs when blood enters the systemic arteries with out traversing ventilated lung
      • Occurs with pulmonary consolidation, atelectasis
    • Hallmark is failure of arterial O2 levels to increase with supplemental O2 (increased A-a)
  • V/Q mismatch
    • PE, pneumonia, asthma, COPD
    • Improves with supplemental O2
    • A-a gradient increased
  • Diffusion impairment
    • Improves with supplemental O2
    • A-a gradient increased
  • Low inspired O2
    • Improves with supplemental O2
    • A-a gradient normal


  1. Increased minute ventilation
  2. Pulmonary arterial vasoconstriction
  3. Increased sympathetic tone

Differential Diagnosis

Acute dyspnea





See Also