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Updated: Feb 13 2019

V/Q Mismatch

  • V/Q Ratio
    • "Normal" V/Q = 0.8
      • V = alveolar ventilation
      • Q = pulmonary blood flow (perfusion)
    • "Normal" V/Q depends on "normal" respiratory rate, tidal volume, and cardiac output
      • PaO2 = 100 mm Hg
      • PaCO2 = 40 mm Hg
    • Ventilation/perfusion matching is essential for ideal gas exchange of O2 and CO2
      • "ideal" V/Q = 1.0
      • exercise → ↑ cardiac output → vasodilation of apical arteries
        • V/Q approaches 1.0
  • Distribution of V/Q in Lung
    • Ventilation and perfusion are nonuniformly distributed in normal, upright lung
    • Zone 1
      • apex of lung
      • decreased ventilation and decreased perfusion
      • V/Q is highest (= 3.0) because of relatively greater decrease in perfusion
        • wasted ventilation
        • PaO2 is highest and PaCO2 is lowest
          • organisms that thrive in high O2 (e.g., TB) flourish in apex of lung
    • Zone 3
      • base of lung
      • V/Q is lowest (= 0.6)
        • wasted perfusion
        • PaO2 is lowest and PaCO2 is highest
  • V/Q Defects
    • Dead Space (V/Q → ∞)
      • e.g., pulmonary embolism (blood flow obstruction)
      • ventilation of lung regions that are not perfused
        • wasted ventilation
        • physiologic dead space
        • no gas exchange occurs
          • PAO2 has same composition as humidified, inspired air (= 150 mm Hg)
          • PACO2 = 0 mm Hg
      • 100% O2 improves PaO2
    • Shunt (V/Q = 0)
      • e.g., airway obstruction,
      • dependent portion of lung in ARDS can act as "shunt"
      • perfusion of lung regions that are not ventilated
        • wasted perfusion
        • no gas exchange
          • pulmonary capillary blood has same composition as venous blood
        • 100% O2 does not improve PaO2
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