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