QS Study

Ventilation-Perfusion Ratio

Definition: The ratio of alveolar ventilation and alveolar blood flow is called ventilation-perfusion ratio. The ratio of alveolar ventilation to instantaneous alveolar capillary blood flow in any division of the lung; because both ventilation and perfusion are articulated per unit volume of tissue and per unit of time, which cancel, the units become liters of gas per liter of blood.

Normal value: 0.84

This is expressed as VA/Q = Alveolar ventilation (VA) / Cadial output (Q)

= (4.2 L/min) / (5 L/m) = 0.84


  • When VA and Q as normal for a given alveolus. The VA /Q will be normal.
  • When VA is 0, Q of the alveolus is normal, then, VA/Q will be Zero.
  • When VA is normal but Q is Zero, then VA/Q will be infinity.

The ratio of either Zero or infinity, there is no exchange of gases through the respiratory membrane of the affected alveoli.

A decrease in ventilation means we are not bringing in enough oxygen to meet our metabolic need for oxygen as well as not blowing enough Carbon Dioxide to get rid of the Carbon Dioxide we produced. An increase in perfusion will have the similar consequence on the blood gases because an increase in perfusion means more blood cells are coming to remove oxygen from the alveolus as they deliver more Carbon Dioxide than will be exhaled.

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