What is the role of blood flow and ventilation in the pathophysiology of respiratory failure?

Updated: Apr 07, 2020
  • Author: Ata Murat Kaynar, MD; Chief Editor: Michael R Pinsky, MD, CM, Dr(HC), FCCP, FAPS, MCCM  more...
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During ideal gas exchange, blood flow and ventilation would perfectly match each other, resulting in no alveolar-arterial oxygen tension (PO2) gradient. However, even in normal lungs, not all alveoli are ventilated and perfused perfectly. For a given perfusion, some alveoli are underventilated, while others are overventilated. Similarly, for known alveolar ventilation, some units are underperfused, while others are overperfused.

The optimally ventilated alveoli that are not perfused well have a large ventilation-to-perfusion ratio (V/Q) and are called high-V/Q units (which act like dead space). Alveoli that are optimally perfused but not adequately ventilated are called low-V/Q units (which act like a shunt).

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