What's a physiologic shunt?

In this article on physiologic shunts, learn about the dangers of ventilation-perfusion imbalances in disease.
Last update4th Dec 2020

More common than no ventilation relative to perfusion in a lung region is an imbalance of ventilation and perfusion, where the ventilation-perfusion ratio falls below the ideal of one but remains greater than zero. These units can be described as if they were the functional equivalent of units with a ventilation-perfusion ratio of zero—this is what is known as shunt physiology. Units with a low ventilation-perfusion ratio, as well as those with a ventilation-perfusion ratio of zero, contribute to the lowering of PaO2 in disease.

Figure 1. Situations where the ventilation-perfusion ratios fall below one include anatomic shunts and physiologic shunts. Anatomic shunts cause a ventilation-perfusion ratio of zero, and physiologic shunts cause a low ventilation-perfusion ratio, contributing to the lowering of partial pressure of oxygen (PaO2) in disease.

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When shunt physiology is the basis for hypoxemia, administration of supplemental oxygen fails to completely correct the hypoxemia, because no matter how high the PAO2, pulmonary capillary blood flowing through the shunt vessel fails to come in contact with the alveoli. Consequently, PaO2 does not improve to the degree expected with administration of pure oxygen.

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Recommended reading

  • Grippi, MA. 1995. “Gas exchange in the lung”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Grippi 1995, 137–149)
  • Grippi, MA. 1995. “Clinical presentations: gas exchange and transport”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Grippi 1995, 171–176)
  • Grippi, MA and Tino, G. 2015. “Pulmonary function testing”. In: Fishman's Pulmonary Diseases and Disorders, edited by MA, Grippi (editor-in-chief), JA, Elias, JA, Fishman, RM, Kotloff, AI, Pack, RM, Senior (editors). 5th edition. New York: McGraw-Hill Education. (Grippi and Tino 2015, 502–536)
  • Tino, G and Grippi, MA. 1995. “Gas transport to and from peripheral tissues”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Tino and Grippi 1995, 151–170)
  • Wagner, PD. 2015. The physiologic basis of pulmonary gas exchange: implications for clinical interpretation of arterial blood gases. Eur Respir J45: 227–243. PMID: 25323225

About the author

Michael A. Grippi, MD
Vice Chairman, Department of Medicine | Pulmonary, Allergy, and Critical Care Division, Perelman School of Medicine, University of Pennsylvania, USA.
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