What defines physiologic dead space?

In this short Medmastery article, read about how pulmonary blockage affects ventilation-perfusion ratios in diseased states.
Last update25th Nov 2020

Other alveolar units have ventilation, but no perfusion. These units have a ventilation-perfusion ratio of infinity and are what is referred to as physiologic dead space.

Figure 1. Physiologic dead space occurs when the pulmonary capillary of an alveolar-pulmonary structure has little or no perfusion, resulting in high ventilation-perfusion ratios.

Conceptually, alveolar units with ventilation-perfusion ratios of greater than one, but not infinity, can be described as if the units were functionally equivalent to areas of physiologic dead space.

In diseases characterized by areas of high ventilation-perfusion ratios, hypercapnia usually does not occur because individuals can compensate for the increase in dead space by increasing the overall level of ventilation.

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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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