Fink effect

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The Fink effect, also known as "diffusion anoxia",[1] "diffusion hypoxia",[2] or the "third gas effect",[3] is a factor that influences the pO2 (partial pressure of oxygen) within the alveolus. When water-soluble gases such as anesthetic agent N2O (nitrous oxide) are breathed in large quantities they can be dissolved in body fluids rapidly. This leads to a temporary increase in both the concentrations and partial pressures of oxygen and carbon dioxide in the alveolus.

The effect is named for Bernard Raymond Fink (1914–2000), whose 1955 paper first explained it.[1][4] When a patient is recovering from N2O anaesthesia, large quantities of this gas cross from the blood into the alveolus (down its concentration gradient) and so for a short period of time, the O2 and CO2 in the alveolus are diluted by this gas. A sufficiently large decrease in the partial pressure of oxygen leads to hypoxia. The decrease in CO2 pressure can also potentiate this effect when ventilation is suppressed, leading to potential hypoxaemia. Nonetheless, this effect only lasts a couple of minutes and hypoxia can be avoided by increasing the fractional inspired oxygen concentration when recovering from N2O anaesthesia.[5] It is for this reason that Entonox, a 50:50 combination of nitrous oxide and oxygen, is suitable for use by para-medical staff such as ambulance officers: it provides sufficient nitrous oxide for pain relief with sufficient oxygen to avoid hypoxia.[6][7]


  1. 1.0 1.1 J. Roger Maltby (2002). Notable Names in Anaesthesia. Royal Society of Medicine Press. p. 63. ISBN 978-1-85315-512-3.
  2. S. Ahanatha Pillai (2007). Understanding Anaesthesiology. Jaypee Brothers Publishers. p. 101. ISBN 978-81-8448-169-3.{{cite book}}: CS1 maint: url-status (link)
  3. Steven M. Yentis; Nicholas P. Hirsch; Gary B. Smith (2009). Anaesthesia and Intensive Care A–Z: An Encyclopedia of Principles and Practice. Elsevier Health Sciences. ISBN 978-0-443-06785-3.
  4. Bernard R. Fink (1955). "Diffusion Anoxia". Anesthesiology. 16 (4): 511–519. doi:10.1097/00000542-195507000-00007. PMID 13238868.
  5. Andrew B. Lumb; John F. Nunn (2005). Nunn's Applied Respiratory Physiology (6th ed.). Elsevier/Butterworth Heinemann. p. 169. ISBN 978-0-7506-8791-1. Archived from the original on 2021-08-28. Retrieved 2016-09-23.
  6. "Entonox". AnaesthesiaUK ( 26 January 2009. Archived from the original on 31 October 2007. Retrieved 27 February 2017.
  7. Joanne D. Fisher; Simon N. Brown; Matthew W. Cooke (October 2006). UK Ambulance Service Clinical Practice Guidelines (2006) (PDF). Joint Royal Colleges Ambulance Liaison Committee. ISBN 1-84690-060-3. Archived (PDF) from the original on 5 June 2011. Retrieved 27 February 2018.