The Fink effect, also known as , "diffusion hypoxia", or the "third gas effect", is a factor that influences the pO2 (partial pressure of oxygen) within the alveolus. When soluble gases (e.g. nitrous oxide, N2O) are breathed in large quantities they can be dissolved in body fluids rapidly. This can lead to a temporary increase in the concentration of oxygen and carbon dioxide in the alveolus, causing an increase in their respective partial pressures.
The effect is named for Bernard Raymond Fink (1914–2000), whose 1955 paper first explained it. 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. This could cause the partial pressure of oxygen to decrease and could temporarily lead to hypoxia. The decrease in CO2 could also potentiate this effect as ventilation would be suppressed, leading to potential hypoxaemia. Nonetheless, this effect would only last a couple of minutes and hypoxia can be avoided by increasing the fractional inspired oxygen concentration when recovering from N2O anaesthesia.