The Fink effect, also known as "diffusion anoxia", "diffusion hypoxia", or the "third gas effect", is a factor that influences the pO2 (partial pressure of oxygen) within the alveolus. When water-insoluble 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. 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. 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.