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The [[helium dilution technique]] for measuring lung volumes uses a closed, rebreathing circuit.<ref name="pmid5475674">{{cite journal | vauthors = Hathirat S, Mitchell M, Renzetti AD | title = Measurement of the total lung capacity by helium dilution in a constant volume system | journal = The American Review of Respiratory Disease | volume = 102 | issue = 5 | pages = 760–70 | date = November 1970 | pmid = 5475674 | doi = 10.1164/arrd.1970.102.5.760 | doi-broken-date = 31 January 2024 }}</ref> This technique is based on the assumptions that a known volume and concentration of helium in air begin in the closed [[spirometer]], that the patient has no helium in their lungs, and that an equilibration of helium can occur between the spirometer and the lungs.{{cn|date=November 2023}}
==== Nitrogen
{{Main|Nitrogen washout}}
The [[nitrogen washout]] technique uses a non-rebreathing open circuit. The technique is based on the assumptions that the nitrogen concentration in the lungs is 78% and in equilibrium with the atmosphere, that the patient inhales 100% oxygen and that the oxygen replaces all of the nitrogen in the lungs.<ref>{{cite journal | vauthors = Boren HG, Kory RC, Syner JC | title = The Veterans Administration-Army cooperative study of pulmonary function: II. The lung volume and its subdivisions in normal men. | journal = The American Journal of Medicine | date = July 1966 | volume = 41 | issue = 1 | pages = 96–114 | doi = 10.1016/0002-9343(66)90008-8 }}</ref>
===Plethysmography===
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