Editing Heliox (section)

== Mechanism of action ==

Heliox generates less airway resistance than air and thereby requires less mechanical energy to ventilate the lungs.<ref name="BOCheox21">{{cite web |url=http://www.bochealthcare.co.uk/en/products/heliox/index.shtml |title=Heliox21 |publisher=Linde Gas Therapeutics |date=27 January 2009 |access-date=13 April 2011}}</ref> "[[Work of breathing]]" (WOB) is reduced by two mechanisms:
# increased tendency to [[laminar flow]];
# reduced resistance in [[turbulent flow]] due to lower density.
Heliox 20/80 diffuses 1.8 times faster than oxygen, and the flow of heliox 20/80 from an oxygen flowmeter is 1.8 times the normal flow for oxygen.<ref>{{cite journal | vauthors = Hess DR, Fink JB, Venkataraman ST, Kim IK, Myers TR, Tano BD | title = The history and physics of heliox | journal = Respiratory Care | volume = 51 | issue = 6 | pages = 608–612 | date = June 2006 | pmid = 16723037 | url = https://rc.rcjournal.com/content/respcare/51/6/608.full.pdf | url-status = live | archive-url = https://ghostarchive.org/archive/20221009/https://rc.rcjournal.com/content/respcare/51/6/608.full.pdf | archive-date = 2022-10-09 }}</ref>

Heliox has a similar [[viscosity]] to air but a significantly lower [[density]] (0.5&nbsp;g/L versus 1.25&nbsp;g/L at [[Standard temperature and pressure|STP]]). Flow of gas through the airway comprises laminar flow, transitional flow and turbulent flow. The tendency for each type of flow is described by the [[Reynolds number]]. Heliox's low density produces a lower Reynolds number and hence higher probability of laminar flow for any given airway. Laminar flow tends to generate less resistance than turbulent flow.

In the small airways where flow is laminar, resistance is proportional to gas viscosity and is not related to density and so heliox has little effect. The [[Hagen–Poiseuille equation]] describes laminar resistance. In the large airways where flow is turbulent, resistance is proportional to density, so heliox has a significant effect.

=== History ===
Heliox has been used medically since the early 1930s. It was the mainstay of treatment in acute [[asthma]] before the advent of [[bronchodilators]]. Currently, heliox is mainly used in conditions of large airway narrowing (upper airway obstruction from tumors or foreign bodies and [[vocal cord dysfunction]]).