View source for Heliox

{{Short description|Breathing gas mixed from helium and oxygen}}
{{cs1 config|name-list-style=vanc|display-authors=6}}
{{about|the breathing gas|the cryogenic system|Heliox (cryogenic equipment)}}
{{Infobox drug
| drug_name          = Heliox
| CAS_number = 58933-55-4 
| PubChem            = 123812
| chemical_formula   = He.O<sub>2</sub>
| molecular_weight   = 36.001
}}

'''Heliox''' is a [[breathing gas]] mixture of [[helium]] (He) and [[oxygen]] (O<sub>2</sub>). It is used as a medical treatment for patients with difficulty breathing because this mixture generates less resistance than atmospheric air when passing through the airways of the lungs, and thus requires less effort by a patient to breathe in and out of the lungs. It is also used as a breathing gas for deep ambient pressure diving as it is not narcotic at high pressure, and for its low [[work of breathing]].

Heliox has been used medically since the 1930s, and although the medical community adopted it initially to alleviate symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, mostly because of the low density of the gas.<ref>{{cite journal | vauthors = Barach AL, Eckman M | title = The effects of inhalation of helium mixed with oxygen on the mechanics of respiration | journal = The Journal of Clinical Investigation | volume = 15 | issue = 1 | pages = 47–61 | date = January 1936 | pmid = 16694380 | pmc = 424760 | doi = 10.1172/JCI100758 }}</ref><ref name="BOCMprod">{{cite web |url=http://www.bocmedical.co.uk/product_information/heliox.asp |title=Heliox product information |publisher=BOC Medical |archive-url=https://web.archive.org/web/20081121031022/http://www.bocmedical.co.uk/product_information/heliox.asp |archive-date=21 November 2008}}</ref> Heliox is also used in [[saturation diving]] and sometimes during the deep phase of [[technical diving|technical dives]].<ref name=usn>{{cite book |title=US Navy Diving Manual, 6th revision |year=2008 |publisher=US Naval Sea Systems Command |location=United States |url=http://www.supsalv.org/00c3_publications.asp?destPage=00c3&pageID=3.9 |access-date=2008-07-08 |archive-date=2008-05-02 |archive-url=https://web.archive.org/web/20080502023541/http://www.supsalv.org/00c3_publications.asp?destPage=00c3&pageId=3.9 |url-status=dead }}</ref><ref name=Brubakk>{{cite book |title=Bennett and Elliott's physiology and medicine of diving | edition = 5th Rev | vauthors = Brubakk AO, Neuman TS |year=2003 |publisher=Saunders Ltd. |location=United States |isbn=0-7020-2571-2 |pages=800 }}</ref><ref>{{cite web | url = https://www.divingheritage.com/moderncomex.htm | title = COMEX PRO }}</ref>

== Medical uses ==

There is also some use of heliox in conditions of the medium airways ([[croup]], asthma and [[chronic obstructive pulmonary disease]]). A recent trial has suggested that lower fractions of helium (below 40%){{snd}} thus allowing a higher fraction of oxygen{{snd}} might also have the same beneficial effect on upper airway obstruction.<ref>{{cite journal | vauthors = Truebel H, Wuester S, Boehme P, Doll H, Schmiedl S, Szymanski J, Langer T, Ostermann T, Cysarz D, Thuermann P | title = A proof-of-concept trial of HELIOX with different fractions of helium in a human study modeling upper airway obstruction | journal = European Journal of Applied Physiology | volume = 119 | issue = 5 | pages = 1253–1260 | date = May 2019 | pmid = 30850876 | doi = 10.1007/s00421-019-04116-7 | s2cid = 71715570 }}</ref>

Patients with these conditions may develop a range of symptoms including [[dyspnea]] (breathlessness), hypoxemia (below-normal oxygen content in the arterial blood) and eventually a weakening of the respiratory muscles due to [[Fatigue|exhaustion]], which can lead to respiratory failure and require intubation and mechanical ventilation. Heliox may reduce all these effects, making it easier for the patient to breathe.<ref name="BOCMdata">{{cite web |url=http://www.bocmedical.co.uk/product_information/Medical_Helium_Oxygen_Mix_172.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.bocmedical.co.uk/product_information/Medical_Helium_Oxygen_Mix_172.pdf |archive-date=2022-10-09 |url-status=live |title=Heliox data sheet |author=BOC Medical}}</ref> Heliox has also found utility in the weaning of patients off mechanical ventilation, and in the nebulization of inhalable drugs, particularly for the elderly.<ref name="AnaesUK">{{cite journal | vauthors = Lee DL, Hsu CW, Lee H, Chang HW, Huang YC | title = Beneficial effects of albuterol therapy driven by heliox versus by oxygen in severe asthma exacerbation | journal = Academic Emergency Medicine | volume = 12 | issue = 9 | pages = 820–827 | date = September 2005 | pmid = 16141015 | doi = 10.1197/j.aem.2005.04.020 | doi-access = free }}</ref> Research has also indicated advantages in using helium–oxygen mixtures in delivery of [[anaesthesia]].<ref name="BJA">{{cite journal | vauthors = Buczkowski PW, Fombon FN, Russell WC, Thompson JP | title = Effects of helium on high frequency jet ventilation in model of airway stenosis | journal = British Journal of Anaesthesia | volume = 95 | issue = 5 | pages = 701–705 | date = November 2005 | pmid = 16143576 | doi = 10.1093/bja/aei229 | doi-access = free }}</ref>

=== Available forms ===
In [[medicine]], heliox may refer to a mixture of 21% O<sub>2</sub> (the same as [[air]]) and 79% He, although other combinations are available (70/30 and 60/40).

== 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]]).

== Usage in diving ==
<!--target for redirect from [[Heliox diving]]-->
{{Listen|right|filename=Helium article read with helium.ogg|title=Effect of helium on a human voice|description=The effect of helium on a human voice|format=[[Ogg]]}}

Helium diluted breathing gases are used to eliminate or reduce the effects of [[inert gas narcosis]], and to reduce [[work of breathing]] due to increased gas density at depth. From the 1960s saturation diving physiology studies were conducted with helium from {{convert|45|to|610|m|ft|abbr=on}} over several decades by a Hyperbaric Experimental Centre operated by the French company [[Compagnie maritime d'expertises|COMEX]] specializing in engineering and deep diving operations.<ref name="Hydra 8">{{cite web |url=http://www.comex.fr/suite/ceh/histo/histo%20anglais.html |title=Extreme Environment Engineering Departement Hyperbaric Experimental Centre - History |access-date=2009-02-22 |url-status=dead |archive-url=https://web.archive.org/web/20081005073910/http://www.comex.fr/suite/ceh/histo/histo%20anglais.html |archive-date=October 5, 2008 }}</ref> Owing to the expense of helium,<ref name="Fills">{{cite web |url=http://www.fillexpress.com/fills.shtml#prices |title=Example pricing for filling cylinders |access-date=2008-01-10 |archive-url=https://web.archive.org/web/20080116133910/http://fillexpress.com/fills.shtml#prices |archive-date=2008-01-16 |url-status=dead }}</ref> heliox is most likely to be used in deep [[saturation diving]]. It is also sometimes used by [[Technical diving|technical divers]], particularly those using [[Diving rebreather|rebreathers]], which conserve the breathing gas at depth much better than [[open circuit scuba]]. Use of heliox mixtures is known as heliox diving, a subs-category of mixed gas diving, also known simply as gas diving.<ref name="Lee et al 2020" >{{cite journal | vauthors = Lee DW, Jung SJ, Ju JS | title = The effects of heliox non-saturation diving on the cardiovascular system and cognitive functions | journal = Undersea & Hyperbaric Medicine | volume = 47 | issue = 1 | pages = 93–100 | date = First Quarter 2020 | pmid = 32176950 | doi = 10.22462/01.03.2020.10 }}</ref>

[[File:IMCA Heliox shoulder quartered.svg|thumb|[[Diving_cylinder#Surface_finish,_colour-coding_and_labeling|Heliox  diving cylinder color coding]]: Illustration of cylinder shoulder painted in brown and white quarters]]
[[File:IMCA Heliox shoulder.svg|thumb|Illustration of cylinder shoulder painted in brown (lower) and white (upper) bands]]

The proportion of oxygen in a diving mix depends on the maximum depth of the dive plan, but it is often [[wiktionary:hypoxic|hypoxic]] and may be less than 10%. Each mix is custom made using [[gas blending]] techniques, which often involve the use of [[booster pump]]s to achieve typical [[diving cylinder]] pressures of {{convert|200 to 300|bar|psi|abbr=on|lk=on}} from lower pressure banks of oxygen and helium cylinders.

Because [[sound]] travels faster in heliox than in air, [[human voice|voice]] [[formant]]s are raised, making divers' speech very high-pitched and hard to understand to people not used to it.<ref>{{cite journal | vauthors = Ackerman MJ, Maitland G | title = Calculation of the relative speed of sound in a gas mixture | journal = Undersea Biomedical Research | volume = 2 | issue = 4 | pages = 305–310 | date = December 1975 | pmid = 1226588 | url = http://archive.rubicon-foundation.org/2738 | access-date = 2008-07-08 | url-status = usurped | archive-url = https://web.archive.org/web/20110127113335/http://archive.rubicon-foundation.org/2738 | archive-date = 2011-01-27 }}</ref> Surface personnel often employ a piece of communications equipment called a "helium de-scrambler", which electronically lowers the pitch of the diver's voice as it is relayed through the communications gear, making it easier to understand.

[[Trimix (breathing gas)|Trimix]] is a less expensive alternative to heliox for deep diving, which uses only enough helium to limit narcosis and gas density to tolerable levels for the planned depth.<ref name=stone>{{cite journal | vauthors = Stone WC |year=1992 |title=The case for heliox: a matter of narcosis and economics. |journal=AquaCorps |volume=3 |issue=1 |pages=11–16 }}</ref> Trimix is often used in [[technical diving]], and is also sometimes used in [[professional diving]].

In 2015, the [[United States Navy Experimental Diving Unit]] showed that decompression from bounce dives using trimix is not more efficient  than dives on heliox.<ref name=NEDU2015-4>{{cite journal |vauthors=Doolette DJ, Gault KA, Gerth WA |year=2015 |title=Decompression from He-N2-O2 (trimix) bounce dives is not more efficient than from He-O2 (heliox) bounce dives. |journal=US Navy Experimental Diving Unit Technical Report 15-4 |url=http://archive.rubicon-foundation.org/10576 |access-date=2015-12-30 |archive-date=2017-07-07 |archive-url=https://web.archive.org/web/20170707084659/http://archive.rubicon-foundation.org/xmlui/handle/123456789/10576 |url-status=usurped }}</ref>

== See also ==
* {{annotated link|Argox}}
* {{annotated link|Nitrox}}
* {{annotated link|Hydreliox}}
* {{annotated link|Hydrox (breathing gas)|Hydrox}}
* {{annotated link|Trimix (breathing gas)|Trimix}}

== References ==
{{Reflist}}

== Further reading ==
* {{cite journal | vauthors = Hashemian SM, Fallahian F | title = The use of heliox in critical care | journal = International Journal of Critical Illness and Injury Science | volume = 4 | issue = 2 | pages = 138–142 | date = April 2014 | pmid = 25024941 | pmc = 4093964 | doi = 10.4103/2229-5151.134153 | doi-access = free }}

== External links ==
* {{cite web | url = https://druginfo.nlm.nih.gov/drugportal/name/heliox | archive-url = https://web.archive.org/web/20210127210104/https://druginfo.nlm.nih.gov/drugportal/name/heliox | url-status = dead | archive-date = January 27, 2021 | publisher = U.S. National Library of Medicine | work = Drug Information Portal | title = Heliox }}

{{Respiratory system procedures}}
{{Underwater diving|divequ}}
{{Portal bar | Medicine}}

[[Category:Breathing gases]]
[[Category:Asthma]]
[[Category:Helium]]
[[Category:Medical treatments]]
[[Category:Respiratory therapy]]
[[Category:Underwater diving safety equipment]]

[[ro:Heliox]]