Heliox

Heliox is a breathing gas composed of a mixture of helium (He) and oxygen (O₂).

Heliox is a medical treatment for patients with difficulty breathing. The 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.

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.^[1][2] Heliox is also used in saturation diving and sometimes during the deep phase of technical dives.^[3][4]

Medical uses

In medicine heliox may refer to a mixture of 21% O₂ (the same as air) and 79% He, although other combinations are available (70/30 and 60/40).

Heliox generates less airway resistance than air and thereby requires less mechanical energy to ventilate the lungs.^[5] "Work of Breathing" (WOB) is reduced. It does this by two mechanisms:

1. increased tendency to laminar flow;
2. reduced resistance in turbulent flow.

Heliox has a similar viscosity to air but a significantly lower density (0.5 g/l versus 1.25 g/l at 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.

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). There is also some use of heliox in conditions of the medium airways (croup, asthma and chronic obstructive pulmonary disease).

Patients with these conditions may suffer 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 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.^[6] Heliox has also found utility in the weaning of patients off mechanical ventilation, and in the nebulization of inhalable drugs, particularly for the elderly.^[7] Research has also indicated advantages in using helium–oxygen mixtures in delivery of anaesthesia.^[8]

Diving uses

	Effect of helium on a human voice The effect of helium on a human voice
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Owing to the expense of helium,^[9] heliox is most likely to be used in deep commercial diving. It is also sometimes used by diving enthusiasts, particularly those using rebreathers, which conserve the breathing gas at depth much better than open circuit scuba.

The proportion of oxygen in a diving mix depends on the maximum depth of the dive plan, but it is often hypoxic and typically 10%. Each mix is bespoke and is created using gas blending techniques, which often involve the use of booster pumps to achieve typical diving cylinder pressures of 200 bar (2,900 psi) from lower pressure banks of oxygen and helium cylinders.

Because sound travels faster in heliox than in air, voice formants are raised, making divers' speech very high-pitched and hard to understand to people not used to it.^[10] 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 is a slightly less expensive alternative to heliox for deep diving.^[11] Trimix is often used in commercial diving and in technical diving.

In 2015, the United States Navy Experimental Diving Unit showed that bounce dives using trimix are not a more efficient decompression than dives on heliox.^[12]

See also

• Argox
• Nitrox
• Hydreliox
• Hydrox
• Trimix

References

Diving decompression
General	Decompression (diving) Decompression equipment Decompression practice Decompression theory Equivalent air depth Equivalent narcotic depth History of decompression research and development Maximum operating depth
Algorithms	Bühlmann decompression algorithm Reduced gradient bubble model Thalmann algorithm Varying Permeability Model
Diving	Deep diving Professional diving Recreational diving Saturation diving Scuba diving Technical diving
Equipment	Decompression buoy Decompression tables Decompression trapeze Dive computer Diving chamber Diving shot Hyperbaric stretcher Jonline Recreational Dive Planner
Gases	Air Breathing gas Gas blending Heliox Nitrox Oxygen Trimix
Injury and treatment	Decompression sickness Diving chamber Hyperbaric medicine Hyperbaric treatment schedules In-water recompression
Procedures	Pyle stop Ratio decompression
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