Heliox

See Heliox (cryogenic equipment) for another use of the word Heliox

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

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 generally refers to a mixture of 21% O₂ (the same as air) and 79% He, although other combinations are available.

Airway resistance is dictated by the diameter of the airways and by the density of the inspired gas. Therefore when nitrogen (of air) is replaced by helium, airway resistance is reduced due to the lower density of the inspired gas. This means that when one breathes Heliox, airway resistance is lowered, thereby requiring less mechanical energy to ventilate the lungs, or the "Work of Breathing" (WOB). Heliox is used mainly in the alleviation of many medical conditions that involve a decrease in airway diameter (and consequently increased airway resistance), such as upper airway obstruction, asthma, chronic obstructive pulmonary disease (COPD), bronchiolitis, vocal cord dysfunction and croup. 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.^[5] Heliox has also found utility in the weaning of patients off mechanical ventilation, and in the nebulization of inhalable drugs, particularly for the elderly.^[6] Research has also indicated advantages in using helium/oxygen mixtures in delivery of anaesthesia.^[7]

Diving uses

Owing to the expense of helium^[8], heliox is most likely to be used in deep commercial diving although it is sometimes used by diving enthusiasts using gas-conserving rebreathers.

The proportion of oxygen in a diving mix depends on the maximum depth of the dive plan but 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.^[9]

Trimix is a close rival and slightly less expensive, deep diving, alternative to Heliox.^[10] Trimix is often used in commercial diving and in technical diving.

See also

• Argox
• Nitrox
• Hydreliox
• Hydrox
• Trimix

References

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