Heliox

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

Heliox has been used in a medical context since the 1930s, and although the medical community adopted its use initially to alleviate the symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, most of which are dependent on the low density of the gas. Heliox is also used in saturation diving and sometimes during the deep phase of technical dives.

Medical uses

In medicine, "heliox" generally describes a mixture that is 21% O₂ (the same as air) and 79% He, although other mixtures 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 less, and therefore the mechanical energy required to ventilate the lungs, or the Work of Breathing (WOB) is decreased. 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 and croup. Patients with these conditions may suffer a range of symptoms including dyspnoea (breathlessness), hypoxaemia (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, and as it will reduce work of breathing, Heliox can help to prevent this respiratory failure. Heliox has also found utility in the weaning of patients off mechanical ventilation, and in the nebulisation of inhalable drugs.

Diving uses

Due to the expense of helium, heliox is most likely to be used in deep commercial diving although it is sometimes used by amateurs in 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 / 3000 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.

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

Cryogenic equipment

Heliox is also a cryogenically cooled system produced by Oxford Instruments. Presently available in 2 varieties, the VL and TL, vertically-loaded and top-loaded respectively. They are both pumped He3 cryostats, the TL capable of magnetic fields of up to 14 T, and the VL capable of achieving magnetic fields of up to 2 T. The base temperature for both systems is ~250 mK. Whilst the basis of operation of system is pumping of liquid Helium 3 below 2.2 K, this low temperature is achieved by first cooling the system to 2.2 K by pumping of Helium 4. A constant supply of liquid He4 is necessary, constituting a typical overhead of ~£1 / litre, whilst He3 is efficiently conserved as it is valued at ~£300 / litre!.

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