Helium and Heliox

Helium

• Helium is the lightest noble gas with a density of 0.18 g/m³, which is much lower than oxygen (1.43 g/m³) and nitrogen (1.25 g/m³).^1,2
• It is colorless, odorless, and tasteless, and unlike most gases, it exists in a monatomic state.³ It is nontoxic at room temperature and is almost physiologically inert.³
• Helium is the second most abundant element in the universe, after hydrogen. It is found in very high amounts in the stars. On Earth, it is found in very low quantities in the atmosphere and in natural gas fields, or it can be extracted from cleveite, a mineral found in uranium deposits.^3,4
• Globally, helium stores are depleting, and there is a call for increased production.²

Heliox

• Heliox is a mixture of helium and oxygen, typically in a 70:30 or 80:20 ratio of helium to oxygen. Its low density reduces airway resistance, especially in turbulent flow, thereby decreasing the work of breathing and improving ventilation in conditions such as upper airway obstruction, asthma, and exacerbations of COPD.^1-5
• It can also be used to deliver bronchodilator therapy in patients with severe asthma. However, higher flows are required.
• Heliox’s benefits are most pronounced when the patient’s oxygen requirements are not high, as increasing the oxygen fraction reduces the mixture’s low-density advantage.^1,2
• Heliox can be delivered via face mask, noninvasive ventilation, or mechanical ventilation, but technical considerations (such as ventilator calibration and gas flow measurement inaccuracies) must be understood to avoid complications.⁵

Heliox Therapy for Airway Obstruction

• Gas flow in the airways can be categorized as laminar and turbulent flow (Figure 1).
  • Laminar flow is characterized by a smooth, parabolic velocity profile of the gas within the airways, with the greatest velocity at the center of the airway.
  • Turbulent flow is characterized by random, radial, and axial air movement.
  • Laminar flow typically occurs distal to the small bronchioles in airways less than 1 mm in diameter, while flow in larger airways is more likely to be turbulent.
  • Turbulent flow is more likely to be observed with higher gas flow, at angles or branch points in the respiratory tree, and abrupt changes in the airway diameter.
  • The Reynolds number predicts whether the flow will be laminar or turbulent. A number <1000 predicts laminar flow, while a number >1500 predicts turbulent flow.

• Laminar flow in the respiratory system is directly proportional to the pressure gradient between the mouth or nose and alveoli and inversely proportional to the resistance.

• • Resistance is determined by Poiseuille’s law:

• • where R is resistance, η is the viscosity of inspired air, l is the length of the airway, and r is the radius of the airway.
• Turbulent flow is less easily modeled mathematically. In turbulent flow, resistance increases in proportion to gas flow and is directly proportional to gas density and inversely proportional to the 5th power of the airway radius:

• • In turbulent flow states, flow becomes less dependent on viscosity and more dependent on density.³
  • Because the amount of pressure needed to produce flow is dependent on density, as a gas with lower density heliox requires less pressure to move through a partially obstructed airway.⁴
• Airway obstruction is associated with an increase in airway resistance, resulting in an increased work of breathing, which can eventually lead to fatigue, hypoxia, and hypercarbia.
• The potential benefits of heliox in patients with airway obstruction include the following:
  • Reduction in respiratory distress: Heliox’s low density reduces airway resistance by facilitating the transition from turbulent to laminar flow by lowering the Reynolds number. This reduces the work of breathing and improves ventilation.^2-5
  • Improved respiratory efficiency: the combination of reduced work of breathing, bulk flow shifting distally, and the fact that gas molecules diffuse four times faster through heliox than through air improves alveolar ventilation.
  • Reduction in PaCO₂: Intrinsic positive end-expiratory pressure, hyperinflation, and PaCO₂ may be reduced secondary to improved alveolar ventilation or decreased CO₂ production due to lower work of breathing.^2-5
  • Enhanced respiratory drug delivery: Heliox increases the deposition of nebulized drugs in the distal airways by improved laminar flow.
  • However, the increased viscosity of helium compared to nitrogen may increase airway resistance in the smaller airways where the flow is primarily laminar, thereby explaining the equivocal benefits of heliox in diseases affecting the distal airways.^2,5

Medical Uses of Helium

• While heliox has been used for several decades in patients with various respiratory diseases, there is very little evidence to guide its use.²
• As mentioned earlier, Heliox’s benefits are most pronounced when the patient’s oxygen requirements are not high, as increasing the oxygen fraction reduces the mixture’s low-density advantage.^1,2

Partial Upper Airway Obstruction

• There are several case reports of heliox use as a bridging therapy in patients with upper airway obstruction resulting from vocal cord dysfunction, malignancies, radiation therapy, and other conditions.^2,5 Heliox potentially provides time for concurrent therapies, such as corticosteroids, bronchoscopy, and chemotherapy, to take effect, thereby reducing the need for escalation of care.^2,5

Asthma

• Similarly, there are several case reports of successful heliox use in patients with acute severe asthma. Authors have reported a greater improvement in peak expiratory flow rates and a reduction in the need for endotracheal intubation compared to the control group.
• However, the level of evidence is too low to recommend the routine use of heliox in this setting.^2,5
• Heliox can also be used as a driving gas to deliver bronchodilator therapy in patients with severe asthma. However, higher flows are required.

COPD

• Several studies have also reported the benefits of using heliox in patients with acute COPD exacerbations, whether breathing spontaneously, during noninvasive ventilation, mechanical ventilation, or during the weaning period.⁵
• A randomized controlled trial aimed to determine if patients with COPD exacerbation requiring noninvasive ventilation had improved outcomes with heliox. This group found that while heliox improved respiratory acidosis and reduced respiratory rate, patients treated with heliox did not have lower rates of intubation compared to those treated with an oxygen/air mixture.⁶

Mechanically Ventilated Patients

• Heliox use may be considered in mechanically ventilated patients with severe asthma with mild hypoxemia and worsening hyperinflation despite standard therapies.²
• It is critical to ensure that the ventilator is compatible with helium. The high thermal conductivity of helium precludes its use in ventilators with hot wire sensors.

Supercooling in Magnetic Resonance Imaging (MRI)

• MRI coils are housed in a double-walled apparatus that is bathed in liquid helium. The apparatus is kept in a vacuum and placed inside a tank filled with liquid nitrogen.
• Quenching is an emergency shutdown of the magnet that involves the rapid boiling of liquid helium from the device. If this liquid helium cannot be dissipated through external vents, it may be released into the scanner room, displacing oxygen and presenting a risk for asphyxiation.³

Anesthesia for Airway Surgery

• Unlike xenon, helium itself does not have anesthetic properties and cannot induce anesthesia, even at high pressures.⁷
• However, it can be used as a carrier gas during anesthesia for airway surgery, especially when spontaneous ventilation is desired, as it may improve gas exchange and reduce airway resistance.⁸