Heliox

Helium is much less dense than either oxygen or nitrogen, and replacing the nitrogen in air with helium decreases its resistance to flow. Helium will increase ventilation in patients with nearly all types of upper airway obstruction. 2 24 It may also restore laminar flow, improving ventilation and ventilation/perfusion matching. The work of breathing also decreases. Carbon dioxide diffuses much faster through a helium-oxygen mixture than it does through a nitrogen-oxygen mixture. This may make ventilation more effective. The indications for the use of heliox use are still somewhat controversial. 24 Most would agree that children not requiring high-flow oxygen should be given a trial of heliox for poor ventilation secondary to upper airway partial obstruction. If nothing else, this may serve as a temporizing measure and allow for a more controlled intubation. Following intubation, helium may be given through the ventilator circuit as a mixture with oxygen. Volume and flow readings from the ventilator will be inaccurate if helium is used,24 since ventilators are calibrated for use with a nitrogen-oxygen mixture.

One easily forgotten point is that oxygen and helium gas cannot occupy the same space. Due to the rule of partial pressures, if the helium concentration is increased, the oxygen concentration must decrease. Helium's therapeutic effects are found only when its concentration makes up 60 to 80% of the inspired gas. Therefore, an individual who requires greater than 35 to 40% oxygen cannot use this modality. The flow of oxygen and helium must be high enough to exceed the patient's minute ventilation to prevent entrainment of room air and increased dead-space ventilation.

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