Heliox Questions

We read with great interest the article by Dr Weber and colleagues¹ comparing heliox and racemic epinephrine in children with moderate to severe croup. Apart from the study limitations stated by the authors in the discussion, there are some points we would like to make about this article.

1. Helium is an inert gas that is one-eighth the density of nitrogen; when blended with 21% oxygen, the resulting gas mixture (heliox, 79%He/21%O₂) has a threefold reduction in density compared with air. This property reduces the pressure gradient associated with gas flow through airways with nonlaminar flow.² The authors seem to have the misconception that because of its lower density, helium is less viscous than air. Actually, the absolute viscosity of helium is slightly higher than that of air, and its kinematic viscosity is about 7 times that of air: from the fluid-dynamical point of view, helium is much more viscous than air.^2,3 These facts have been theoretically validated, and moreover, we now know that heliox does not need to be laminar to provide higher flow rates and that its benefits persist even under turbulent conditions.³

2. We consider it important to highlight the fact that helium, a gas of inert nature, lacks biological properties. Consequently, the use of heliox in any clinical circumstance is not for treating the underlying disease or for influencing the anatomy of the airway. Rather, heliox is used to reduce resistance of the airways to gas flow and to decrease respiratory muscle work until definitive therapies act or the underlying condition spontaneously resolves.² This means that heliox is a temporizing measure whose positive effects are maintained as long as it is held. In their article, Weber et al compared heliox with a biologically active therapy as epinephrine. Namely, epinephrine causes …