A patient receiving 70% helium in oxygen to ease airway obstruction: which gas property change best explains the decrease in airway resistance?

A patient receiving 70% helium in oxygen to ease airway obstruction: which gas property change best explains the decrease in airway resistance?

• A. Decreases viscosity
• B. Decreases friction coefficient
• C. Decreases density
• D. Increases Reynolds number

Answer: (C)

The major idea is that in narrowed airways, flow tends to become turbulent, and resistance is strongly affected by how dense the gas is. Helium-oxygen mixtures are much lighter than air, so the density decreases. In the Reynolds number relationship, resistance to flow through the airways increases with density, because density is in the numerator of Reynolds number. Lower density lowers Reynolds number, making the flow more laminar and reducing energy losses from turbulence, which translates to easier ventilation and less airway resistance. Viscosity changes with a helium-oxygen mix are minimal, so they don’t drive the change in resistance. The friction coefficient is not a primary gas property that explains the effect here, and increasing Reynolds number would actually imply more turbulence, not less.

The major idea is that in narrowed airways, flow tends to become turbulent, and resistance is strongly affected by how dense the gas is. Helium-oxygen mixtures are much lighter than air, so the density decreases. In the Reynolds number relationship, resistance to flow through the airways increases with density, because density is in the numerator of Reynolds number. Lower density lowers Reynolds number, making the flow more laminar and reducing energy losses from turbulence, which translates to easier ventilation and less airway resistance.

Viscosity changes with a helium-oxygen mix are minimal, so they don’t drive the change in resistance. The friction coefficient is not a primary gas property that explains the effect here, and increasing Reynolds number would actually imply more turbulence, not less.