Helium advantages: Small-bore tube

Gas flow through a tube is either laminar (parabolic velocity profile) or turbulent (flat velocity profile). The nature of flow is dependent on several physical properties of the gas (density, viscosity) as well as of the flow itself (velocity, length of tubing), all of which can be combined mathematically into the Reynold’s number, Re (Re = ρUL/visc., where ρ is the density of the fluid, U is the mean flow velocity, L is length of flow, and visc. is the viscosity). When Re > 2100, gas flow becomes turbulent

During laminar flow, resistance depends on gas viscosity (laminar flow is modeled as concentric shells sliding across each other, thus their “stickiness” would be important), however during turbulent flow, resistance depends on gas density (turbulent flow is modeled as semi-random movement of gas molecules [which is energetically inefficient], thus the denser the molecules, the more energy which is required to move them).

Gas Flow: Summary

• Reynold’s Number: Re = ρUL/visc, < 2100 is laminar, > 2100 is turbulent
• Laminar (concentric shells): resistance depends on gas viscosity (air and Heliox are the same viscosity)
• Turbulent (semi-random movement): resistance depends on gas density (Heliox much less dense than air)

Heliox (70% helium, 30% oxygen) therefore affects gas flow in two ways – first, the Reynold’s number is lower (because density is lower), and thus flow is more likely to be laminar, and second, if flow IS turbulent, resistance to flow will be lower, because Heliox is less dense (~ 0.5 g/L versus 1.2 5g/L).

Note that in laminar flow, Heliox offers no advantages, as it has approximately the same viscosity as air. Also, note that Heliox is ~ 70% helium, thus the potential benefit of improved gas flow must be balanced by the knowledge that a lower FiO₂ may be delivered.

Heliox: Summary

• ~ 70% helium, ~ 30% oxygen
• Significantly less dense than air (0.5 vs 1.2 5g/L), but viscosity is about the same
• Improves gas flow by lowering Reynold’s number (converting to laminar flow) or decreasing resistance when flow is turbulent (lower density)