Helium improves flow (decreases resistance) when flow is through an orifice or other obstruction. That is, the flow in both the trachea and the endotracheal tube will be turbulent whether the gas mixture is oxygen and 60% nitrogen or 60% helium. However, flows in adult-sized endotracheal tubes and large airways are so far above their transition Reynolds number values that this difference is of little consequence. This would reduce friction, because friction in a laminar flow is generally less than that in a turbulent flow at the same velocity. Because transition to turbulent flow occurs at a particular value of Re for a given flow geometry (e.g., flow in a tube), the use of helium theoretically could delay the transition to turbulence. Thus for a given flow velocity U and dimension D, the Reynolds number for helium will be less than that for air by a factor of 8 (the ratio of kinematic viscosities).
The difference in kinematic viscosity ( v) between air and helium leads to a difference in the Reynolds number, Re = UD/v. If the flow pattern in the endotracheal tube were laminar, replacing nitrogen with helium actually would increase the pressure loss by about 6%, due to the increased absolute viscosity. Where Q = flow, P = pressure loss, r = radius, μ = viscosity, L = length. Absolute viscosity determines resistance when flow is laminar.
This leads to a corresponding difference in the kinematic viscosity, v, which is the ratio of absolute viscosity to density: 11.69 × 10 -5 m 2/s for helium versus 1.52 × 10 -5 m 2/s for air ( 2). The main physical difference between helium and air is the density: 0.166 kg/m 3 for helium versus 1.21 kg/m 3 for air. Rampil ( 1) incorrectly states that (when helium/oxygen is substituted for nitrogen/ oxygen) the lower viscosity of helium, “will allow the use of a smaller endotracheal tube without turbulence and high resistance to gas flow.” In actual fact, the absolute viscosity, μ, of helium at room temperature (20☌) is slightly greater than that of air: 1.94 × 10 -5 kg/m-s for helium versus 1.83 × 10 -5 kg/m-s for air. In the recent, otherwise excellent review of anesthetic considerations for laser surgery.
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