![]() 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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