In a patient with fixed respiratory obstruction Helium is used along with Oxygen instead of plain oxygen because –

Helium is given as Heliox (HeO2,), a mixture of 20% to 80% He and 80% to 20% O2 . It has about 1/3rd the density of O2 or air. Because helium is highly diffusible, the use of the mixture greatly reduces the work of breathing in a patient with narrowed airways and thus leads to improved aerosol delivery and increased oxygenation in severe asthma. It may also reduce the risk of barotrauma. It decreases turbulence. In COPD, Helium is used along with O2 to decrease the viscosity of gaseous mixture which increases its linearity & decreases resistance in pathway. It has lower Reynaud's number, is 3 times faster, has low specific gravity & low solubility.