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

Heliox is a mixture of 80% Helium and 20% O2. Its density is about ? rd of air and has a low Reynolds number and hence low turbulence. It reduces the workload of breathing, improves dyspnea, and aerial blood gases in patients with fixed respiratory obstruction. It also decreases the chance for pneumothorax. Ref: Clinical Anesthesiology By G. Edward Morgan, 4th Edition, Chapter 49 ; Principles of Critical Care By Jesse B. Hall, 3rd Edition, Chapter 40