In acute severe asthma, the use of heliox can reduce dyspnea, when the patient is spontaneously breathing as well as in mechanical ventilation. This effect is due to a decrease in airway resistance. A better penetration of aerosolized bronchodilators has also been observed. However, the clinical benefit of these physiological measurable effects remains undetermined. Heliox could nevertheless be interesting in emergency situations in order to avoid endotracheal intubation, and in very difficult cases when mechanical ventilation is almost impossible to perform. This gas mixture could also be used with non-invasive mechanical ventilation, but this indication is presently investigated.