The data would also be compatible with a high tracheal or laryngeal area of narrowing/collapse, as after a tracheostomy.

Another cause of this presentation is a high tracheal tumour, which, because of its slow onset, may mimic asthma. However, the wheeze will be fixed and the symptoms constant, unlike in asthma. iii. A mixture of oxygen (21%) and helium (79%) as helium is less dense than nitrogen, allowing a greater flow of gas.