The ability to climb a flight of 15 stairs and walk 50 meters without symptoms is an indication that a patient with cardiac or respiratory disease will cope with the relative hypoxia of air travel.
A health summary and medication list can be helpful.
Supplementary oxygen
If this is needed the airline must be informed well in advance.
Rate of flow and delivery system specified.
Most airlines will insist that they supply the oxygen cylinder and there will be a charge for this.
If a nebuliser is needed in flight it needs to be approved by the airline in advance.