Passive heating may be provided to a flexible tube (202) configured to deliver respiratory gas to a patient. A flexible sheathing body (206, 208, 210) may be removably attached to the flexible tube so as to surround the flexible tube and extend along the flexible tube. A passive heating element (218, 220, 222) integrated with the flexible sheathing body may store non-electrical energy and release the stored non-electrical energy as thermal energy to heat the flexible tube.
