"The comment about the rebreathing bag on portable O2 cylinder is utter rubbish by the way. The bag is there to accommodate the peak inspiratory flow rate during spontaneous respiration. If a patient is not spontaneously breathing then resuscitation is required. Fortunately cabin crew are well trained and know this.
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Just to clear up confusion:a rebreath bag is used when there is
not spontaneous respiration.The bag is squeezed to inflate the lungs when the patient is not able to do so himself, 2 times to 30 chest compressions if there is also cardiac arrest due to inhalation shock.Portable cylinder required,because the patient would need to out of their seat for the chest compressions to be effective.That would be why the purser brought it.