Opioid — pinpoint pupils, slow shallow rate. Ventilate first, naloxone second; titrate to breathing, not to consciousness.
Glucose — check one on everybody. It costs a finger-prick and it is the cause you will otherwise miss.
Postictal or still seizing — apnea after a tonic-clonic is usually short. Support and watch; a prolonged one is status until proven otherwise.
CO₂ narcosis — COPD, obesity hypoventilation, sedatives on board. Ventilate; consider NIV once they are protecting.
Brainstem — stroke, bleed, raised ICP. Irregular pattern, hypertension, bradycardia. Needs a scanner, not a ward bed.
Neuromuscular — Guillain-Barré, myasthenia, high cord injury. The diaphragm tires long before the saturation falls.
Obstruction, not arrest — if the chest is trying and no air moves, this is an obstructed airway. Look, clear, reposition first.
Other toxins — benzodiazepine, alcohol, GHB, organophosphate. Support the breathing; the antidote is rarely the urgent part.