Carbon monoxide poisoning — oxygen, co-oximetry and hyperbaric indications

Immediate management. Remove from source (already done). Give 100% oxygen via non-rebreather at 10–15 L/min without interruption; escalate to RSI and mechanical ventilation with FiO2 1.0 if airway threatened or GCS falls. Resus bay, monitoring, IV access, ECG, troponin, VBG/lactate, FBC, U&E, β-hCG (pregnancy is an HBO indication), CT brain if focal signs. Assess for inhalation injury (soot, singed nasal hair, hoarse voice, stridor) — early intubation if present.^[2]

Why SpO2 lies. Standard pulse oximetry uses two wavelengths and assumes haemoglobin is either oxy- or deoxyhaemoglobin. Carboxyhaemoglobin absorbs like oxyhaemoglobin at 660 nm, so SpO2 reads falsely high (here 98%) despite lethal tissue hypoxia. Only co-oximetry (multi-wavelength) measures COHb directly; PaO2 may also be near normal.^[2]

Half-lives. COHb half-life approximately 4–6 hours on room air, 60–80 minutes on 100% oxygen, and ~20–23 minutes on hyperbaric oxygen at 2.5–3 ATA. This is the pharmacological rationale for immediate high-flow oxygen in every patient.^[2]

Hyperbaric indications (met here). Loss of consciousness / abnormal mental status (GCS 13, confusion), COHb >25% (28%), and often cardiac ischaemia or severe acidosis if present. Pregnancy with symptomatic exposure is an independent lower-threshold indication. Aim first treatment within 6 hours when available; discuss early with the hyperbaric unit. Evidence: Weaver NEJM 2002 supports reduced cognitive sequelae with multi-session HBO; Scheinkestel MJA 1999 (ANZ) found no benefit — use HBO for severe/indicated cases rather than mild poisoning.^[1][3]

Fire-survivor extras. Treat for combined CO + cyanide (burning plastics): high lactate, rapid collapse. Empirical hydroxocobalamin 5 g IV if unconscious/acidaemic. Parallel burns/airway care. Mental-health and bereavement issues after partner death — after medical stabilisation.^[4]