The structure-fire victim slumped by the door isn't just a burn or a respiratory patient — they are a dual poisoning. Cyanide and carbon monoxide are the two toxins killing them, and your pulse ox is lying to you about both.
Two toxins, one patient
Work the next room-and-contents fire as a toxicology call and you'll start treating the right problem. A victim pulled from a smoke-filled structure — and frankly any crew member who took a bad lungful before masking up — is dealing with two poisons at once: carbon monoxide (CO) and hydrogen cyanide (HCN). Modern furnishings are the reason. Polyurethane foam, nylon, wool, and acrylics are nitrogen-containing synthetics, and incomplete combustion of them produces HCN in lethal quantities. This isn't industrial chemistry anymore; it's a living-room couch.
Both toxins do the same final-common-pathway damage — they starve tissue of usable oxygen — but by different mechanisms, and that distinction drives treatment.
| Carbon monoxide | Hydrogen cyanide | |
|---|---|---|
| Mechanism | Binds hemoglobin (~240x O₂ affinity), forms carboxyhemoglobin | Blocks cytochrome c oxidase, halts aerobic ATP production at the mitochondria |
| Effect | Blood can't *carry* oxygen | Cells can't *use* oxygen |
| Pulse ox (SpO₂) | Falsely normal/high — reads COHb as oxyHb | Often normal — oxygen is in the blood, just unused |
| Field marker | SpCO via CO-oximetry | Lactate as a surrogate |
| Treatment | High-flow O₂, consider HBO | Empiric hydroxocobalamin |
Why your pulse ox is lying
A standard two-wavelength pulse oximeter cannot tell oxyhemoglobin from carboxyhemoglobin. COHb absorbs light almost identically to oxyHb at the wavelengths the device uses, so a patient with a COHb of 30% can show a reassuring SpO₂ of 98%. That number is fiction. The patient is hypoxic at the tissue level while your monitor tells you everything's fine.
The fix is multi-wavelength CO-oximetry. The Masimo Rad-57 is the field workhorse — it uses multiwavelength spectrophotometry to report non-invasive SpCO (carboxyhemoglobin) and SpMet (methemoglobin), and it's the FDA-cleared device for non-invasive SpCO. EMS can't run a lab COHb in the back of the rig, and up to half of hospitals don't have on-site COHb testing either, so a field SpCO reading changes triage decisions in real time. CO's half-life is roughly 300 minutes on room air, but drops to around 60–90 minutes on a high-flow non-rebreather — which is why the first move on every smoke-inhalation patient is 100% oxygen, full stop.
Cyanide: treat empirically, don't wait for a number
There is no field test for blood cyanide, and the confirmatory lab takes hours you don't have. So we treat empirically on a clinical picture: structure-fire exposure, altered mental status, hypotension, and soot in the airway. The surrogate marker that backs the decision is lactate. A plasma lactate above 10 mmol/L in a smoke-inhalation victim without major burns is a sensitive indicator of significant cyanide intoxication — it correlates more tightly with blood cyanide than with CO. If you carry point-of-care lactate, a high number on a fire victim is a cyanide flag, not just a perfusion flag.
The antidote is hydroxocobalamin (Cyanokit). It's a precursor of vitamin B12 that binds cyanide to form cyanocobalamin, which is renally excreted. The adult dose is 5 g IV over 15 minutes, with a second 5 g dose for a total of 10 g in severe poisoning. It's hemodynamically friendly — unlike the old nitrite-based kits, it doesn't induce methemoglobinemia, which is exactly why it's the right call in a patient who *also* has CO on board and can't afford more dysfunctional hemoglobin. Heads-up: it turns skin, urine, and plasma deep red and will interfere with some colorimetric lab assays and even pulse CO-oximetry readings after administration.
Tie it into your monitoring
Don't forget waveform capnography on these patients. Metabolic acidosis from cyanide-driven anaerobic metabolism will show up as a falling EtCO₂ trend and a respiratory compensation pattern, and capnography gives you a continuous window on ventilation in a patient whose mental status is deteriorating. The whole picture — soot, altered LOC, normal-looking SpO₂, elevated SpCO, high lactate — points one direction.
Protecting yourself from this same smoke is the other half of the job; the products of combustion poisoning your patient are the ones plating onto your gear, which is the case for aggressive on-scene decon and clean-cab discipline. The smoke is the enemy on both ends of the call.
Sources
- CHEMM / HHS — Hydroxocobalamin Medical Countermeasures
- FDA — Cyanokit (hydroxocobalamin) prescribing information
- Masimo — Carboxyhemoglobin (SpCO) clinical technology
- NEJM — Elevated Blood Cyanide Concentrations in Victims of Smoke Inhalation
- LITFL — Cyanide Toxicity (lactate surrogate)
- StatPearls / NCBI — Hyperbaric Treatment of Carbon Monoxide Toxicity
Curated by Jonathan B
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