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.
Educational reading for clinicians. Follow your current protocols and local guidance.
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
Jonathan B, career paramedic
Published May 13, 2026