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Cyanide toxicity: Dx

Cyanide toxicity causes uncoupling of oxidative phosphorylation, interrupting aerobic metabolism and forcing a shift to anaerobic metabolism, leading to decreased O2 consumption, elevated lactate and severe metabolic acidosis.

Cyanide enters the body as gaseous HCN, water-soluble K-CN and Na-CN, and poorly soluble salts with Hg, Cu, Au, and Ag. Cyanogens, such as cyanogen-Br or –Cl, nitriles, and sodium nitroprusside may be converted to CN inside the body. Iatrogenic CN poisoning may be produced by high-dose or long-tern Na-nitroprusside IV therapy (>10 mcg/kg/min). CRF, pediatric patients and those with low thiosulfate reserves (POST-OP) are at increased risk of developing symptoms even at therapeutic doses.

With attention to the routes of exposure, a patient who presents with generalized weakness, malaise, collapse, neurologic symptoms, GI symptoms, and cardiopulmonary symptoms, CN poisoning must be high on the differential. The differential for these symptoms is broad (ACS, anaphylaxis, angina, MI, apnea, gastroenteritis, HA, meningitis, encephalitis, tachycardia, Hemlock poisoning, PE, cardiogenic shock, ischemic stroke, CO toxicity, hydrogen sulfide toxicity, Fe toxicity, INH toxicity, NSAID toxicity, azide toxicity, methanol toxicity, strychnine toxicity) and in addition to any known history, physical exam may reveal variable vitals, confusion, tachypnea, soot in the mouth and nose (following fire exposure) cherry-red skin, bright red retinal arteries and veins, and the smell of bitter almonds on the breath. Monitoring reveals a falsely reassuring SpO2, as no oxygen is being consumed by the cells hemoglobin remains fully saturated; supraventricular arrhythmias are common.


  • Lactate: high
  • ABG: normal O2; metabolic acidosis may be masked by respiratory compensation
  • VBG: abnormally high O2, similar to ABG

Carboxyhemoglobin level may be checked to rule out co toxicity.

Toxicology screen for methanol, ethylene glycol, FE, ketones, salicylates to rule out confounders

*CN levels may be drawn but are not clinically useful for dx; typically not available in time to aid in diagnosis, may be useful to monitor treatment progress over a period of days