Emergency Medicine · Toxicology
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Most common cause is smoke inhalation from house fires (burning plastics/polyurethane) or iatrogenic prolonged sodium nitroprusside infusions.
First-line antidote is hydroxocobalamin, which binds cyanide to form non-toxic cyanocobalamin (vitamin B12) for renal excretion.
Cyanide binds the ferric ion of cytochrome c oxidase (Complex IV), halting oxidative phosphorylation and causing severe cellular hypoxia despite adequate oxygen.
Diagnostic clues: severe high-anion-gap lactic acidosis (lactate often >8 mmol/L) with abnormally high mixed venous oxygen saturation from failed tissue oxygen extraction.
The classic alternative is the cyanide antidote kit: nitrites induce methemoglobinemia to pull cyanide off cytochrome c oxidase, followed by sodium thiosulfate.
Nitrites are absolutely contraindicated in smoke-inhalation victims because concurrent carbon monoxide poisoning worsens tissue hypoxia.
Buzzwords are bitter almond odor on the breath and cherry-red skin; survivors may develop delayed Parkinsonian features from basal ganglia necrosis.
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A 45-year-old firefighter is pulled from a burning building and arrives confused, tachypneic, and hypotensive. Arterial blood gas shows a profound high-anion-gap metabolic acidosis with a serum lactate of 14 mmol/L. His measured mixed venous oxygen saturation is strikingly elevated. Soot is noted around his nose and mouth.
Which of the following is the most appropriate antidote?
Hydroxocobalamin.
Smoke inhalation with severe lactic acidosis and a paradoxically high venous oxygen saturation indicates cyanide toxicity from impaired oxidative phosphorylation. Hydroxocobalamin is first-line and is safe in fire victims, whereas nitrites are contraindicated because of likely concomitant carbon monoxide poisoning.
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Etiology / Epidemiology
Most commonly caused by smoke inhalation from house fires (burning plastics/polyurethane) or iatrogenically via prolonged sodium nitroprusside infusions.
Clinical Manifestations
Rapid onset of altered mental status and cardiovascular collapse with classic bitter almond odor on breath and cherry-red skin.
Diagnosis
Clinical diagnosis supported by severe lactic acidosis and abnormally elevated mixed venous oxygen saturation.
Treatment
Immediate administration of hydroxocobalamin; alternative is the cyanide antidote kit (nitrites + thiosulfate), but nitrites are contraindicated with concomitant carbon monoxide poisoning.
Prognosis
Rapidly fatal without prompt antidotal therapy; survivors may develop delayed Parkinsonian features due to basal ganglia necrosis.
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Epidemiology & Etiology
The most common etiology in the US is inhalation of smoke from closed-space house fires involving the combustion of synthetic materials like plastics, wool, and polyurethane. Iatrogenic toxicity classically occurs in patients receiving high-dose, prolonged sodium nitroprusside infusions, especially in the setting of renal impairment. Industrial exposures occur in jewelry manufacturing, electroplating, and metal extraction.
Pertinent Anatomy
Cyanide rapidly distributes to highly metabolic tissues, primarily affecting the central nervous system and the myocardium. The basal ganglia are exquisitely sensitive to hypoxic injury, which explains the delayed neurological deficits seen in survivors.
Pathophysiology
Cyanide binds tightly to the ferric (Fe3+) ion of cytochrome c oxidase (Complex IV) in the mitochondrial electron transport chain. This binding completely halts oxidative phosphorylation, preventing cells from utilizing oxygen. The resulting severe cellular hypoxia forces a shift to anaerobic metabolism, generating massive amounts of lactic acid. Because tissues cannot extract oxygen from the blood, venous blood remains highly oxygenated.
Clinical Manifestations
Patients initially present with headache, confusion, and tachypnea, rapidly progressing to seizures, coma, and cardiovascular collapse. Classic buzzword physical exam findings include a bitter almond odor on the breath and cherry-red skin (though the latter is a late and unreliable finding). The presence of bright red venous blood during phlebotomy is a highly suggestive clinical clue.
Diagnosis
Diagnosis is primarily clinical, as specific blood cyanide levels take too long to result for acute management. Arterial blood gas reveals a severe high-anion gap metabolic acidosis driven by massive lactic acid production. A classic laboratory hallmark is an abnormally high mixed venous oxygen saturation (SvO2), reflecting the tissue's inability to extract oxygen. Serum lactate is typically >8 mmol/L.
Treatment
The first-line antidote is hydroxocobalamin, which binds cyanide directly to form non-toxic cyanocobalamin (Vitamin B12) for safe renal excretion. An alternative is the traditional cyanide antidote kit, which uses nitrites (amyl and sodium) to intentionally induce methemoglobinemia (methemoglobin pulls cyanide off cytochrome c oxidase) followed by sodium thiosulfate to enhance renal excretion. Nitrites are absolutely contraindicated in smoke inhalation victims due to the risk of worsening tissue hypoxia in the presence of concomitant carbon monoxide poisoning.
Prognosis
Acute cyanide poisoning is a rapidly fatal medical emergency if left untreated. Survivors of severe toxicity are at high risk for developing anoxic brain injury and delayed neurological sequelae, specifically Parkinsonian features due to selective basal ganglia necrosis.
Differential Diagnosis
Carbon Monoxide Poisoning: Co-occurs in fires; presents with headache and elevated carboxyhemoglobin, but normal venous oxygen extraction.
Methemoglobinemia: Induced by dapsone or local anesthetics; presents with chocolate-brown blood and normal PaO2
Hydrogen Sulfide Poisoning: Industrial exposure (sewers); presents identically to cyanide toxicity but has a classic rotten egg odor.
Salicylate Toxicity: Causes a mixed acid-base disorder (respiratory alkalosis + metabolic acidosis) with classic tinnitus.