Situation: A 40-year-old woman weighing 55 kg is rescued after being trapped for 20 minutes in a closed room of a burning house, where furniture and plastic items were burning. She has partial-thickness burns of the face, neck, anterior trunk, and both arms, estimated at 30% of total body surface area. She does not smoke. Despite 100% oxygen, her serum lactate is 11 mmol/L (normal < 2 mmol/L), and the physician orders hydroxocobalamin intravenously. What is the reason for this order?

Burning plastics can release hydrogen cyanide, which blocks cellular oxygen use and causes severe lactic acidosis even when oxygen supply is adequate. Hydroxocobalamin binds cyanide to form cyanocobalamin, which is excreted in urine.

Why hydroxocobalamin is ordered

The patient was trapped in a closed-space fire with burning furniture and plastics. Under these conditions, incomplete combustion of nitrogen-containing synthetic materials releases hydrogen cyanide (HCN) gas, which is rapidly absorbed through the lungs. Cyanide is a cellular asphyxiant: it binds to the ferric ion of cytochrome c oxidase in the mitochondrial electron transport chain, halting oxidative phosphorylation. The cells can no longer use oxygen even though arterial oxygen content may be normal or high. This forces anaerobic metabolism, producing large amounts of lactate and a severe high-anion-gap metabolic acidosis. The serum lactate of 11 mmol/L (normal < 2 mmol/L) despite 100% oxygen is a strong clinical signal of cyanide toxicity in this setting ^[1][4].

Hydroxocobalamin is given because it binds cyanide directly to form cyanocobalamin (vitamin B12), which is then excreted in the urine. This restores mitochondrial oxygen utilization and helps reverse the lactic acidosis. The European expert consensus identifies enclosed-space fire, altered consciousness, cardiovascular instability, and elevated plasma lactate as findings suggesting cyanide poisoning that should prompt antidote consideration ^[4]. The American Heart Association also recommends hydroxocobalamin for suspected HCN poisoning in smoke inhalation victims ^[1].

Watch out! Hydroxocobalamin does not buffer acid, does not primarily lower methemoglobin, and is not given to replace vitamin B12 lost from burned skin. Its role is specific: chelating cyanide. Key point! A lactate above 10 mmol/L in a closed-space fire victim is considered a strong indicator for empiric cyanide antidote therapy, even before serum cyanide levels return ^[1][4].

The table below compares the four options and why only one fits the mechanism.

Option	Proposed mechanism	Why it is incorrect or correct
1. Buffers acid	Neutralizes lactic acid from hypoperfused cells	Hydroxocobalamin is not a buffer; it does not directly correct acidosis. Acidosis improves only after cyanide is removed and aerobic metabolism resumes.
2. Lowers methemoglobin	Improves oxygen-carrying capacity	Methemoglobinemia is not the primary problem here. Methylene blue, not hydroxocobalamin, is used for methemoglobinemia. Cyanide blocks oxygen use, not oxygen transport.
3. Binds cyanide	Forms cyanocobalamin excreted in urine	Correct. This directly addresses the toxic agent released by burning plastics and restores cellular respiration.
4. Replaces vitamin B12	Restores B12 lost from burned skin	Hydroxocobalamin is a B12 precursor, but it is given here as a cyanide antidote, not for nutritional replacement or burn-related losses.

The clinical picture of a closed-space fire, soot around the face, altered mental status, cardiovascular instability, and a disproportionately elevated lactate should raise suspicion for cyanide toxicity even when the burn size alone does not explain the acidosis ^[4]. In this patient, the 30% partial-thickness burn and 55 kg weight are less immediately relevant than the combination of enclosed-space exposure, burning plastics, and refractory lactic acidosis. Empiric hydroxocobalamin is justified because waiting for confirmatory cyanide levels can delay life-saving treatment ^[1][4].