Cyanide: Choking the Cell's Powerhouse

The one enzyme cyanide was built to jam

Every breath you take exists to feed one enzyme at the end of a chain. Deep inside every mitochondrion runs the electron transport chain, a line of four protein complexes that pass electrons down a slope of energy and, at the very end, hand them to oxygen. That final hand-off is done by complex IV — cytochrome c oxidase. It is the only step where oxygen is actually consumed; it is the reason we breathe at all. Cyanide has a lethal affinity for the ferric iron (Fe3+) buried in that enzyme. It binds there, locks the enzyme shut, and the whole chain grinds to a halt. Electrons back up, oxygen goes unused, and oxidative phosphorylation — the process that makes almost all of the cell's ATP — simply stops.

Now comes the cruel paradox that defines cyanide. The oxygen is still there. The lungs still load it onto haemoglobin, the heart still pumps it to every organ, delivery is completely intact. But the cell can no longer use it. This is histotoxic hypoxia — hypoxia at the level of the tissue, with a normal PaO2 and a normal arterial saturation. Starved of ATP but forced to keep living, cells switch to anaerobic metabolism, burning glucose without oxygen and pouring out lactic acid. A severe, rapidly deepening lactic acidosis is the metabolic signature of the poisoning, and it explains almost everything that follows.

Where cyanide comes from

Cyanide is not an exotic laboratory curiosity; it is disturbingly common. The single most important source in the emergency department is smoke from house and building fires: when synthetics, foams, plastics and wool combust, they release hydrogen cyanide gas. A fire victim pulled from an enclosed space almost always has a combined exposure — carbon monoxide and cyanide together — and this combination shapes the whole antidote decision below. Industrial and laboratory exposures follow: electroplating, metal extraction, plastics manufacture and certain chemical labs. Medicine itself is a source: prolonged or high-dose infusions of sodium nitroprusside, a potent vasodilator, release cyanide as the molecule breaks down — a link explored in the Cardiovascular chapter, where nitroprusside is used for hypertensive emergencies. And nature contributes amygdalin, the cyanogenic compound in apricot kernels, bitter almonds and cassava, occasionally taken as a bogus "anti-cancer" remedy.

The clinical picture: fast, and full of clues

Cyanide kills the organs that need ATP the fastest — the brain and the heart. A large exposure can drop a victim within seconds to minutes; smaller doses give headache, anxiety, dizziness and a sense of breathlessness that quickly escalates. Because the brain and myocardium are the most oxygen-hungry tissues, the earliest and worst effects are neurological and cardiovascular: agitation giving way to confusion, seizures and coma, and a heart that races and then fails as cardiovascular collapse sets in. The breathing is dramatic — the patient gasps and hyperventilates, driven by the acidosis — yet, and this is the diagnostic heart of it, there is dyspnoea without cyanosis. The blood is fully oxygenated, so the patient does not turn blue; some describe a cherry-red or simply normal-pink appearance instead. A minority of examiners can detect a faint bitter-almond odour on the breath, but this is an unreliable clue: the ability to smell it is genetically determined and absent in many people, so its absence never rules cyanide out.

The laboratory tells the real story, and two findings should make cyanide leap to mind. First, a severe, otherwise unexplained high-anion-gap lactic acidosis — a lactate that is high and climbing, out of proportion to anything else, is one of the most reliable pointers (the anion gap and lactic acidosis are worked through in the Principles of toxicology chapter). Second, a paradoxically high venous oxygen saturation. Normally tissues extract a large fraction of the oxygen delivered to them, so venous blood is much darker and less saturated than arterial. In cyanide poisoning the tissues extract almost nothing, so the venous blood stays bright and highly saturated — "arterialised" venous blood, with a narrowed arteriovenous oxygen difference. Blood cyanide levels confirm the diagnosis but take far too long to return; the poisoning is treated on clinical suspicion long before any level is available.

The antidotes: three mechanisms, one enzyme to free

Everything else is supportive; oxygen and a specific antidote are what save the patient. Start with 100% oxygen at once — high concentrations help even though delivery is not the primary problem, and it is essential in the fire victim who also has carbon monoxide poisoning. Then choose an antidote, and the choice hinges on mechanism. Hydroxocobalamin (vitamin B12a) is the safest first-line agent: its cobalt centre grabs a cyanide ion directly to form cyanocobalamin (ordinary vitamin B12), which is harmless and cleared by the kidneys. Because it works by simply binding cyanide, it does not compromise oxygen-carrying capacity — which is exactly why it is preferred in smoke-inhalation victims who may also be carrying carbon monoxide. Its harmless signature is a deep red discolouration of the skin, urine and body fluids, and it can interfere with some colorimetric lab assays.

The older cyanide antidote kit works by a cleverer, riskier two-step logic. First, nitrites — amyl nitrite by inhalation, then intravenous sodium nitrite — are given deliberately to induce methaemoglobinaemia. It sounds mad to poison the blood on purpose, but the ferric iron (Fe3+) of methaemoglobin is itself a cyanide magnet: it competes for the cyanide and pulls it off cytochrome oxidase onto the methaemoglobin instead, freeing the mitochondrial enzyme to breathe again. Second, sodium thiosulfate is given as a sulfur donor. The body's own detoxifying enzyme, rhodanese, uses sulfur to convert cyanide into thiocyanate, a far less toxic compound excreted in the urine — but rhodanese runs short of sulfur substrate, and thiosulfate resupplies it. Nitrites shift the cyanide to a safer holding site; thiosulfate then permanently disposes of it.

The nitrite step is exactly what makes the old kit dangerous in fire victims. Here the mechanisms collide. In a fire victim who has inhaled smoke, a large slice of the haemoglobin is already knocked out by carbon monoxide (as carboxyhaemoglobin). If you now give a nitrite and deliberately convert yet more haemoglobin into methaemoglobin — which also cannot carry oxygen — you have crippled oxygen delivery from two directions at once, and can tip a struggling patient into fatal hypoxia. So in smoke inhalation, nitrites are avoided; hydroxocobalamin (which spares the haemoglobin entirely) is the antidote of choice, often given alongside sodium thiosulfate. Nitrites remain reasonable in a pure, isolated cyanide exposure with no carbon monoxide in the picture. This oxidation of haem iron — and its interplay with oxygen carriage — is the same chemistry taught in the Haematology section on haem iron oxidation states and methaemoglobin.