Describe carbon monoxide poisoning.

Sources and Mechanism

• Carbon monoxide is a colourless, odourless, tasteless and non-irritant gas produced by the incomplete combustion of any carbon-containing fuel. Its complete absence of warning properties is what makes it dangerous — the victim has no reason to leave the room
• Sources — charcoal and coal braziers (angithi, sigri) used for heating in closed rooms in winter, which is the classical Indian setting and causes deaths of whole families every year; gas geysers in bathrooms, a recurring and preventable cause of death in young people; faulty heaters and chimneys; motor exhaust; and smoke inhalation in fires, which is much the commonest cause overall
• Mechanism — carbon monoxide binds haemoglobin with an affinity about 200 to 250 times that of oxygen, forming carboxyhaemoglobin; it also shifts the oxygen dissociation curve to the left, so that the remaining oxyhaemoglobin does not release its oxygen to the tissues; and it binds cytochrome oxidase and myoglobin, adding a histotoxic and a cardiac component. The tissue hypoxia is therefore far worse than the carboxyhaemoglobin figure alone suggests

Findings, Investigation and Treatment

• The classical sign is cherry-pink discolouration of the skin, lividity, mucous membranes, blood, muscles and viscera — though it is often absent or unappreciated in life, and it appears reliably only at high saturations. The absence of cyanosis in a hypoxic patient is itself a clue
• Pulse oximetry IS falsely normal and is dangerously misleading, because the ordinary oximeter cannot distinguish carboxyhaemoglobin from oxyhaemoglobin and reports a high saturation in a profoundly hypoxic patient. A CO-oximeter or direct carboxyhaemoglobin estimation is required
• Delayed neuropsychiatric sequelae — occurring in a substantial minority days to weeks after apparent recovery: cognitive impairment, personality change, parkinsonism, incontinence and psychosis, with necrosis of the globus pallidus characteristically seen on imaging and at autopsy. This is what makes follow-up essential
• Postmortem — cherry-pink lividity, blood and viscera; the blood remains fluid and bright; and soot in the airway with burns where death occurred in a fire, which together with a raised carboxyhaemoglobin proves the person was alive and breathing in the fire
• Sampling — blood in a clean, airtight container, filled to the brim, with NO preservative, refrigerated
• Treatment — remove from exposure; 100% oxygen by tight-fitting mask, which reduces the half-life of carboxyhaemoglobin from about 4 to 5 hours in air to about 1 hour; and hyperbaric oxygen, which reduces it to about 20 minutes and is considered for severe poisoning, loss of consciousness, neurological signs, cardiac involvement and pregnancy — the fetus being especially vulnerable, since fetal haemoglobin binds carbon monoxide still more avidly

Applied Aspects

• Do not trust the pulse oximeter; it reads normal in carbon monoxide poisoning and has given false reassurance in fatal cases. Ask for a carboxyhaemoglobin
• Ask about the heating and the geyser in any winter cluster of headache and nausea, and about whether symptoms improve away from home
• Give high-flow oxygen immediately and continue it; it is the treatment, and it should not wait for confirmation
• Treat the pregnant patient more aggressively and consider hyperbaric oxygen; fetal haemoglobin binds carbon monoxide more strongly and the fetus is at greater risk than the mother
• Warn about delayed sequelae and arrange follow-up; a patient who recovers completely may deteriorate cognitively at two to four weeks, and the family must know to return
• Measure carboxyhaemoglobin in every fire death; a raised level with soot below the larynx proves the person was breathing in the fire, and a normal level in a burnt body suggests death occurred beforehand
• Allow for smoking when interpreting a modestly raised level, and for the time elapsed and any oxygen already given, which lower it rapidly
• Soot below the larynx with a raised carboxyhaemoglobin proves the person was alive and breathing in the fire