The charcoal stove in a closed room: carbon monoxide
On a cold harmattan night in Bamenda, a family of five closes the door and windows of their single room and brings in a charcoal stove that is still burning, so that the room will be warm. By morning the mother has a severe headache and is vomiting, the father is confused and unsteady, and their four-year-old son is unconscious, breathing about 30 times a minute, with bright cherry-red lips. The neighbour's pulse oximeter clipped onto the boy's finger reads 97%. The father wants to wait until the rain stops before going to hospital and says the boy 'only needs fresh air'. An aunt suggests giving the child strong coffee to wake him up.
1. Explain why carbon monoxide is far more dangerous than the carbon dioxide also produced by the burning charcoal. (5 marks)
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Carbon dioxide
It is a normal product of respiration, carried in the blood mainly as bicarbonate and removed by the lungs. In high concentration it lowers blood pH and stimulates breathing, which produces obvious distress and drives people out of the room. It does not bind to haemoglobin. Its danger is real but its warning signs are loud.
Carbon monoxide
- It is colourless and odourless, so there is no warning: the family simply fall asleep.
- It binds haemoglobin at the same site as oxygen but with an affinity roughly 200-250 times greater, forming carboxyhaemoglobin. Even a small concentration in the air therefore occupies a large share of the haemoglobin, and oxygen has little chance to compete.
- It also shifts the dissociation curve of the remaining oxyhaemoglobin to the left, so the little oxygen that is carried is released to tissues less readily.
- It binds to cytochrome oxidase in mitochondria, blocking the final stage of aerobic respiration directly.
The result is tissue hypoxia without the usual signs of suffocation. The cherry-red lips are carboxyhaemoglobin showing through the skin, not a sign of good oxygenation.
2. The pulse oximeter reads 97%. Explain why this reading is misleading, and why the four-year-old is more severely affected than his parents. (5 marks)
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Why the oximeter lies
The device shines red and infrared light through the finger and compares how much each is absorbed, then reports the percentage of haemoglobin that appears to be carrying oxygen. Carboxyhaemoglobin absorbs light very similarly to oxyhaemoglobin at these wavelengths, so it is counted as if it were oxygenated. The number is therefore a false 'saturation'. It also reports saturation, not oxygen content: with much of the haemoglobin occupied by carbon monoxide, the oxygen content can be dangerously low while the percentage looks reassuring.
Why the child is worse
- He has a higher metabolic rate per kilogram, so his tissues consume oxygen faster and tolerate a low supply for less time.
- His small blood volume and smaller total haemoglobin mean a modest amount of carbon monoxide occupies a larger fraction of it.
- Fetal haemoglobin, still present in a young child, binds carbon monoxide even more avidly.
- He is smaller and probably closer to the stove, and he has less reserve to compensate by increasing heart rate and breathing.
Adults had headache and confusion first because their brains were affected before their muscles, and they were able to get up; the unconscious child could not.
3. Explain the treatment the boy needs and why 'fresh air' and strong coffee are inadequate. (5 marks)
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The treatment
The boy needs hospital oxygen: 100% oxygen through a tight-fitting mask, immediately. The half-life of carboxyhaemoglobin is about four hours when breathing room air, falls to roughly 40-60 minutes on 100% oxygen, and to about 20-30 minutes in a hyperbaric chamber. Severe cases (unconsciousness, neurological signs, pregnancy) are referred for hyperbaric oxygen. Support the airway and breathing, monitor the heart, treat dehydration and acidosis, and keep him under observation because delayed brain injury can appear days later. All five family members must be assessed, because the ones who seem well may still have high levels.
Why 'fresh air' is not enough
Room air is only 21% oxygen. Opening the door helps in principle but removes the carbon monoxide far too slowly, and in an unconscious child with a raised breathing rate the window of time is short.
Why coffee is useless and risky
Caffeine does not displace carbon monoxide or restore oxygen carriage; it cannot reverse a mitochondrial block. Giving fluid by mouth to an unconscious child risks choking and aspiration, and it wastes the time needed to reach oxygen. The stove must also be carried outside and the room ventilated, so that nobody else is poisoned while he is being treated.