Clinical pharmacology and therapeutics for MRCP Part 1

A 40-year-old man and his wife both have several days of headache, nausea and dizziness that are worse at home and ease when they are out. It is winter and they have been using an old gas heater. His pulse oximetry reads 99 percent, but a blood gas shows a carboxyhaemoglobin of 22 percent. What is the most likely diagnosis?

1. Carbon monoxide poisoning correct Correct. Symptoms shared by the household, worse at home, with a raised carboxyhaemoglobin level, point to carbon monoxide poisoning.
2. Acute coronary syndrome Chest pain from a starved heart muscle. Here, symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin points elsewhere.
3. Alcohol or drug intoxication Confusion from alcohol or a drug taken by mouth. But symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin does not point to it.
4. Cyanide poisoning Cells starved of oxygen from breathing in smoke, often alongside carbon monoxide. It does not fit symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin.
5. Diabetic or other metabolic coma A drop in consciousness from a body-chemistry problem such as very high or very low blood sugar. That does not match symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin.

The point: Carbon monoxide is a gas with no colour and no smell. It forms when fuel burns without enough air, so poisoning usually comes from a faulty gas boiler, heater or fire, from breathing in smoke, or from car exhaust fumes. Carbon monoxide sticks to haemoglobin (the protein in red blood cells that carries oxygen) roughly two hundred and fifty times more strongly than oxygen does. This forms carboxyhaemoglobin, haemoglobin joined to carbon monoxide instead of oxygen. That cuts how much oxygen the blood can carry, and it also makes the blood hold on to the oxygen it does have instead of handing it over to the tissues. On top of this, the gas poisons the cells' own breathing machinery, so the net effect is severe oxygen starvation in the tissues. The symptoms are vague and are easily mistaken for a viral illness: headache is the commonest, with nausea, dizziness and, as the poisoning gets worse, confusion, breathlessness, chest pain, seizures, an abnormal heart rhythm and coma. Two clues point to the diagnosis: several people or pets in the same household fall ill together, and the symptoms ease away from home and come back on returning. Two teaching points come up again and again in the exam. Pulse oximetry, the finger-probe reading of oxygen levels, is unreliable here and gives a falsely normal result, because the probe cannot tell carboxyhaemoglobin apart from ordinary oxygen-carrying haemoglobin. This means the diagnosis rests on measuring the carboxyhaemoglobin level directly, with a co-oximeter or a blood gas machine. The well-known cherry-red skin colour is a late sign and is not reliable. Treatment starts with getting the patient away from the source and giving high-flow one hundred percent oxygen through a non-rebreathe mask, which shortens the half-life of carboxyhaemoglobin (the time it takes for the level to fall by half) from around four hours in room air to about one hour. Hyperbaric oxygen, oxygen given in a pressurised chamber, is considered for severe poisoning, such as loss of consciousness, nerve signs, heart involvement, pregnancy or a very high carboxyhaemoglobin level. Survivors are at risk of delayed problems with memory, mood or movement weeks later. The exam themes are the falsely normal pulse oximetry, measuring carboxyhaemoglobin directly, high-flow oxygen as the first treatment, and hyperbaric oxygen for severe poisoning.

Source: TOXBASE (National Poisons Information Service) — carbon monoxide TOXBASE (National Poisons Information Service) · tier 3, national formulary

A 40-year-old man and his wife both have several days of headache, nausea and dizziness that are worse at home and ease when they are out. It is winter and they have been using an old gas heater. His pulse oximetry reads 99 percent, but a blood gas shows a carboxyhaemoglobin of 22 percent. What is the most appropriate management?

1. Follow up for delayed neuropsychiatric effects Memory, mood or movement problems can appear after the patient seems to have recovered. That does not match symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin.
2. Remove the patient from the source of exposure This stops more carbon monoxide getting into the blood. But symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin does not point to it.
3. High-flow one hundred percent oxygen by non-rebreathe mask correct Correct. High-flow oxygen cuts the time it takes for the carboxyhaemoglobin level to fall by half.
4. Advise inspection of the source appliance This stops the same poisoning happening again. It does not fit symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin.
5. Cardiac monitoring and supportive care This watches for an abnormal heart rhythm and other problems. Here, symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin points elsewhere.

The point: Carbon monoxide is a gas with no colour and no smell. It forms when fuel burns without enough air, so poisoning usually comes from a faulty gas boiler, heater or fire, from breathing in smoke, or from car exhaust fumes. Carbon monoxide sticks to haemoglobin (the protein in red blood cells that carries oxygen) roughly two hundred and fifty times more strongly than oxygen does. This forms carboxyhaemoglobin, haemoglobin joined to carbon monoxide instead of oxygen. That cuts how much oxygen the blood can carry, and it also makes the blood hold on to the oxygen it does have instead of handing it over to the tissues. On top of this, the gas poisons the cells' own breathing machinery, so the net effect is severe oxygen starvation in the tissues. The symptoms are vague and are easily mistaken for a viral illness: headache is the commonest, with nausea, dizziness and, as the poisoning gets worse, confusion, breathlessness, chest pain, seizures, an abnormal heart rhythm and coma. Two clues point to the diagnosis: several people or pets in the same household fall ill together, and the symptoms ease away from home and come back on returning. Two teaching points come up again and again in the exam. Pulse oximetry, the finger-probe reading of oxygen levels, is unreliable here and gives a falsely normal result, because the probe cannot tell carboxyhaemoglobin apart from ordinary oxygen-carrying haemoglobin. This means the diagnosis rests on measuring the carboxyhaemoglobin level directly, with a co-oximeter or a blood gas machine. The well-known cherry-red skin colour is a late sign and is not reliable. Treatment starts with getting the patient away from the source and giving high-flow one hundred percent oxygen through a non-rebreathe mask, which shortens the half-life of carboxyhaemoglobin (the time it takes for the level to fall by half) from around four hours in room air to about one hour. Hyperbaric oxygen, oxygen given in a pressurised chamber, is considered for severe poisoning, such as loss of consciousness, nerve signs, heart involvement, pregnancy or a very high carboxyhaemoglobin level. Survivors are at risk of delayed problems with memory, mood or movement weeks later. The exam themes are the falsely normal pulse oximetry, measuring carboxyhaemoglobin directly, high-flow oxygen as the first treatment, and hyperbaric oxygen for severe poisoning.

Source: TOXBASE (National Poisons Information Service) — carbon monoxide TOXBASE (National Poisons Information Service) · tier 3, national formulary

A 40-year-old man and his wife both have several days of headache, nausea and dizziness that are worse at home and ease when they are out. It is winter and they have been using an old gas heater. His pulse oximetry reads 99 percent, but a blood gas shows a carboxyhaemoglobin of 22 percent. What is the most likely complication?

1. Cardiac ischaemia and arrhythmia Low oxygen puts a strain on the heart. It does not fit symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin.
2. Metabolic acidosis with a raised lactate This shows the cells are starved of oxygen. Here, symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin points elsewhere.
3. Delayed neuropsychiatric sequelae correct Correct. Survivors can develop memory, thinking and nerve problems weeks later.
4. Death or re-exposure from an uncorrected source An appliance that is not fixed puts the whole household in danger again. But symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin does not point to it.
5. Seizures and coma in severe poisoning Severe lack of oxygen in the tissues affects the brain. That does not match symptoms shared by the household, worse at home, with a normal finger-probe reading but a raised carboxyhaemoglobin.

The point: Carbon monoxide is a gas with no colour and no smell. It forms when fuel burns without enough air, so poisoning usually comes from a faulty gas boiler, heater or fire, from breathing in smoke, or from car exhaust fumes. Carbon monoxide sticks to haemoglobin (the protein in red blood cells that carries oxygen) roughly two hundred and fifty times more strongly than oxygen does. This forms carboxyhaemoglobin, haemoglobin joined to carbon monoxide instead of oxygen. That cuts how much oxygen the blood can carry, and it also makes the blood hold on to the oxygen it does have instead of handing it over to the tissues. On top of this, the gas poisons the cells' own breathing machinery, so the net effect is severe oxygen starvation in the tissues. The symptoms are vague and are easily mistaken for a viral illness: headache is the commonest, with nausea, dizziness and, as the poisoning gets worse, confusion, breathlessness, chest pain, seizures, an abnormal heart rhythm and coma. Two clues point to the diagnosis: several people or pets in the same household fall ill together, and the symptoms ease away from home and come back on returning. Two teaching points come up again and again in the exam. Pulse oximetry, the finger-probe reading of oxygen levels, is unreliable here and gives a falsely normal result, because the probe cannot tell carboxyhaemoglobin apart from ordinary oxygen-carrying haemoglobin. This means the diagnosis rests on measuring the carboxyhaemoglobin level directly, with a co-oximeter or a blood gas machine. The well-known cherry-red skin colour is a late sign and is not reliable. Treatment starts with getting the patient away from the source and giving high-flow one hundred percent oxygen through a non-rebreathe mask, which shortens the half-life of carboxyhaemoglobin (the time it takes for the level to fall by half) from around four hours in room air to about one hour. Hyperbaric oxygen, oxygen given in a pressurised chamber, is considered for severe poisoning, such as loss of consciousness, nerve signs, heart involvement, pregnancy or a very high carboxyhaemoglobin level. Survivors are at risk of delayed problems with memory, mood or movement weeks later. The exam themes are the falsely normal pulse oximetry, measuring carboxyhaemoglobin directly, high-flow oxygen as the first treatment, and hyperbaric oxygen for severe poisoning.

Source: TOXBASE (National Poisons Information Service) — carbon monoxide TOXBASE (National Poisons Information Service) · tier 3, national formulary