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. Shared household symptoms worse at home with a raised carboxyhaemoglobin are carbon monoxide poisoning.
2. Acute coronary syndrome Chest pain from cardiac ischaemia. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin raises it.
3. Alcohol or drug intoxication Confusion from a toxic ingestion. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin makes it a consideration.
4. Cyanide poisoning Cellular hypoxia from smoke inhalation, often together with carbon monoxide. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin makes it a consideration.
5. Diabetic or other metabolic coma Reduced consciousness from a metabolic cause. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin points elsewhere.

The point: Carbon monoxide is a colourless, odourless gas produced by incomplete combustion, and poisoning most often results from faulty gas boilers, heaters and fires, from smoke inhalation, and from car exhaust fumes. Carbon monoxide binds haemoglobin with an affinity roughly two hundred and fifty times that of oxygen to form carboxyhaemoglobin, which both reduces the oxygen-carrying capacity of the blood and shifts the oxygen dissociation curve to the left so that the remaining oxygen is released less readily to the tissues, and it also poisons cellular respiration, so the net effect is severe tissue hypoxia. The symptoms are non-specific and are easily mistaken for a viral illness: headache is the commonest, with nausea, dizziness and, as poisoning worsens, confusion, breathlessness, chest pain, seizures, arrhythmia and coma. Two clues point to the diagnosis: several people or pets in the same household are affected together, and the symptoms improve away from the home and return on going back. Two teaching points are heavily examined. Pulse oximetry is unreliable and reads a falsely normal saturation, because the oximeter cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so the diagnosis rests on measuring the carboxyhaemoglobin level directly with a co-oximeter or blood gas analyser. The classic cherry-red appearance of the skin is a late and unreliable sign. Management begins by removing the patient from the source and giving high-flow one hundred percent oxygen through a non-rebreathe mask, which shortens the half-life of carboxyhaemoglobin from around four hours in room air to about one hour. Hyperbaric oxygen is considered in severe poisoning, such as loss of consciousness, neurological signs, cardiac involvement, pregnancy or a very high carboxyhaemoglobin level. Survivors are at risk of delayed neuropsychiatric complications. The exam themes are the falsely normal pulse oximetry, measuring carboxyhaemoglobin, high-flow oxygen as first-line 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 investigation?

1. Recognise that pulse oximetry reads falsely normal The oximeter cannot distinguish carboxyhaemoglobin from oxyhaemoglobin. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin makes it decisive.
2. Arterial blood gas for metabolic acidosis and lactate They indicate the severity of tissue hypoxia. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin makes it important.
3. Assess for features of severe poisoning They determine the need for hyperbaric oxygen. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin makes it selective.
4. Carboxyhaemoglobin level on a co-oximeter or blood gas correct Correct. The carboxyhaemoglobin level confirms the diagnosis that oximetry misses.
5. ECG for ischaemia and arrhythmia The heart is sensitive to hypoxia. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin makes it useful.

The point: Carbon monoxide is a colourless, odourless gas produced by incomplete combustion, and poisoning most often results from faulty gas boilers, heaters and fires, from smoke inhalation, and from car exhaust fumes. Carbon monoxide binds haemoglobin with an affinity roughly two hundred and fifty times that of oxygen to form carboxyhaemoglobin, which both reduces the oxygen-carrying capacity of the blood and shifts the oxygen dissociation curve to the left so that the remaining oxygen is released less readily to the tissues, and it also poisons cellular respiration, so the net effect is severe tissue hypoxia. The symptoms are non-specific and are easily mistaken for a viral illness: headache is the commonest, with nausea, dizziness and, as poisoning worsens, confusion, breathlessness, chest pain, seizures, arrhythmia and coma. Two clues point to the diagnosis: several people or pets in the same household are affected together, and the symptoms improve away from the home and return on going back. Two teaching points are heavily examined. Pulse oximetry is unreliable and reads a falsely normal saturation, because the oximeter cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so the diagnosis rests on measuring the carboxyhaemoglobin level directly with a co-oximeter or blood gas analyser. The classic cherry-red appearance of the skin is a late and unreliable sign. Management begins by removing the patient from the source and giving high-flow one hundred percent oxygen through a non-rebreathe mask, which shortens the half-life of carboxyhaemoglobin from around four hours in room air to about one hour. Hyperbaric oxygen is considered in severe poisoning, such as loss of consciousness, neurological signs, cardiac involvement, pregnancy or a very high carboxyhaemoglobin level. Survivors are at risk of delayed neuropsychiatric complications. The exam themes are the falsely normal pulse oximetry, measuring carboxyhaemoglobin, high-flow oxygen as first-line 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 They can appear after apparent recovery. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin makes it important.
2. Remove the patient from the source of exposure It stops further absorption. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin makes it foundational.
3. High-flow one hundred percent oxygen by non-rebreathe mask correct Correct. High-flow oxygen shortens the carboxyhaemoglobin half-life.
4. Advise inspection of the source appliance It prevents re-exposure. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin makes it preventive.
5. Cardiac monitoring and supportive care It manages arrhythmia and complications. Shared household symptoms worse at home with a normal oximetry but a raised carboxyhaemoglobin makes it prudent.

The point: Carbon monoxide is a colourless, odourless gas produced by incomplete combustion, and poisoning most often results from faulty gas boilers, heaters and fires, from smoke inhalation, and from car exhaust fumes. Carbon monoxide binds haemoglobin with an affinity roughly two hundred and fifty times that of oxygen to form carboxyhaemoglobin, which both reduces the oxygen-carrying capacity of the blood and shifts the oxygen dissociation curve to the left so that the remaining oxygen is released less readily to the tissues, and it also poisons cellular respiration, so the net effect is severe tissue hypoxia. The symptoms are non-specific and are easily mistaken for a viral illness: headache is the commonest, with nausea, dizziness and, as poisoning worsens, confusion, breathlessness, chest pain, seizures, arrhythmia and coma. Two clues point to the diagnosis: several people or pets in the same household are affected together, and the symptoms improve away from the home and return on going back. Two teaching points are heavily examined. Pulse oximetry is unreliable and reads a falsely normal saturation, because the oximeter cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so the diagnosis rests on measuring the carboxyhaemoglobin level directly with a co-oximeter or blood gas analyser. The classic cherry-red appearance of the skin is a late and unreliable sign. Management begins by removing the patient from the source and giving high-flow one hundred percent oxygen through a non-rebreathe mask, which shortens the half-life of carboxyhaemoglobin from around four hours in room air to about one hour. Hyperbaric oxygen is considered in severe poisoning, such as loss of consciousness, neurological signs, cardiac involvement, pregnancy or a very high carboxyhaemoglobin level. Survivors are at risk of delayed neuropsychiatric complications. The exam themes are the falsely normal pulse oximetry, measuring carboxyhaemoglobin, high-flow oxygen as first-line treatment, and hyperbaric oxygen for severe poisoning.

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