QuestionSBA
QuestionChoice
Lung function tests are unlikely to be useful in this acute scenario
32861
d
Lung function tests
QuestionChoice
Alcohol intoxication is a common differential of carbon monoxide poisoning, and the two can occur concomitantly. However, the patient is unlikely to complain of vertigo, and there is no mention of alcohol use in the history
32862
e
Blood ethanol levels
QuestionChoice
A urine dipstick would be a reasonable investigation to perform in this case. However, based on the history, although delirium secondary to a urinary tract infection is a differential, it is not the most likely and would be unlikely to lead to headache or nausea
32859
b
Urine dipstick
QuestionChoice
A chest x-ray would be a reasonable investigation to perform in this case. However, based on the history, although delirium secondary to pneumonia is a differential, it is not the most likely
32860
c
Chest x-ray
QuestionChoice
Carboxyhaemaglobin levels can be taken when there is a clinical suspicion of carbon monoxide poisoning. This patient is displaying classic signs and symptoms. Other classic signs are cherry red lips and peripheral cyanosis, which are more common in textbooks than in practice
32858
a
Carboxyhaemaglobin levels
Concept
Chapter
# Summary

Carbon monoxide is a tasteless, odourless and invisible gas that may be produced by fires, faulty gas appliances and fuel-burning heaters. Signs and symptoms include confusion, nausea, vomiting, hypotension and dizziness. Pulse oximetry typically shows close to 100% oxygen saturations as monitors cannot differentiate between haemoglobin bound to oxygen or carbon monoxide. A blood gas will show high levels of carboxyhaemoglobin, and breath tests can be used to measure exhaled carbon monoxide. High-flow oxygen is the mainstay of treatment, with hyperbaric oxygen considered for certain high risk patients. 
 

# Definition
 
Carbon monoxide poisoning is a serious and potentially fatal condition caused by acute or chronic inhalation of carbon monoxide gas. This gas is colourless and odourless, and can only be detected by equipment such as carbon monoxide alarms. It is produced by the incomplete combustion of fuel (when insufficient oxygen is present), for example in faulty heaters or cooking equipment.
 
# Epidemiology
 
In England, there are approximately 4000 A&E attendances per year with carbon monoxide poisoning and approximately 40 deaths. Around half of cases are due to accidental exposure and around 40% are due to self-harm (e.g. from intentional inhalation of exhaust fumes in an enclosed space). 

Poisoning is more common in winter when heating equipment is more likely to be used, and is more common in groups affected by socio-economic deprivation.

Some patient groups are more sensitive to the effects of carbon monoxide poisoning, including:

- The elderly
- Young children
- Pregnant women 
- People with anaemia
- People with cardiovascular disease

# Aetiology

Inhaled carbon monoxide binds to haemoglobin with a much greater affinity than oxygen, forming carboxyhaemoglobin. This causes tissue hypoxia, especially affecting organ systems with a high oxygen demand such as the central nervous system and the cardiovascular system.

**Prevention of carbon monoxide poisoning:**

- Carbon monoxide alarms should be fitted in the home
- Gas appliances should be correctly installed and be regularly serviced
- Vulnerable patients (e.g. disabled or elderly patients) may be entitled to free annual gas safety checks
- Landlords have a legal duty to ensure gas appliances are checked at least yearly
- Do not burn charcoal indoors for cooking or heating
- Do not use gas appliances if you suspect they may be faulty

# Signs and Symptoms

- Headache
- Dizziness
- Lethargy
- Flushing
- Myalgia and weakness
- Confusion
- Nausea and vomiting
- Cherry red skin (rare)
- Tachycardia and hypotension
- Seizures

# Differential Diagnosis
 
- **Migraine:** headache is the predominant symptom of many cases of carbon monoxide poisoning; migraine is more likely to be associated with aura, photophobia and phonophobia
- **Viral infections:** cause overlapping symptoms of headache and myalgia, may have other symptoms such as cough or fever not seen in carbon monoxide poisoning
- **Diabetic ketoacidosis:** may have similar symptoms of nausea and vomiting, lethargy and weakness; blood glucose will be high with raised ketones
 
# Investigations

**Bedside tests:**

- **Pulse oximetry** is likely to show close to 100% oxygen saturations; this is artifactual as the devices cannot differentiate carboxyhaemoglobin and oxyhaemoglobin.
- A **blood gas** (either venous or arterial) should be taken to confirm the diagnosis by measuring carboxyhaemoglobin
    - The normal level in non-smokers is 1–2%
    - In smokers 5-10% is normal (heavy smokers can tolerate up to 15%)
    - Carboxyhaemoglobin of 10% or more is usually indicative of carbon monoxide poisoning
    - Toxicity usually manifests at levels of 15-20%
    - Carboxyhaemoglobin of 20% or higher is usually associated with severe cerebral or cardiac ischaemia
- An **ECG** should be done to look for ischaemic changes
- **Capillary blood glucose** to rule out hypoglycaemia or hyperglycaemia as a cause for lethargy and confusion

**Blood tests:** 

- **FBC** for anaemia
- **U&Es** for renal impairment
- **CK** for rhabdomyolysis
- **Troponin** for myocardial ischaemia
- **Lactate** - may indicate concurrent cyanide exposure if high in the context of smoke exposure

# Management
 
- Take an **A to E approach** and ensure the airway is protected in the first instance
- Administer **100% oxygen** via a tight-fitting face mask (if high-flow nasal cannulae available these can be used to administer up to 60L/min oxygen)
- Continue high-flow oxygen until any symptoms have resolved and carboxyhaemoglobin levels are 2% or lower in non-smokers or 10% or lower in smokers
- Correct hypotension with **IV fluids**
- Contact the **National Poisons Information Service** (NPIS) if advice required
- In certain severe cases (e.g. carboxyhaemoglobin > 25%) **hyperbaric oxygen therapy** may be considered however its use is currently not recommended by the NPIS
- **Inform the local Health Protection Team** who will coordinate services to address the source of carbon monoxide poisoning

# Complications

Most patients will recover fully from carbon monoxide poisoning, although in a minority of people (generally those who have had severe or prolonged exposure) neuropsychiatric complications may develop, including:

- Emotional lability and personality change
- Difficulty concentrating
- Insomnia
- Lethargy
- Movement disorders such as chorea and Parkinsonism
- Memory impairment and dementia
- Psychosis
- Neuropathy

# NICE Guidelines

[NICE CKS - Carbon Monoxide Poisoning](https://cks.nice.org.uk/topics/carbon-monoxide-poisoning/)

# References
 
[RCEM Learning - Carbon Monoxide Poisoning](https://www.rcemlearning.co.uk/reference/carbon-monoxide-poisoning/)

[London Fire Brigade - Carbon Monoxide Safety](https://www.london-fire.gov.uk/safety/the-home/carbon-monoxide-safety/)
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Emergency Management of Carbon Monoxide poisoning
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Quesmed Tutorial: Drug Toxicity and Overdose
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Elevated carboxyhemoglobin (COHb) levels in carbon monoxide poisoning occur as CO binds to hemoglobin, reducing oxygen delivery; levels above 3-4% in non-smokers or >20-30% indicate severe exposure and symptoms.
An 89-year-old woman attends A&E. She moved into a new ground-floor flat two weeks ago and calls her daughter as she says her heating appears to have broken. Her daughter finds her confused, and she is complaining of a headache, nausea, vertigo, and dizziness.

Which of these investigations is most likely to lead to the correct diagnosis?
a