Recommendations
Urgent
Suspect acute carbon monoxide poisoning in a patient with: headache (often tension-type), nausea, vomiting, vertigo, drowsiness, dizziness, dyspnoea, tachypnoea, chest pain, confusion, syncope, irritability, diarrhoea, abdominal pain.[1][10][17][19][21][22]
Have a high index of suspicion as symptoms are non-specific.[2][17][19][23] Misdiagnosis can lead to the patient being sent home, possibly returning to the source of the carbon monoxide exposure.[18]
Use the COMA questions to quickly assess the likelihood of carbon monoxide exposure:[21]
C: Cohabitees/companions - Is anyone else in the property affected (including pets)?
O: Outdoors - Do the symptoms improve when out of the building?
M: Maintenance - Are any fuel-burning appliances and vents properly maintained?
A: Alarm - Does the affected building have a carbon monoxide alarm?
Be alert to features of severe poisoning, including:
Key Recommendations
Take a thorough history to assess the likelihood of carbon monoxide exposure.[22]
Make a clinical diagnosis based on the symptoms and a possible history of exposure to carbon monoxide.
Use a blood gas analysis to confirm diagnosis based on the carboxyhaemoglobin level.[17][22]
Do not use standard pulse oximetry as a measure of oxygen saturation.[18] It is an unreliable test in carbon monoxide poisoning.[1][19]
Perform a cardiovascular and neurological examination.[18][22]
Have a high index of clinical suspicion for carbon monoxide poisoning. Symptoms may be non-specific.[2][17][19]
The differential diagnoses include more common presentations, such as influenza, gastroenteritis, migraine, or alcohol toxicity.[22] See Differential diagnosis for more information.
Misdiagnosis can lead to the patient being sent home, possibly returning to the source of the carbon monoxide exposure.[18]
Most patients experience no or minor symptoms only.[7]
Practical tip
Patients may delay presentation, believing that they have flu or food poisoning, as the presenting symptoms may be similar.[22]
Be alert to features of severe poisoning, including:
Cardiac arrest
Severe cardiovascular features such as arrhythmias, or myocardial infarction[1][19]
Neurological features such as seizures, coma, cortical blindness or cerebral oedema[1][19]
Elevated lactate
Liver ischaemia
Acute kidney injury[1]
Cheyne-Stokes breathing.[24]
Acute carbon monoxide poisoning
Suspect acute carbon monoxide poisoning in a patient with: headache (often tension-type), nausea, vomiting, vertigo, drowsiness, dizziness, dyspnoea, tachypnoea, chest pain, confusion, syncope, irritability, diarrhoea, or abdominal pain.[1][2][10][17][19][21][22][23]
Other features include non-cardiogenic pulmonary oedema, lactic acidosis, rhabdomyolysis, hyperglycaemia, disseminated intravascular coagulation, and skin lesions such as blisters and sweat gland necrosis.[10][22][25][26][27]
Symptoms may fluctuate within hours, therefore making severity difficult to determine.[22]
Exposure to high concentrations of carbon monoxide leads to collapse and death within minutes.[18]
Chronic carbon monoxide poisoning
Chronic carbon monoxide poisoning may occur over weeks to months. Be aware that chronic poisoning is frequently misdiagnosed because the clinical features are non-specific.[10] These clinical features include:
Headache[10]
Nausea[10]
Memory change[10]
Flu-like symptoms[10]
Confusion[22]
Stomach pains/non-specific pain[22]
Shortness of breath[22]
Bear in mind that symptoms may be similar to those of acute poisoning, but with a more gradual and insidious onset.
Always suspect chronic poisoning if other members of the same household have similar symptoms.[10]
Sub-lethal levels of carbon monoxide poisoning can lead to chronic health problems, such as delayed neuropsychiatric features.[18]
Note that blood levels may not accumulate to a life-threatening level in a patient experiencing chronic exposure because the patient may not always be in proximity to the source. Prolonged exposure will cause blood levels to accumulate beyond a symptomatic threshold, leading to an acute presentation.[22]
Delayed features of carbon monoxide poisoning
Bear in mind that a patient previously exposed to carbon monoxide might display delayed neuropsychiatric features.[10]
Take a thorough history to assess the likelihood of carbon monoxide exposure.[22] Use the COMA questions:[21]
C: Cohabitees/companions - Is anyone else in the property affected (including pets)?
O: Outdoors - Do the symptoms improve when out of the building?
M: Maintenance - Are any fuel-burning appliances and vents properly maintained?
A: Alarm - Does the affected building have a carbon monoxide alarm?
Ascertain the type and duration of exposure to carbon monoxide.[17] Potential sources of carbon monoxide include:[10][18]
Stoves[19]
Heaters
Boilers[1]
Fires and portable fuel burners[19]
Exhaust fumes
Cigarette smoke
Gas-powered tools or generators[19]
House fires.[1]
Cigarette smoke.
[Figure caption and citation for the preceding image starts]: Common sources of carbon monoxideAshcroft J, et al. Carbon monoxide poisoning. BMJ 2019;365:l2299 (used with permission) [Citation ends].
Poorly installed fuel burning appliances, or those that are faulty or used inappropriately (including with inadequate ventilation) are common sources of carbon monoxide.[1]
Examples include home boilers that are incorrectly installed, and barbecues or portable generators used inside homes, caravans and tents.[1]
Ask if the patient has noticed soot around appliances, smoke, or an increase in condensation. All of which may indicate a faulty appliance.[18]
Poisoning may be more common during the winter months when heating devices are more likely to be used.[10]
Faulty flues from gas appliances are a common source of exposure. Ask about the use of a caravan, boat, or mobile home as these may use portable heating devices. Also consider exhaust fumes from vehicle engines and generators.[1][18]
Ask about any recent time spent away from the suspected source of carbon monoxide. Leaving doors and windows shut allows carbon monoxide to accumulate, causing acute and severe symptoms when the person returns to this environment.[22]
Ask whether any members of the same house have had similar symptoms.[10]
Ascertain whether the patient has any comorbidities.
Consider any indications that lead you to suspect that poisoning was intentional as an act of self-harm and refer to senior colleagues for assessment.[10]
A large proportion of deaths from carbon monoxide poisoning have been linked to suicide.[29]
See our topic Suicide risk mitigation.
Ask about any pets having sudden illness or unexpectedly dying. Pets are often the first to show the signs of carbon monoxide poisoning due to their small size.[22]
Determine the length of time since the patient left the carbon monoxide environment.[19]
Consider whether any personal protective equipment (PPE) is needed before undertaking any examination.[10]
In a patient exposed to a toxic substance by inhalation the risk of contamination to a healthcare provider in hospital is very low.
Wear standard hospital PPE as a precaution.
Wear the appropriate PPE for chemical exposure where the patient has surface contamination and has not yet been decontaminated.
Work in a well-ventilated area.[30]
Assess for signs of respiratory failure, non-cardiogenic pulmonary oedema, and Cheyne-Stokes breathing, which are features of severe poisoning.[1][10][24]
Check the patient’s skin for lesions such as blisters and sweat gland necrosis.[10][27]
Perform a neurological examination and test the patient’s coordination and balance.
Carbon monoxide poisoning may cause sensory changes and ataxia.[22]
Seizures, coma, cortical blindness, or cerebral oedema may be present in people with severe poisoning.[1][10][17][19][22]
Use the mini-mental state examination or a similar test of short-term memory and cognitive function.[18][19]
Assess blood pressure as the patient may be hypotensive or hypertensive.[1][19][22][31]
The ‘cherry red’ skin tone often described in textbooks is very rarely seen in practice.[1][2][18][22]
Use venous or arterial blood gas analysis to measure carboxyhaemoglobin levels.[17][19][22] This is the gold standard test.
An initial carboxyhaemoglobin level >30% indicates severe poisoning; however, significant poisoning cannot be excluded at lower concentrations.[1][10] Toxic effects of carbon monoxide poisoning appear at carboxyhaemoglobin levels of 15% to 20%.[22]
Non-smokers will have a baseline carboxyhaemoglobin of 1% to 3%.[1][10][22]
People who smoke will tend to have a baseline carboxyhaemoglobin of 5% to 10%.[1][10] This may reach 15% in those who smoke heavily.[22] Acute carbon monoxide poisoning can lead to a carboxyhaemoglobin level of 50% or more in people who smoke.[32]
Be aware that carboxyhaemoglobin levels do not correlate well with clinical outcome.[1][19][17]
Determine the length of time since the patient left the carbon monoxide environment.[19]
Carboxyhaemoglobin levels may decrease with time and treatment and may not reflect the true severity of the exposure.[15]
If the patient has been breathing room air for several hours, carboxyhaemoglobin levels may be unreliable.[19]
Practical tip
Note that some patients may have a physiologically high carboxyhaemoglobin level due to endogenous production as a result of heme metabolism (e.g., haemolytic anaemia or haemolysis in patients with sickle cell disease).[1]
Do not use breath analysis to rule out carbon monoxide poisoning.[22] The results do not correlate well with the severity of the poisoning.[33]
Breath tests are commonly used to detect carbon monoxide in smoking cessation programmes. They may detect a high concentration of exhaled carbon monoxide, which indicates excess carbon monoxide exposure. Confirm any positive results with a blood sample to measure carboxyhaemoglobin levels.[10]
Recent cigarette smoking will affect the result.
Lactose intolerance may affect the result as these patients will have a higher concentration of hydrogen in expired breath.[18]
Perform a cardiovascular examination with a 12-lead ECG to check for sinus tachycardia.[1][19][22]
Check cardiac rhythm, QRS duration, and QT interval.[10]
Carbon monoxide poisoning may be associated with angina, cardiac ischaemia and, less commonly, dysrhythmias and myocardial infarction.[1][22]
Test cardiac biomarkers and use cardiac monitoring to check for arrhythmias or other cardiovascular effects.
As well as for blood gas analysis, take blood for:
Full blood count[10]
Look for leukocytosis
Urea and electrolytes[10]
Check for results outside of normal range
Creatinine
Acute kidney injury is a feature of carbon monoxide poisoning.[10] See our topic Acute kidney injury.
Creatine kinase (CK)[10]
May be elevated
Provides evidence of skeletal muscle damage
CK level greater than 5 times the normal level, or more than 1000 IU/L indicates rhabdomyolysis. See our topic Rhabdomyolysis.
Elevated troponin-I level may indicate cardiomyopathy and predict poor outcome[34]
Lactate
May be elevated in severe carbon monoxide poisoning[22]
Check for hypoglycaemia in an obtunded patient
Blood glucose is often normal or elevated in patients with carbon monoxide poisoning[25]
Liver function tests
Consider in patients with severe poisoning
Derangement may indicate liver ischaemia.
Do not use pulse oximetry as a measure of oxygen saturation.[18]
Pulse oximetry readings are not reliable in people with suspected carbon monoxide poisoning.[1][19] Carboxyhaemoglobin may cause an artificially ‘normal’ reading because it has similar light absorption characteristics as oxyhaemoglobin.
Carboxyhaemoglobin levels above 2% may cause falsely elevated SpO 2 measurements.[35]
Carbon monoxide poisoning impairs the ability of haemoglobin to bind oxygen, thereby reducing oxygen-carrying capacity.[32]
A specific carbon monoxide pulse oximeter may be used.[1][10][18]
Order a pregnancy test for all women of childbearing age with suspected carbon monoxide poisoning.[17][19] In practice, this is particularly important for moderate or severe poisoning.
Exposure to carbon monoxide during pregnancy can cause harm to the fetus.[1]
Request a CT scan of the head when patients present with severe carbon monoxide poisoning (especially loss of consciousness or cardiopulmonary signs and symptoms), or acute neurological symptoms, such as confusion, headache, blurred vision, and/or seizure.[9][19]
This is to rule out cerebral infarction that is directly due to carbon monoxide poisoning, or differential diagnoses.[9][19]
Consider a chest X-ray in severe carbon monoxide poisoning, especially for patients with loss of consciousness or cardiopulmonary symptoms and signs.[19] Be aware that patients can develop pulmonary complications at a later stage.
Consider magnetic resonance spectroscopy and discuss with the neuroradiology consultant. This may indicate sequelae from carbon monoxide poisoning.[36][37]
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