Carbon monoxide inhalation

Carbon monoxide can be common cause of poisoning depending on your location or care of lower socioeconomic groups. It can be quite obvious if a fire has occurred or the patient admits to a suicide attempt. Symptoms can be a little more insidious and subtle if exposure has been chronic and hyper-vigilance is required.

  • Carbon monoxide deaths almost exclusively occur pre-hospital.
  • A thorough neurological exam and MMSE is required to identify those who have neuropsychological sequelae.
  • All patients are at risk of neuropsychological sequelae and should be warned of this on discharge.

High risk features include

  • Significant loss of consciousness
  • Persistent neurological dysfunction
  • Metabolic acidosis
  • Myocardial ischaemia
  • Age over 55
  • The unborn child (I know we all talk about fetal Hb having an even higher affinity for CO but that might be a little simplistic)
Reveal the fetal Hb Explanation

Traditionally, it was thought that fetal hemoglobin had a high affinity for CO. Pregnant ewe studies show a delayed but substantive increase in COHb levels in fetuses, exceeding the level and duration of those in the mothers. Thus, it appeared that fetuses are a sink for CO and could be poisoned at levels lower than mothers. However, such data may not apply to humans because it vitro work shows that as opposed to sheep, human fetal hemoglobin actually has less affinity for CO than maternal hemoglobin, at a ratio of 0.8. Under conditions of low oxygenation and high 2,3-BPG, as in serious CO poisoning, the affinity of human fetal hemoglobin starts to approach that of maternal. The more important issue with maternal CO exposure is the precipitous decrease in fetal arterial oxygen content that occurs within minutes at CO concentrations of 3000 ppm. Therefore, the ensuing hypoxia of the fetus, rather than increase in fetal COHb, is of more concern.

Tox Tute AUDIO – Carbon Monoxide (Quick version/Case Scenario)
Tox Tute AUDIO – Carbon Monoxide

Correlation of COHb levels and clinical features 

Used to confirm the diagnosis but are a poor indicator for outcome and are altered by any previous oxygen that has been applied or delayed presentation.

  • <10% = Background level in a smoker
  • 10% = Usually asymptomatic, slight headache
  • 20% = Dizziness, nausea, dyspnoea, headache
  • 30% = Vertigo, ataxia, visual disturbance
  • 40% = Confusion, coma, seizures, syncope
  • 50% = Cardiovascular compromise, respiratory failure, seizures, death

The show notes are presented as a show and reveal‘ mini quiz.

Question 1

Resus: What is the mainstay of resuscitation in CO poisoning?

Reveal the Answer

The application of oxygen. Patients who make it to hospital are very unlikely to die and require oxygen to reduce the half life of carbon monoxide. Those in the field or someone that comes in with a reduced GCS may require specific attention to their airway i.e. intubation but the overall objective is the same.

Question 2

Risk assessment: Can you name the CNS and CVS symptoms associated with carbon monoxide exposure?

Reveal the Answer
  • CNS: Headahce, nausea, dizziness, confusion, mini mental status examination errors, incorrdination, ataxia, seizures and finally coma.
  • CVS: Dysrhythmias, Ischaemia, hyper or hypotension.

Question 3

Risk assessment: What other symptoms might the patient experience with carbon monoxide exposure beyond the CNS and CVS symptoms?

Reveal the Answer
  • Non-cardiogenic pulmonary oedema
  • Lactic acidosis
  • Rhabdomylysis
  • Hyperglycaemia
  • Disseminated intravascular coagulation
  • Bullae
  • Alopecia
  • Sweat gland necrosis

Question 4

Risk assessment: In regards to long term neuropsychiatric complications what is the main difference/risk between an acute exposure and a chronic exposure?

Reveal the Answer
  • The acute exposure is usually a higher concentration of carbon monoxide for a brief period and is less likely to result in longterm sequelae.
  • Chronic exposures have a lower dose of carbon monoxide for a long period and these are high risk for developing complications. These symptoms are usually non-specific but can include personality changes, poor concentration, dementia, psychosis, Parkinsonism, ataxia, peripheral neuropathy and hearing loss.

Question 5

Investigations: You have done your COHb levels, checked lactate levels and looked for ischaemic complications. You then subject your patient to an MMSE and they have a number of errors. What or where should this patient be referred to for follow up? If you were to do a CT or MRI what are the classical findings in severe cases of carbon monoxide exposure?

Reveal the Answer
  • Anyone with a neurological deficit will require neuropsychiatric testing in 1-2 months. Complications are present in 30% of survivors at 1 month and 6-10% at 12 months.
  • A CT/MRI may demonstrate cerebral oedema, cerebral atrophy, basal ganglia injury or cortical demyelination.

Question 6

Investigations: What additional investigations should a pregnant woman receive?

Reveal the Answer
  • Foetal monitoring in the form of a CTG +/- an obstetric consultation.

Question 7

Enhanced elimination: Although currently controversial, which patients would you consider for hyperbaric oxygen?

Reveal the Answer
  • All pregnant patients
  • Significant LOC
  • Signs of ischaemia
  • Significant neurological deficit
  • Metabolic acidosis

Question 8

Disposition: How long should the patient receive oxygen for? What about if they are pregnant?

Reveal the Answer
  • 8 hours of high flow.
  • 24 hours of high flow if pregnant.

Question 9

Disposition: Your patient has some mild pass pointing so you refer him on for neuropsychiatric testing in the next couple of weeks and warn him to return if he has progressive symptoms. He tells you his exposure was from work, what should you do now?

Reveal the Answer

This patient could be an index case and you need to inform his work and occupational health as their could be more victims. Similarly if you find someone has been poisoned at home, you need to ask about other members of the household and they should also be investigated.


Additional Resources

toxicology library antidote 700 1

Toxicology Library


Dr Neil Long BMBS FACEM FRCEM FRCPC. Emergency Physician at Kelowna hospital, British Columbia. Loves the misery of alpine climbing and working in austere environments (namely tertiary trauma centres). Supporter of FOAMed, lifelong education and trying to find that elusive peak performance.

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