Carbon Monoxide Poisoning Author: Jeffrey T. Chapman, MD
Editorial changes - 2009-10-30
Author information and module status
Prevention
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Diagnosis
Consultation for Diagnosis
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Diagnosis
  • Obtain a careful history of neurologic and cardiac symptoms attributable to CO poisoning, sick housemates, and possible CO exposures.
  • Use physical exam to confirm neurologic or cardiac signs and to exclude alternative diagnoses and concomitant illnesses.
  • Use diagnostic tests to evaluate the severity of poisoning and assess concomitant problems.
  • Determine levels of blood HbCO.
  • Consider determination of exhaled CO as an evolving alternative test in patients with suspected CO exposure.
  • Consider using neuroimaging to exclude alternative diagnoses or prognosticate about future recovery after initial treatment.
  • Consider other diseases with neurologic or cardiac symptoms that mimic CO poisoning.
History and Physical Examination Elements for CO Poisoning (table)
Laboratory and Other Studies for CO Poisoning (table)
Differential Diagnosis of CO Poisoning (table)

Obtain a careful history of neurologic and cardiac symptoms attributable to CO poisoning, sick housemates, and possible CO exposures. BC

  • Ask about:
    • Neurologic complaints, including:
      • Nausea
      • Frontal headache
      • Lightheadedness
      • Difficulty concentrating
      • Motor difficulties
      • Visual changes
      • Confusion
      • Seizures
      • Visual agnosia
      • Psychotic episode
      • Delirium
      • Coma
    • Cardiopulmonary symptoms, including:
      • Dyspnea
      • Chest pain
      • CHF
      • Dysrhythmia
    • Other symptoms, such as:
      • Abdominal pain
      • Myalgias
      • Muscle cramping
      • Skin blistering
      • Renal failure
  • Ask about housemates with similar unexplained flu-like symptoms.
  • Ask about potential exposure to elevated CO levels from combustion vehicles and appliances.
  • Ask contacts about potential CO exposure if the patient is unable to give a history because of delirium or coma.
  • See table History and Physical Examination Elements for CO Poisoning.
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Use physical exam to confirm neurologic or cardiac signs and to exclude alternative diagnoses and concomitant illnesses. C

  • Look for:
    • Neurologic signs such as confusion, decreased acute memory, poor coordination, and poor visual perception
    • Cardiac signs suggesting ischemia, such as dysrhythmia or CHF
    • Other causes of the patient's symptoms, such as infections or primary neurologic or cardiac disease
  • See table History and Physical Examination Elements for CO Poisoning.
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Use diagnostic tests to evaluate the severity of poisoning and assess concomitant problems. C

  • Obtain:
    • CBC
    • Complete metabolic profile
    • ABGs
    • Urinalysis
    • ECG in patients with dyspnea, dysrhythmia, chest pain, or over age 40
    • CXR in all patients with dyspnea, chest pain, or smoke exposure and in all cases of severe poisoning
    • ABG in all patients with neurologic changes, dyspnea, chest pain, or smoke exposure
  • Note that pulse oximetry data are unreliable because the oximeter is unable to differentiate HbCO from HbO2
  • See table Laboratory and Other Studies for CO Poisoning.
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Determine levels of blood HbCO. C

  • Determine HbCO level:
    • With a CO oximeter in all persons with a significant exposure to enclosed combustion appliances or uncontrolled fire, even if they are asymptomatic
    • In all persons with symptoms or physical findings consistent with CO poisoning and any combustion or fire exposure
    • In comatose patients, or those unable to give a history, with a possible CO exposure
  • Recognize that:
    • HbCO at presentation may not correlate with symptoms or outcomes and that HbCO interpretation must take into account the clinical situation.
    • HbCO levels immediately after cessation of exposure correlate with symptoms:
      • 0.3% to 0.7%, no symptoms
      • 20% to 30%, headache, nausea, impaired cerebellar function
      • 50% to 60%, seizures, coma
    • Length of time since exposure cessation and ventilation amount greatly affect HbCO levels because significant delay or hyperventilation can lead to normal HbCO levels but continued symptoms.
    • High HbCO levels suggest significant exposure and tissue injury, but low levels do not necessarily indicate insignificant exposure or injury.
    • HbCO level above 25% in any patient is diagnostic of severe acute CO poisoning.
    • HbCO levels between 3% and 25% in nonsmokers are diagnostic of CO poisoning.
    • HbCO level is elevated in current smokers, with number of packs of cigarettes smoked per day correlating with HbCO, and heavy smokers achieving HbCO levels of 12% without sequelae.
  • See table Laboratory and Other Studies for CO Poisoning.
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Consider determination of exhaled CO as an evolving alternative test in patients with suspected CO exposure. C

  • Encourage the use of exhaled CO as a screening tool at the location of exposure by prehospital personnel and in rural hospitals unable to perform HbCO determination.
  • Determine the current smoking status in all patients who have exhaled CO determined.
  • Confirm elevated levels (>5 ppm) in nonsmokers by HbCO determination.
  • Recognize that low CO levels (<5 ppm) either exclude CO exposure or indicate a significant period of time since being removed from the CO source.
  • Recognize that smokers can have elevated exhaled CO levels (up to 20 ppm immediately after smoking a cigarette).
  • See table Laboratory and Other Studies for CO Poisoning.
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Consider using neuroimaging to exclude alternative diagnoses or prognosticate about future recovery after initial treatment. C

  • Do not use CT or MRI as primary diagnostic tests in uncomplicated cases because the delay typically associated with obtaining these test results may impair treatment of CO poisoning.
  • Use CT or MRI scanning of the head only when an alternative diagnosis to CO poisoning is more likely.
  • See table Laboratory and Other Studies for CO Poisoning.
Background | Back to top


Consider other diseases with neurologic or cardiac symptoms that mimic CO poisoning. C

  • In all patients quantify the exposure to CO in terms of source, duration, and time since removal from exposure.
  • Consider alternative diagnoses in patients without a history of exposure to CO or with normal HbCO findings.
  • Question all patients about ingestion of sedative medications and previous psychiatric illness, especially depression.
  • Question all patients about history of CAD, dysrhythmias, or risk factors for such.
  • See table Differential Diagnosis of CO Poisoning.
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FAQs
Jeffrey T. Chapman, MD has no financial relationships with pharmaceutical companies, biomedical device manufacturers, or health-care related organizations.
Steven E. Weinberger, MD, FACP, Acting Editor, PIER, has stock holdings in Glaxosmithkline and Abbott.

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