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Editors

MikaValtonen
JouniKurola

Carbon Monoxide Poisoning

Essentials

  • Carbon monoxide (CO) is formed by incomplete combustion. Air CO levels exceeding 0.01% will cause poisoning symptoms and levels exceeding 0.2% cause immediate death.
  • Carbon monoxide poisoning leading to death is most often caused by a domestic fire or suicide. Accidental poisonings are caused by petrol-driven motors running at idle in a closed space, by various heaters that use fuels or gas, and also by traditional wood-burning heaters. The severity of poisoning is dependent both on the concentration of CO and on the exposure time.
  • Regular pulse oximetry does not distinguish between carboxyhaemoglobin (COHb) and oxyhaemoglobin (OHb) and thus gives false normal results even for a severely anoxic victim.
  • If carbon monoxide poisoning is associated with a burn injury, a burn centre should be consulted.

Mechanisms of poisoning

  • As the binding affinity of CO for haemoglobin is 210 times that of oxygen, CO reduces the blood oxygen transport capacity.
  • The release of oxygen from haemoglobin to tissues is impaired.
    • The dissociation curve is shifted to the left.
  • A direct cyanide-like effect blocking cell respiration

Symptoms and findings

  • Symptoms are nonspecific and vary.
  • Headache, dyspnoea, nausea
  • Confusion, unconsciousness, convulsions

Diagnosis

  • Clinical suspicion based on the history of events is of primary importance.
    • Fire victim
    • Person found unconscious in a wood-heated apartment, a car or garage
  • The level of carboxyhaemoglobin (COHb %) in arterial blood should be measured.
    • For equivalent carbon monoxide and carboxyhaemoglobin levels, see Table T1.
    • A COHb level exceeding 10% always signifies poisoning.
  • The partial pressure of oxygen in arterial blood is normal.
  • There is often also respiratory alkalosis Blood Gas Analysis and Acid-Base Balance.

Treatment Hyperbaric Oxygen Therapy for Carbon Monoxide Poisoning

  • Remember the possibility of cyanide poisoning.

Oxygen therapy

  • Administration of 100% oxygen should be started immediately.
    • Reservoir mask, securing the airway, as necessary
  • Oxygen therapy should be continued until symptoms subside and COHb levels fall below 5%.
    • The half-life of COHb is about 5 h when breathing air, 80-100 min. when breathing 100% oxygen, and 20 min. with hyperbaric oxygen therapy.
    • COHb levels should be extrapolated back to the end of exposure (levels falling by about 50% an hour when breathing 100% oxygen).

Hyperbaric oxygen therapy (HBOT)

  • There is no clear evidence for the benefit of HBOT in carbon monoxide poisoning.
  • HBOT is recommended for severe carbon monoxide poisoning.
  • Minimizing the delay to treatment will improve the prognosis.
  • Primarily, carboxyhaemoglobin should be measured in the emergency unit and other severe causes of disturbed consciousness should be excluded before transfer.
  • Find out about locally available units with HBOT and relevant policies.

Therapy indications

  • The patient is or has been unconscious.
  • Besides headache, the patient has other neurological symptoms.
  • The patient has severe cardiovascular symptoms.
  • The blood concentration of COHb extrapolated back to the time of exposure is > 40%, even if the patient is asymptomatic.
  • The blood concentration of COHb is > 20% and the patient is pregnant.
    • There is a high risk of foetal poisoning.

Equivalent carbon monoxide (CO) and carboxyhaemoglobin (COHb) levels

CO (parts per million, ppm)COHb (%)
6010
12020
18029
22033
23037
35040
52050

Evidence Summaries