Information ⬇
Editors
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.
- 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 (%) |
---|
60 | 10 |
120 | 20 |
180 | 29 |
220 | 33 |
230 | 37 |
350 | 40 |
520 | 50 |
Evidence Summaries ⬆