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Tight affinity
All bound same vicinity
Space heater on fritz
Household red, barely alive
Force free heme, O₂, deep dive    
@DrCindyCooper

CO poisoning is not apparent on a pulse oximeter and needs an ABG if suspected. Key to prognosis is removal from the source of carbon monoxide as quickly as possible and instigation of high flow oxygen treatment. Normally CO is <10%. In city dwellers or smokers, levels can be raised up to 10%. A carboxyhaemoglobin level of 30% indicates severe exposure.

About

• CO binds avidly to Hb x 240 affinity than Oxygen
• CO is an odourless colourless gas
• Saturation probes treat CoHb as HbO₂ - false normal SaO₂
• The O₂ sat probe can be normal despite severe tissue hypoxia

Aetiology

• Result is tissue hypoxia and metabolic acidosis
• Carbon monoxide also inhibits cytochrome oxidase a3.
• The result is leftward shift in oxygen haemoglobin dissociation curve.

Clinical

• The patient is rosy and pink and not blue/cyanosed at all
• Drowsiness, headaches, fatigue, breathless
• Confusion: Mild disorientation or difficulty thinking clearly.
• Coma and death, Pink rosy colouration
• Arrhythmias and cardiac symptoms

Investigations

• FBC, U&E, ↑ lactate
• CK, troponin if needed
• Glucose: always exclude hypoglycaemia as cause of coma
• ECG - ischaemia/infarction
• Arterial blood gas: Measure CoHb and Normally CO is <10%. In city dwellers or smokers, levels can be raised up to 10%. A carboxyhaemoglobin level of 30% indicates severe exposure.
• CT head - exclude other causes of coma until diagnosis is made, cerebral oedema, ICH

Assessing COHb

• Less than COHb 1 to 3% for non-smokers is normal
• Up to COHb 5% in women who are pregnant, or people with anaemia.
• Normally COHb is <10%. Seen in city dwellers or smokers
• COhB up to 10% in smokers, and up to 13% in heavy smokers.
• CoHb level >10% : poisoning, poorly ventilated boilers or old heating systems.
• CoHb level > 10-20% : nausea, headache vomiting, dizziness predominant.
• CoHb level > 20-30% severe exposure. Arrhythmias, angina, respiratory failure and seizures. cherry-red skin, lethargy, hypotension, poor capillary refill, unconsciousness, syncope, seizures, coma, and cardiac arrest

Indications for Hyperbaric oxygen

• CoHb > 40%, Coma, ECG changes
• Neurological or psychiatric problems
• Pregnancy NB fetal CoHB is to the left of mothers

Differentials

• Opiate overdose
• Subdural haematoma
• Intracranial lesion

Management

Administer 100% oxygen via a non-rebreather mask. This helps to displace CO from haemoglobin and speed up the elimination of CO from the body.

• ABC, 15 L/min O₂. Give high FiO₂. Give 100% O₂ at least 12 hours or until carboxyhaemoglobin is normal and symptoms resolved is based on expert opinion in a narrative review. When breathing air, the half-life of carboxyhaemoglobin is 320 minutes. This can be reduced to 80 minutes when breathing 100% O₂
• Hyperbaric O₂ is ideal but the logistics in transferring a patient are significant and improved outcome has not been proven.
• Patients are also prone to Cerebral oedema so do neuro observations and they may need Mannitol and neuro ICU. There may be long term neuropsychiatric damage and parkinsonism.
• Pregnancy: fetus is susceptible to the toxic effects of maternal hypoxia. Fetal Hb has an even greater affinity than adult Hb for CO binding. COHb levels above 15% represent severe poisoning

References

• Carbon Monoxide Toxicity