184 Tox icolog y
SPECIFIC POISONSCarbon monoxide
DIAGNOSIS
1 Carbon monoxide poisoning is usually associated with the combustion of
fuel with an inadequate f lue, e.g. a blocked domestic heater, or from the
fumes of a car exhaust. It is a colourless odourless gas and the most common
poison used for successful suicide in the UK and Australia.
2 Clinical manifestations are directly related to early ABG carboxyhaemo-
globin (COHb) concentration levels around the time of exposure. Later
COHb levels lack prognostic value:
(i) 0–10%: asymptomatic (may be seen in smokers).
(ii) 10–25%: throbbing frontal headache, nausea, shortness of breath
on exertion.
(iii) 25–40%: cognitive impairment, auditory and visual disturbances,
dizziness, aggression and psychosis.
(iv) 40–50%: confusion, coma and seizures.
(v) 50–70%: hypotension, respiratory failure, cardiac arrhythmias
and cardiac arrest.
(vi) >70%: death.
3 A strong clinical suspicion is important in making the diagnosis. Suspect
carbon monoxide toxicity if several members of one household present in a
similar fashion.
4 Remember a pulse oximeter does not distinguish between carboxyhaemoglobin
and ox yhaemoglobin, so will record misleadingly norma l ox ygen saturations.
(i) Therefore send an ABG sample in all cases. Look for evidence of
metabolic acidosis and an elevated carboxyhaemoglobin level.
5 Gain i.v. access and send blood for FBC, U&Es, LFTs, troponin, serum lactate
and blood sugar level. Check a -human chorionic gonadotrophin (hCG)
pregnancy test in women.
6 Perform an ECG. Look for evidence of cardiac arrhythmias or myocardial
ischaemia.
7 Request a CXR and arrange a CT brain scan in a comatose patient.MANAGEMENT
1 Secure the airway and give 100% oxygen by tight-fitting mask with reservoir
bag.
(i) Call the senior ED doctor and prepare for endotracheal
intubation in comatose patients, to protect and maintain the
airway and to optimize ventilation with 100% oxygen.
2 Commence f luid resuscitation for hypotension and to correct acid–base
disturbances. Hypotension usually responds to f luids, but may require
inotropic support.