Intended for healthcare professionals

Rapid response to:

Endgames Case report

Headache and drowsiness in a 22 year old student

BMJ 2008; 337 doi: (Published 24 September 2008) Cite this as: BMJ 2008;337:a1481

Rapid Response:

Carbon Monoxide poisoning

This endgames article dangerously indicates that arterial paO2 needs
to be abnormal in cases of carbon monoxide(CO) poisoning.
Partial pressure merely describes the tendancy for the gas molecules to
escape the solvent they are in. If the solubility of oxygen(O2) in blood
is altered by binding of CO to haemoglobin, the O2 content will vary but
the partial pressure will stay the same(as long as the gas mixture the
blood equilibrates against remains constant).

The lethal problem in CO poisoning is a reduced O2 CONTENT in blood
because oxygen's binding sites on haemoglobin are blocked by CO with
consequent insufficient oxygen delivery to tissues.
This will produce a lactic acidosis as a consequence of anaerobic
respiration, which may be the only abnormality on blood gas analysis if
carboxyhaemoglobin is not measured directly.

To be even more obtuse the paO2 MAY be slightly decreased at the scene of
the poisoning as a high CO concentration will dilute the concentration of
O2 in the inspired gas mixture that the victim is breathing. However as
soon as the patient is moved away the paO2 will reflect the new (normal)
ambient pO2 in air or hopefully the enriched oxygen mixture the ambulance
crew are administering!

Competing interests:
None declared

Competing interests: No competing interests

28 September 2008
Graham J Walker
consultant anaesthetist
Oxford radcliffe hospitals NHS trust