Nejm 2009;360:1217; 1998;339:1603
Cause:Carbon monoxide: O = C 
O– = C+
Pathophys:
C = O binds with 2 electrons to the Fe++, just as O2 does, to bring its total electron complement to a stable 36. Carboxyhemoglobin has a high association rate but a slow dissociation rate (200 times oxygen's affinity to bind to hemoglobin)—ie, it shifts the hemoglobin dissociation curve to the left (increased affinity) and thus decreases hemoglobin's carrying capacity despite normal pO2. Since CO also greatly increases the other 3 hemes' affinity for O2, even one CO per hemoglobin molecule really slows up O2 dissociation. Also poisons mitochondrial cytochrome cellular respiration system.
Children physiologically more susceptible to injury.
Oxidation of fuels in presence of insufficient O2 forms carboxyhemoglobin, eg, car exhaust, fire smoke, gas appliances in campers and enclosed-cabin pleasure boats (Jama 1995;274:1615) especially at high altitudes (Sci Am Text Med 1982). Rarely from methylene chloride, absorbed cutaneiously from paint thinner and metabolized to CO in liver 600 accidental and 3000-6000 intentional suicidal deaths/yr in US.
Sx:
If >10% of hemoglobin is carboxyhemoglobin, sx include headache and dizziness; get level in patients with these sx if others in house have similar sx and home heating system would be compatible with carbon monoxide poisoning (Ann IM 1987;107:174).
If 30-40% carboxyhemoglobin: severe headache, easy fatigability. If 40-60% carboxyhemoglobin: unconscious.
Si:Retinal hemorrhages, flame-shaped (Sci Am Text Med 1982); pink skin and mucous membranes
CNS damage including permanent memory loss and seizures, and peripheral neuritis from the anoxia, hyperthermia from sweat gland necrosis; precipitation of ischemic heart disease, MI, and incr long-term mortality (Jama 2006;295:398; Nejm 1995;332:48). Delayed (3-240 days) neuropsych sx and si, including personality changes, dementia, incontinence, psychosis, all w 50-75% 1 yr recovery
r/o cyanide poisoning from combustible plastics in burn patients w oxygen refractory acidosis
Lab:
ABGs:Normal pO2; depressed O2 saturation, but not pulse oximetry, which is falsely high (Ann Emerg Med 1994;24:252); metabolic acidosis if severe
Chem:Carboxyhemoglobin level, >10-15; can be up to 15% in smokers
Rx: Prevent w home CO detectors (Jama 1998;279:685)
Support oxygenation with 100% O2 as soon as suspect, at 1 atm pressure allows 2 vol % O2 to be carried in plasma alone
Hyperbaric chamber (Nejm 2002;347:1057 [NNT=6], 1105, vs no help—Med J Austr 1999;170:203) if available and practical, tid 100% O2 at 3 atm results in plasma carrying 6 vol %; consider especially if levels >25%, pregnant, unconscious, angina, other mental status changes
Exchange transfusion?