Mechanism and Pharmacokinetics
Carbon monoxide (CO) binds to hemoglobin with an affinity 250 times that of oxygen, resulting in reduced oxyhemoglobin saturation and decreased oxygen carrying capacity, shifting the oxygen-hemoglobin dissociation curve to the left and causing tissue hypoxia.
Signs and Symptoms
Related to level of carboxyhemoglobin. All are manifestations of tissue hypoxia, predominantly in organs with high oxygen consumption.
- Majority report headache, dizziness, and nausea. Confusion, loss of consciousness also possible.
- No reliable exam findings. Cherry red skin coloration or bright red venous blood is possible, but not sensitive (generally a post-mortem finding). Severe exposures lead to impaired thinking, seizures, arrhythmias, pulmonary edema, myocardial infarction, coma, and death.
- SpO2 via standard pulse oximetry typically normal.
Evaluation
Diagnosis is not difficult if there is a clear history of exposure, but can easily be missed due to nonspecific symptoms. Consider especially in winter months (use of indoor heating, use of a portable generator), or when other household members have similar symptoms.
- Obtain carboxyhemoglobin level (specifically request VBG or ABG with “co-oximetry”). Pulse oximetry gives falsely normal readings because it cannot distinguish between oxyhemoglobin and carboxyhemoglobin, and a non-co-oximeter ABG will give a normal PaO2 with a falsely elevated estimate of SaO2.
- Check ECG, CBC, CMP.
Management
- 100% oxygen via NRB or HFNC speeds the elimination of CO to approximately 90 minutes (6 hours RA).
- If comatose or severely impaired mental status, intubate immediately and ventilate with 100% FiO2.
- Treat until the carboxyhemoglobin level is less than 5%.
- Hyperbaric treatment is controversial, but may be considered in the most severe cases (e.g., coma due to CO, carboxyhemoglobin >25%, severe metabolic acidosis, myocardial ischemia, cerebellar dysfunction). Contact Poison Control for guidance.
- In smoke inhalation, concomitant cyanide poisoning is also possible. If severely ill, consider empiric treatment with hydroxycobalamin. Contact Poison Control for guidance.
Key Points
- ABG/VBG with co-oximetry is key to making the diagnosis.
- Main treatment is 100% oxygen.
- Call Poison Control (800-222-1222) or Medical Toxicology consult service (415-443-0122) for assistance.
Poisoning and Drug Overdose, 6th ed. 2012.