• Diagnosis
o History of exposure
• Confirmation
o Co-oximeter
o Arterial or venous line samples
• CO levels are elevated for days
• Actual CO levels measured on arrival to the hospital correlates poorly with clinical status
• Should not be used as sole criterion to determine need for treatment
o CO levels on exhaled air
• Pulse oximetry with only two light sources (wavelengths) will always show a normal oxygen saturation of >92%
o Always will be an overestimate of oxygen saturation
o Carboxyhemoglobin (carbon monoxide bound to hemoglobin) absorbs more infrared (940 nm) than red (660 nm) wavelength of light just like oxygenated hemoglobin (HgbO2) → infrared to red light absorption ratio is the same as oxygenated hemoglobin (HgbO2)
o Hemoglobin’s affinity for carbon monoxide is much greater than it is for oxygen → increased carbon monoxide saturation of hemoglobin → less oxygen saturation of hemoglobin → decreased oxygen carrying capacity of blood
• Left shift in oxygen-hemoglobin dissociation curve
o Increased affinity of Hgb for oxygen → decreased oxygen release to tissue (tissue hypoxia)
o Lower p50 – less PaO2 required to saturate 50% of Hgb with oxygen
o Hemoglobin’s affinity for carbon monoxide is 200x that of oxygen → carboxyhemoglobin → carboxyhemoglobin has increased affinity for oxygen → left shift of hemoglobin dissociation curve
• Treatment
o 100% oxygen
• Reverses carboxyhemoglobinemia within an hour
o Hyperbaric oxygen
• Carboxyhemoglobin >25%
• Neurologic impairment
• Cardiac abnormalities
• Differential diagnoses
o Methemoglobinemia (met-Hb)
• Pulse oximetry always shows ~85%
• Causes falsely low oxygen saturations readings on pulse ox when SaO2 is actually greater than 85% and vice versa
• Methemoglobin absorbs more red (660 nm) than infrared (940 nm) wavelength light (opposite of oxygenated hemoglobin)
• Methemoglobin absorbs both red and infrared wavelengths of light equally – 1:1 ratio
• Left shift in oxygen-hemoglobin dissociation curve
• Increased affinity of Hgb for oxygen → decreased oxygen release to tissue (tissue hypoxia)
• Lower p50 – less PaO2 required to saturate 50% of Hgb with oxygen
o Alcohol intoxication
• Differentiae by blood ethanol levels
o Sedative hypnotic overdose
• Differentiate by urine toxicology screens
o Cyanide poisoning
• Differentiate by increased levels of serum cyanide
Keyword history
Percentage correct 74% Year asked 2014
