Rationale:

• The diverse symptoms testify to the ubiquity of CO-binding proteins and the dependence on oxygen utilization within all cells.
• Housemates frequently have symptoms similar to the index case because of common exposure.

Evidence:

• Three recent expert reviews describe that acute CO intoxication manifests with variable symptoms (16), but most relate to neurologic (17) or cardiopulmonary dysfunction (18).
• The mechanism of organ dysfunction is from decreased oxygen delivery and even more importantly impairment of intracellular cytochromes as noted in a recent review (19).
• An observational study noted that of 89 patients presenting with a flu-like illness in the winter months, the 4 with CO poisoning had a similarly affected housemate and heated with an alternative indoor fuel source (20).

Comments:

• Adult hemoglobin binds CO with high affinity, approximately 200 to 250 times stronger than oxygen. Elevated blood CO levels displace oxygen from hemoglobin and left shift the oxygen-hemoglobin dissociation curve, both of which cause decreased delivery of oxygen to tissue.
• In addition, elevated intracellular levels of CO cause mitochondrial and other types of cell dysfunction through CO binding of cytochromes. This is an important concept and explains why end-organ dysfunction persists despite normalization of tissue oxygen delivery.
• Neurologic and cardiopulmonary function are most frequently impaired because of the fixed energy requirements of these organs.