COHb

Carboxyhemoglobin (COHb) is the form of hemoglobin in which carbon monoxide (CO) is bound to the iron of the heme group. CO has a much higher affinity for hemoglobin than oxygen, so inhaled CO rapidly forms COHb, reducing the blood’s capacity to carry oxygen and shifting the oxygen-hemoglobin dissociation curve to the left, which impairs oxygen release to tissues.

The primary toxic effect of CO is tissue hypoxia due to decreased oxygen delivery. CO also impairs cellular respiration by binding to mitochondrial cytochrome oxidase at high concentrations and by increasing the affinity of remaining hemoglobin for oxygen, further reducing tissue oxygen use. The result is a mismatch between oxygen supply and demand, especially in the brain and heart. COHb levels are used to assess exposure, and symptoms correlate with both COHb percentage and exposure duration.

Common sources include combustion exhaust from motor vehicles, faulty furnaces or boilers, smoke inhalation from fires, and cigarette smoke. Occupational and environmental exposures can occur in confined or poorly ventilated spaces. Diagnosis relies on co-oximetry to measure COHb levels; pulse oximetry cannot distinguish COHb from oxyhemoglobin.

Treatment begins with removing the person from the exposure and administering 100% oxygen to accelerate CO clearance and improve oxygen delivery. Hyperbaric oxygen therapy may be indicated for patients with COHb above specific thresholds, pregnancy, neurological symptoms, cardiovascular dysfunction, or loss of consciousness. Prolonged recovery can include delayed neurological sequelae, making prompt recognition and treatment important. COHb levels decrease with time, with shorter half-lives on high fractions of inspired oxygen.