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Carboxyhemoglobin (COHb) – Definition & CO Poisoning

Carboxyhemoglobin (COHb) is a compound formed when hemoglobin binds to carbon monoxide, blocking oxygen transport in the blood and causing CO poisoning.

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Things worth knowing about "Carboxyhemoglobin"

Carboxyhemoglobin (COHb) is a compound formed when hemoglobin binds to carbon monoxide, blocking oxygen transport in the blood and causing CO poisoning.

What is Carboxyhemoglobin?

Carboxyhemoglobin (COHb) is formed when hemoglobin -- the red blood cell protein responsible for carrying oxygen -- binds to carbon monoxide (CO). Carbon monoxide is a colorless, odorless gas produced by incomplete combustion of fuels. Because hemoglobin binds carbon monoxide approximately 200 to 250 times more strongly than oxygen, even small amounts of CO can severely disrupt the delivery of oxygen to the body tissues.

Causes and Sources

Carboxyhemoglobin forms whenever carbon monoxide is inhaled. Common sources include:

Motor vehicle exhaust in poorly ventilated spaces (e.g., garages)
Faulty heating systems, gas stoves, or furnaces
Smoke from fires or wildfires
Cigarette smoke (smokers routinely have elevated COHb levels)
Industrial environments with CO exposure

Small amounts of COHb are also produced naturally during the normal breakdown of heme in the body. In non-smokers, physiological COHb levels are typically below 1–2%.

Mechanism of Action and Effects

Under normal conditions, hemoglobin picks up oxygen in the lungs and delivers it to tissues throughout the body. When carbon monoxide binds to hemoglobin, it forms carboxyhemoglobin, which is unable to carry oxygen. Furthermore, COHb impairs the release of any remaining oxygen to tissues -- a phenomenon related to the Bohr effect. The result is internal suffocation (hypoxia), even when atmospheric oxygen levels are normal.

Symptoms of Elevated Carboxyhemoglobin (CO Poisoning)

Symptoms depend on the percentage of COHb in the blood:

5–10% COHb: Headache, mild nausea, reduced physical performance
10–30% COHb: Dizziness, confusion, rapid heartbeat, shortness of breath
30–50% COHb: Severe confusion, loss of consciousness, seizures
Above 50% COHb: Coma, cardiac failure, life-threatening emergency

A classic but not always present sign of CO poisoning is a cherry-red discoloration of the skin and mucous membranes.

Diagnosis

Elevated carboxyhemoglobin is diagnosed through arterial or venous blood gas analysis, which directly measures the percentage of COHb in the blood. Standard pulse oximeters are unreliable in this context, as they cannot distinguish COHb from oxyhemoglobin and may display falsely normal oxygen saturation values. CO-oximetry or laboratory blood analysis is required for accurate diagnosis.

Treatment

Treatment focuses on eliminating carbon monoxide from the blood as quickly as possible:

Fresh air and supplemental oxygen: Moving the patient to fresh air and administering 100% oxygen via a non-rebreather mask reduces the half-life of COHb from approximately 5 hours (on room air) to about 60–90 minutes.
Hyperbaric oxygen therapy (HBO): In severe cases, high-pressure oxygen delivered in a hyperbaric chamber shortens the half-life to approximately 20–30 minutes and helps prevent long-term neurological damage.
Supportive care: Cardiac monitoring, neurological assessment, and intensive care observation as needed.

Reference Values and Clinical Significance

COHb is expressed as a percentage of total hemoglobin:

Non-smokers: below 2% (physiological baseline)
Smokers: 3–10% (chronically elevated depending on smoking intensity)
Clinically significant: above 10–15%
Life-threatening: above 40–50%

Elevated COHb may also indicate conditions involving increased heme breakdown, such as hemolytic anemia.

References

Weaver LK. - Carbon Monoxide Poisoning. New England Journal of Medicine, 2009; 360(12):1217-1225.
Hampson NB, Piantadosi CA, Thom SR, Weaver LK. - Practice Recommendations in the Diagnosis, Management, and Prevention of Carbon Monoxide Poisoning. American Journal of Respiratory and Critical Care Medicine, 2012; 186(11):1095-1101.
World Health Organization (WHO) - Carbon monoxide poisoning. Fact Sheet. Geneva 2023. Available at: https://www.who.int

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