Carbon Monoxide Has an Affinity More Than 200 Times That of Oxygen for Binding Hemoglobin
Carbon monoxide (CO) is the substance that has an affinity approximately 220 times that of oxygen for binding to hemoglobin. 1 This extremely high binding affinity is a key factor in CO's toxicity and explains why even small amounts of CO can cause significant clinical effects.
Mechanism of Carbon Monoxide Binding to Hemoglobin
- CO binds to the same sites on the hemoglobin molecule as oxygen but with much greater affinity
- The binding of CO to hemoglobin forms carboxyhemoglobin (COHb), which:
- Reduces oxygen-carrying capacity of the blood
- Shifts the oxyhemoglobin dissociation curve to the left, further impairing oxygen delivery to tissues 1
- Creates a state of functional anemia without actual decrease in hemoglobin concentration
Comparison with Other Substances
- Carbon dioxide (option B) does not have this high binding affinity for hemoglobin
- Arsenic (option C) is toxic through different mechanisms, primarily by inhibiting cellular enzymes
- Cyanide (option D) is highly toxic but works primarily by binding to cytochrome c oxidase in mitochondria, not by binding to hemoglobin with such high affinity 1
Clinical Implications of CO's High Binding Affinity
The 220-fold greater affinity of CO for hemoglobin explains several important clinical observations:
- Even low concentrations of CO in inspired air can lead to significant COHb levels
- CO poisoning can occur rapidly in enclosed spaces with incomplete combustion
- Conventional pulse oximetry cannot differentiate between oxyhemoglobin and carboxyhemoglobin, potentially leading to falsely normal readings in CO poisoning 2
- Co-oximetry is required for accurate measurement of different hemoglobin species in suspected CO poisoning 2
Treatment Considerations Based on CO's Binding Properties
- Administration of 100% oxygen speeds the elimination of CO from the body by:
- Decreasing the elimination half-life from 4-5 hours to approximately 1 hour 1
- Competing with CO for hemoglobin binding sites
- Hyperbaric oxygen therapy further decreases the elimination half-life to about 20 minutes 1
The extremely high binding affinity of CO for hemoglobin (220 times that of oxygen) is a critical concept in understanding the pathophysiology and management of carbon monoxide poisoning.