What is Carboxyhemoglobinemia?
Carboxyhemoglobinemia is the presence of carboxyhemoglobin (COHb) in the blood—a compound formed when carbon monoxide (CO) binds to hemoglobin—resulting in impaired oxygen delivery to tissues and direct cellular toxicity. 1
Pathophysiology
Carboxyhemoglobinemia develops through two primary mechanisms:
Exogenous Carbon Monoxide Exposure
- CO binds hemoglobin with approximately 220 times the affinity of oxygen, displacing oxygen from hemoglobin binding sites 2
- This creates a functional anemia by reducing oxygen-carrying capacity 3
- CO also shifts the oxyhemoglobin dissociation curve to the left, further impairing oxygen release to tissues 1
- The elimination half-life of CO is 4-5 hours breathing room air, reduced to 85 minutes with high-flow oxygen, and 20 minutes with hyperbaric oxygen 2
Beyond Simple Hypoxia
The toxicity extends far beyond just COHb formation. Modern understanding recognizes that CO poisoning causes both tissue hypoxia AND direct cellular damage through multiple mechanisms 1:
- Binding to intracellular proteins (myoglobin, cytochrome a,a3)
- Nitric oxide generation leading to peroxynitrite production
- Lipid peroxidation by neutrophils
- Mitochondrial oxidative stress
- Apoptosis (programmed cell death)
- Immune-mediated injury
- Delayed inflammation
A critical clinical pitfall: COHb levels do NOT correlate with symptom severity or clinical outcomes 1, 3. Patients can be severely poisoned with relatively low COHb levels, and conversely, some patients tolerate high levels with minimal symptoms.
Normal vs. Pathologic Levels
Normal COHb levels:
Special populations:
- Neonates can have physiologically elevated COHb from normal fetal hemoglobin breakdown by heme oxygenase 4
- Patients with hemolytic anemia produce endogenous CO during hemoglobin metabolism, leading to elevated COHb 5, 6
Clinical Significance
CO poisoning results in approximately 50,000 emergency department visits annually in the United States and is a leading cause of poisoning death 1, 2. The presentation varies dramatically from minimal symptoms to:
- Headache, dizziness, nausea, vomiting
- Shortness of breath, chest pain
- Altered mental status, seizures
- Cardiovascular collapse
- Death
Long-term Morbidity
Neurologic sequelae occur in 12-68% of poisoned patients, manifesting as 2:
- Memory loss
- Impaired concentration or language
- Depression or other affective changes
- Parkinsonism
- Virtually any neurologic abnormality in severe cases
These can be persistent from exposure or delayed in onset by 2-21 days. Poisoned patients have up to a 3-fold increase in mortality compared to matched controls at median 7.6-year follow-up 2.
Diagnostic Considerations
COHb cannot be measured by conventional pulse oximetry 3. Accurate measurement requires:
- Laboratory CO-oximeter (most accurate)
- Pulse CO-oximetry (approximation only)
- Arterial and venous samples are clinically equivalent 3
- Samples are stable during transport 3
The classic "cherry-red" appearance is rarely seen clinically 3.