Causes of Elevated Carboxyhemoglobin Levels
Carbon monoxide exposure is the primary cause of elevated carboxyhemoglobin (COHb) levels, with smoking being the most common everyday source in the general population. 1, 2
Common Sources of Carbon Monoxide Exposure
External Environmental Sources
- Exhaust fumes from motor vehicles 3
- Smoke from fires 3
- Faulty heating systems or furnaces 3
- Improperly ventilated fuel-burning appliances 1
- Indoor use of generators 2
- Indoor charcoal burning 4
Tobacco-Related Sources
- Cigarette smoking: Each pack per day increases COHb by approximately 2.5% 1, 2
- Cigar smoking: Can cause COHb levels as high as 13.0-38.6% 5
- Waterpipe/hookah smoking: Typically results in COHb levels of 10.1% ±2.5%, with cases reported up to 39.2% 5
Normal vs. Abnormal COHb Levels
Normal Range
Abnormal Levels
- Non-smokers: ≥3-4% indicates abnormal exposure 1
- Smokers: ≥10% is considered outside the expected physiological range 1
- Heavy smokers: May reach 15% routinely 6
- Extreme cases: Documented cases of 35% from heavy cigarillo smoking 6
Clinical Correlation with COHb Levels
It's important to note that symptoms correlate poorly with COHb levels 1, 2. This presents a diagnostic challenge as:
- Patients with high COHb levels (16-23%) may sometimes be asymptomatic 7
- Symptoms like headache, dizziness, and nausea are nonspecific 1, 2, 3
- More severe cases may present with tachycardia, tachypnea, and CNS depression 3
Measurement Considerations
When measuring COHb levels, be aware of these important technical considerations:
- Laboratory spectrophotometry (CO-oximeter) is required for accurate measurement 1, 2
- Either arterial or venous blood can be used 1
- Standard pulse oximeters cannot differentiate COHb from oxyhemoglobin, potentially giving falsely normal oxygen saturation readings 1
- Fingertip pulse CO oximetry should be confirmed with laboratory testing 1
Clinical Pitfalls to Avoid
- Don't rely on "cherry red" skin coloration as a diagnostic sign - this is rare and typically only appears with lethal COHb levels 1
- Don't dismiss CO poisoning due to normal pulse oximetry readings, as standard pulse oximeters cannot detect COHb 1
- Don't assume symptoms will always correlate with COHb levels 1, 2
- Don't overlook the possibility of CO exposure in patients with nonspecific symptoms, especially during cold weather 1, 2
- Remember that COHb levels may be lower than at the time of exposure if there was a delay in measurement or if oxygen therapy was initiated 1
Maintaining a high index of suspicion for CO exposure is crucial, particularly during cold weather months and in patients presenting with nonspecific symptoms like headache, dizziness, fatigue, and nausea 1, 2.