Evaluation for Carbon Monoxide Exposure
Immediately administer 100% high-flow oxygen via non-rebreather mask at 15 L/min to any patient with suspected CO exposure, without waiting for diagnostic confirmation. 1, 2
Initial Diagnostic Approach
Carboxyhemoglobin Measurement
- Obtain COHb level immediately via co-oximetry on venous or arterial blood – this is the definitive diagnostic test 1, 2
- Normal COHb is <3% in nonsmokers and <10% in smokers 3
- Critical pitfall: Standard pulse oximetry is unreliable – it cannot differentiate between oxyhemoglobin and carboxyhemoglobin, showing falsely normal SpO2 readings (>90%) even with COHb levels as high as 25% 1, 2
- Fingertip pulse CO-oximetry can provide rapid screening at the scene but requires laboratory confirmation before making treatment decisions about hyperbaric oxygen 2, 3
Arterial Blood Gas Analysis
- Obtain ABG with co-oximetry capability – older analyzers that calculate SaO2 based only on PaO2 and pH will report falsely normal oxygen saturation despite high COHb levels 2
- PaO2 typically remains normal in CO poisoning because it measures dissolved oxygen in plasma, which is unaffected by CO binding to hemoglobin 2
- Check for severe metabolic acidosis or elevated lactate, which may indicate concomitant cyanide poisoning in house fire victims 3
Clinical Assessment
History Elements
- Source identification is mandatory – determine the CO exposure source (faulty heating systems, indoor propane equipment, charcoal burning, gasoline generators, vehicle exhaust) 1, 4
- Duration and intensity of exposure 1
- Time elapsed from exposure to presentation – COHb levels may be normal or low if several hours have passed or if oxygen was already administered 3
- Loss of consciousness during or after exposure 3
- Ambient CO levels from emergency personnel if available 2
Symptom Assessment
- Early symptoms (COHb 15-30%): headache, dizziness, nausea, fatigue, impaired manual dexterity, shortness of breath, chest pain 1, 4
- Severe symptoms (COHb 30-70%): loss of consciousness, confusion, seizures 4
- Cardiac symptoms: chest pain and decreased exercise tolerance can occur at COHb levels as low as 1-9% in patients with ischemic heart disease 4
Physical Examination Priorities
- Neurologic status: assess for confusion, memory impairment, focal deficits, altered mental status 1, 4
- Cardiovascular assessment: obtain 12-lead ECG to monitor for cardiac ischemia and arrhythmias – CO has direct toxic effects on myocardium 2, 5
- Respiratory status: evaluate for signs of respiratory distress, pulmonary edema 1
Additional Diagnostic Testing
Laboratory Workup
- Toxicology screening for intentional CO poisoning (present in up to 44% of cases) 3
- Blood alcohol level if mental status changes are disproportionate to COHb level 3
- Cardiac biomarkers if ischemic changes present 3
Imaging Considerations
- Brain MRI may show white-matter damage in centrum semiovale and periventricular areas, or abnormalities in globus pallidus, though neuroimaging is not required for initial diagnosis 4
Critical Clinical Pearls
COHb levels correlate poorly with symptoms or prognosis – clinical severity does not reliably match COHb percentages, and patients may have significant toxicity despite relatively low COHb levels 2, 3
Do not delay oxygen therapy while awaiting laboratory confirmation – the elimination half-life of COHb is 320 minutes on room air versus 74 minutes on 100% oxygen 1, 3
Environmental assessment is mandatory before discharge – the CO source must be identified and eliminated to prevent re-exposure 1, 3