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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Carbon monoxide is odorless and colorless; suspect with headache, dizziness, nausea, confusion, or multiple affected people.
- Standard pulse oximetry is falsely normal because it cannot distinguish oxyhemoglobin from carboxyhemoglobin.
- Treat immediately with 100% oxygen; do not wait for carboxyhemoglobin level.
- Consider hyperbaric oxygen for loss of consciousness, neurologic deficits, ischemia, severe acidosis, pregnancy, or high carboxyhemoglobin level.
- Smoke inhalation may also involve cyanide poisoning, airway burns, and trauma.
Overview
Odorless gas poisoning causing tissue hypoxia through carboxyhemoglobin formation, treated immediately with high-flow oxygen and selected hyperbaric oxygen. Emergency management focuses on early recognition, stabilization, targeted investigation, and prompt definitive therapy while avoiding common pitfalls tested on USMLE Step 2 CK.
Epidemiology
This presentation is encountered in emergency and acute care settings. Risk varies by exposure, comorbidity, age, mechanism, and timeliness of treatment. Morbidity and mortality increase when recognition is delayed or when airway, breathing, circulation, antidotal therapy, or definitive source control is postponed.
Clinical Features
Symptoms
Headache, dizziness, weakness, nausea, vomiting, or flu-like illness without fever
Confusion, syncope, seizure, coma, or focal neurologic deficit
Chest pain, dyspnea, palpitations, or myocardial ischemic symptoms
Multiple people or pets ill in the same environment
Pregnancy with exposure is high risk even if maternal symptoms are mild
Signs
Normal pulse oximetry despite significant poisoning
Tachycardia, tachypnea, hypotension, altered mental status
Signs of smoke inhalation: soot, facial burns, hoarseness, wheeze
Cherry-red skin is classic but rare and not reliable
Neurologic or cardiac abnormalities indicate severe toxicity
Investigations
First-line
Carboxyhemoglobin by co-oximetryVenous or arterial sample acceptable. Smokers may have baseline elevation; level does not perfectly correlate with severity.
ECG and troponinEvaluate myocardial ischemia/injury, especially older adults, chest pain, cardiovascular disease, or severe exposure.
Blood gas with lactateAssess acidosis and coexisting cyanide poisoning in smoke inhalation; PaO2 may be normal despite tissue hypoxia.
Second-line
Pregnancy assessmentPregnant patients require lower threshold for hyperbaric consultation because fetal hemoglobin binds CO strongly and clears slowly.
Chest X-rayIf smoke inhalation, hypoxemia, respiratory symptoms, or associated trauma.
Neurocognitive assessmentBaseline mental status and neurologic deficits guide hyperbaric consideration and follow-up.
Specialist
Hyperbaric medicine / poison center consultationFor severe symptoms, pregnancy, high levels, cardiac ischemia, acidosis, or neurologic findings.
1
Immediate care
- Remove patient from exposure and ensure scene safety.
- Give 100% oxygen by nonrebreather mask immediately; intubate if airway/ventilation compromised.
- Continue high-flow oxygen until symptoms resolve and carboxyhemoglobin falls substantially.
- Evaluate and treat smoke inhalation, burns, trauma, and cyanide poisoning when relevant.
2
Hyperbaric oxygen consideration
- Consider for loss of consciousness, persistent neurologic symptoms, abnormal cerebellar testing, severe metabolic acidosis, cardiovascular dysfunction, ischemia, or pregnancy.
- Consider for high carboxyhemoglobin levels, commonly >25% in adults or >15-20% in pregnancy, depending on protocol.
- Do not delay high-flow normobaric oxygen while arranging hyperbaric therapy.
3
Disposition and follow-up
- Admit patients with neurologic symptoms, cardiac injury, severe acidosis, pregnancy requiring monitoring, persistent symptoms, or social safety concerns.
- Warn about delayed neuropsychiatric syndrome.
- Ensure source remediation before return home.
Complications
- Myocardial injury: CO poisoning can trigger ischemia, arrhythmia, and long-term mortality risk
- Delayed neuropsychiatric syndrome: Cognitive, mood, movement, and gait problems days to weeks later
- Fetal injury: Fetal CO clearance is delayed
- Smoke inhalation complications: Airway edema, cyanide poisoning, burns, and ARDS
- Death: From severe hypoxic injury or associated trauma
USMLE Step 2 CK Exam Tips
- 1Headache + multiple household members sick + winter/heater/generator exposure = CO poisoning.
- 2Pulse oximetry is falsely normal; order co-oximetry/carboxyhemoglobin.
- 3Treat with 100% oxygen immediately before labs return.
- 4Cherry-red skin is rare — do not rely on it.
- 5Pregnancy is a hyperbaric oxygen consultation trigger at lower thresholds.
- 6CO poisoning can cause myocardial ischemia — check ECG/troponin.
practicetest your knowledge on toxic ingestions — carbon monoxideApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — emergency medicine and beyond.
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