Test Description

Pathophysiology of CO Poisoning

Carbon monoxide binds hemoglobin with 230-270 times the affinity of oxygen, forming carboxyhemoglobin (COHb). This reduces the oxygen-carrying capacity of hemoglobin AND shifts the oxygen-hemoglobin dissociation curve to the LEFT, impairing oxygen release at the tissue level. The result is cellular hypoxia disproportionate to the COHb level.

CO also has direct cellular toxicity beyond hemoglobin binding:

  • Binds myoglobin — causes direct cardiac toxicity and myocardial depression
  • Binds cytochrome oxidase — disrupts mitochondrial electron transport chain
  • Triggers inflammatory cascades — lipid peroxidation, delayed neurologic injury
  • Binds fetal hemoglobin with even greater affinity — fetus is at extreme risk

How to Measure COHb

Carboxyhemoglobin must be measured by co-oximetry, which is available on most arterial (or venous) blood gas analyzers. It spectrophotometrically differentiates between oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin.

Standard pulse oximetry reads COHb as oxyhemoglobin. A patient with 40% COHb may display SpO2 of 99%. The pulse ox is FALSELY REASSURING. You must order a co-oximetry blood gas to diagnose CO poisoning. Some newer pulse oximeters (e.g., Masimo RAD-57) can estimate COHb noninvasively but are screening tools, not definitive.
Quick Reference
  • Normal COHb: Non-smoker <3%; Smoker up to 10-15%
  • Symptomatic: Typically >10-15%; headache, nausea, dizziness
  • Severe Poisoning: >25% — confusion, syncope, cardiac ischemia
  • Life-Threatening: >40-50% — seizures, coma, death
  • Pulse Oximetry: UNRELIABLE — SpO2 reads falsely normal in CO poisoning
  • Treatment: 100% O2 via NRB (half-life reduces to 60-90 min); consider hyperbaric O2 (HBO) for severe cases
  • Key Test: Co-oximetry on arterial or venous blood gas (NOT standard pulse oximetry)
  • Don't Forget: Co-exposure to cyanide in structural fire victims
COHb Levels & Clinical Correlation
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COHb Level Expected Findings
<3% Normal (non-smoker)
3-10% Normal for smoker; non-smokers may have mild headache; neonates may have slightly higher baseline
10-20% Headache (throbbing, frontal), nausea, dizziness, dyspnea on exertion
20-30% Severe headache, impaired judgment, visual disturbance, confusion, vomiting, tachycardia
30-40% Syncope, tachypnea, altered mental status, chest pain/cardiac ischemia (even in young patients)
40-60% Seizures, coma, cardiovascular collapse, lactic acidosis
>60-70% Usually fatal
Symptoms correlate poorly with COHb level. Levels may have dropped significantly by the time blood is drawn (especially if O2 was given prehospital). A "normal" or mildly elevated level does NOT exclude significant CO exposure. Treat based on history, symptoms, and duration of exposure.
Common Sources of CO Exposure

Intentional/Suicidal

  • Car exhaust in enclosed space (less common with catalytic converters)
  • Charcoal grills or hibachis burning indoors
  • Chemical CO generators (formic acid + sulfuric acid)

Accidental

  • House fires — most common cause of severe CO poisoning; always consider concurrent cyanide exposure
  • Furnace/heater malfunction — classic winter presentation; often affects multiple family members
  • Generators used indoors — common after power outages
  • Water heaters with inadequate ventilation
  • Operating gas-powered equipment in enclosed spaces (forklifts, saws, pressure washers)
  • Methylene chloride exposure (paint stripper) — metabolized to CO in the liver; delayed and prolonged COHb elevation
"Cluster presentation" clue: If multiple members of the same household present to the ED with headache, nausea, and dizziness — especially in winter — think CO poisoning until proven otherwise. Ask about shared living space, heating type, and recent appliance changes.
Treatment & Hyperbaric Oxygen

Immediate Treatment — All Patients

  1. Remove from exposure
  2. 100% oxygen via non-rebreather mask (NRB) at 15 L/min
    • COHb half-life: ~4-6 hours on room air → ~60-90 minutes on 100% O2 → ~20-30 minutes on hyperbaric O2 at 2.5-3 ATA
  3. Continuous cardiac monitoring — CO causes myocardial ischemia and arrhythmias
  4. Check COHb level via co-oximetry (ABG or VBG)
  5. Obtain troponin — cardiac injury is common and correlates with mortality
  6. ECG — look for ischemia, arrhythmias, QT prolongation
  7. Obtain metabolic panel, lactate — lactic acidosis indicates tissue hypoxia
  8. Pregnancy test in women of reproductive age (fetal hemoglobin binds CO even more avidly)

Hyperbaric Oxygen (HBO) Therapy

Indications for HBO referral (variable by institution, but commonly accepted):

  • Loss of consciousness at any point during exposure
  • Neurologic symptoms — confusion, ataxia, seizures, focal deficits
  • COHb >25% (some use >20%)
  • Pregnancy — low threshold due to fetal risk (some centers treat all pregnant CO patients)
  • Cardiac ischemia/arrhythmia secondary to CO
  • Metabolic acidosis (pH <7.1)
  • Persistent symptoms despite normobaric 100% O2

COHb Half-Life by Oxygen Delivery

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Oxygen Delivery COHb Half-Life
Room air (21% O2) 4-6 hours
100% O2 via NRB 60-90 minutes
Hyperbaric O2 (2.5-3 ATA) 20-30 minutes
Continue 100% O2 until COHb <5% AND symptoms have resolved. Do not stop O2 just because the COHb is trending down — the goal is complete elimination and symptom resolution. In practice, most patients need at least 4-6 hours of high-flow O2.
Delayed Neurologic Sequelae (DNS)

One of the most feared complications of CO poisoning is delayed neuropsychiatric syndrome (DNS), which develops days to weeks after apparent recovery:

  • Onset: 2-40 days after exposure (most commonly 2-3 weeks)
  • Incidence: 10-30% of moderate-to-severe CO poisoning
  • Manifestations:
    • Cognitive deficits — memory impairment, difficulty concentrating
    • Personality changes — irritability, impulsivity
    • Movement disorders — parkinsonism, gait disturbance
    • Dementia
    • Psychosis, depression
  • Imaging: MRI may show bilateral globus pallidus necrosis (classic finding) and white matter demyelination
  • Prognosis: 50-75% recover within 1 year, but some have permanent deficits
The primary argument for HBO therapy is prevention of delayed neurologic sequelae. The evidence is mixed, with the Weaver et al. (2002) trial showing benefit and other trials showing no difference. Most toxicologists still recommend HBO for significant exposures.
Clinical Pearls
"Cherry red" skin is a myth for clinical practice: While cherry-red discoloration of skin (and retinal vessels) is classically described, it is actually a late/postmortem finding seen only at very high COHb levels. Most living CO patients appear pale or cyanotic. Do NOT rely on skin color to diagnose or exclude CO poisoning.
VBG is adequate for COHb measurement: Venous blood gas co-oximetry provides essentially the same COHb value as arterial. You do NOT need an ABG specifically for CO level. VBG is less painful and equally diagnostic for this purpose.
Fire victims: always consider cyanide co-exposure. Structural fires produce both CO and hydrogen cyanide (HCN) from combustion of synthetic materials (plastics, carpet, furniture). If a fire victim has persistent lactic acidosis despite O2 and COHb treatment, empirically treat for cyanide with hydroxocobalamin (Cyanokit).
Pregnancy and CO: Fetal hemoglobin binds CO more avidly than adult hemoglobin, and the fetus has a higher baseline COHb. A maternal COHb that appears only mildly elevated may represent significant fetal exposure. Most experts recommend a lower threshold for HBO in pregnancy — some centers treat any symptomatic pregnant patient with HBO.
Methylene chloride (paint stripper): This is metabolized by the liver to CO, producing a delayed and prolonged COHb elevation that can last 24+ hours (unlike inhalation CO, which clears much faster). These patients may need prolonged O2 therapy and repeated COHb monitoring.
The "saturation gap": If pulse oximetry shows SpO2 of 99% but the patient's calculated SaO2 from ABG (based on PaO2) should be lower, or if the measured SaO2 on co-oximetry is significantly lower than the SpO2, suspect CO (or methemoglobin) poisoning. This "gap" between pulse ox and true saturation is a classic diagnostic clue.
References
  1. Weaver, L. K., Hopkins, R. O., Chan, K. J., et al. (2002). Hyperbaric oxygen for acute carbon monoxide poisoning. New England Journal of Medicine, 347(14), 1057-1067.
  2. Rose, J. J., Wang, L., Xu, Q., et al. (2017). Carbon monoxide poisoning: pathogenesis, management, and future directions of therapy. American Journal of Respiratory and Critical Care Medicine, 195(5), 596-606.
  3. Hampson, N. B., Piantadosi, C. A., Thom, S. R., & Weaver, L. K. (2012). Practice recommendations in the diagnosis, management and prevention of carbon monoxide poisoning. American Journal of Respiratory and Critical Care Medicine, 186(11), 1095-1101.
  4. Wolf, S. J., et al. (2008). Clinical policy: critical issues in the management of adult patients presenting to the emergency department with acute carbon monoxide poisoning. Annals of Emergency Medicine, 51(2), 138-152.
  5. Nelson, L. S., Howland, M. A., Lewin, N. A., et al. (Eds.). (2019). Goldfrank's Toxicologic Emergencies (11th ed.). McGraw-Hill. Chapter: Carbon Monoxide.
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