Carbon monoxide toxicity

Epidemiology & Risk Factors

• Etiology: Incomplete combustion of carbon-containing materials (hydrocarbons).
• Common Sources:
  • Smoke inhalation from house fires (often co-exists with cyanide toxicity).
  • Faulty home heating appliances (furnaces, gas/wood heaters) in winter.
  • Poorly ventilated running motor vehicles, generators, or charcoal grills indoors.
  • Hookah / water-pipe smoking.
• Pathophysiology:
  • High affinity for hemoglobin (Hb) (~200–250× greater than O₂), forming carboxyhemoglobin (COHb).
  • Causes a left shift in the oxygen-hemoglobin dissociation curve (decreased O₂ unloading to tissues).
  • Inhibits cytochrome c oxidase in the mitochondrial electron transport chain $\to$ cellular hypoxia & lactic acidosis.

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Clinical Features

• General Rule: Multiple family members or co-habitants presenting simultaneously with identical, non-specific symptoms during winter months.
• Mild-to-Moderate Toxicity:
  • Tension-type headache (most common presenting symptom).
  • Nausea, vomiting, dizziness, weakness, malaise, confusion (frequently misdiagnosed as viral gastroenteritis or influenza).
• Severe Toxicity:
  • Syncope, seizures, altered mental status (AMS), coma.
  • Myocardial ischemia/infarction, dysrhythmias, hypotension, pulmonary edema.
• Physical Exam:
  • “Cherry-red” skin and mucous membranes (classic NBME buzzword; rare in vivo, typically a post-mortem finding).
  • Retinal hemorrhages (flame-shaped) on fundoscopy.
  • Tachypnea, tachycardia.

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Diagnosis

• Initial / Screening Trap:
  • Standard pulse oximetry ($Sp{O}_2$) is FALSELY NORMAL (conventional pulse oximeters cannot differentiate between oxyhemoglobin and COHb).
• Confirmatory / Gold Standard:
  • Co-oximetry on ABG or VBG: Directly quantifies Carboxyhemoglobin (COHb) level.
    • COHb > 3% in non-smokers = Diagnostic.
    • COHb > 10–15% in chronic smokers = Diagnostic.
• Key Laboratory Workup:
  • ABG: Normal $Pa{O}_2$, normal calculated $Sa{O}_2$, but severely reduced actual $O_2$ content; anion gap metabolic acidosis (due to elevated lactic acid).
  • Cardiac Enzymes & ECG: Serial troponins and 12-lead ECG to rule out myocardial ischemia/infarction (CO is highly toxic to myocardium).
  • Serum Lactate & CK: Elevated lactate (tissue hypoxia); elevated CK (rhabdomyolysis in prolonged immobilization/coma).
  • Toxicology Screen & Pregnancy Test: Rule out co-ingestions; urine $\beta$-hCG in all females of childbearing age (fetus is at extremely high risk).
• Imaging:
  • Brain CT/MRI: Bilateral, symmetric globus pallidus T2/FLAIR hyperintensity or hypodensity (characteristic ischemic necrosis). c
  • CXR: Normal or non-cardiogenic pulmonary edema in severe smoke exposure.

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Differential Diagnostics

• Cyanide Poisoning:
  • Diff: Also presents post-smoke inhalation with severe lactic acidosis ($\text{lactate}>8\text{–}10\text{ mmol/L}$), but COHb is normal (unless co-exposed). Breath smells like bitter almonds. Treated with hydroxocobalamin.
• Methemoglobinemia:
  • Diff: Induced by oxidants (dapsone, benzocaine/lidocaine, nitrates). Cyanosis unresponsive to $O_2$, “chocolate-brown blood”, $Sp{O}_2$ fixed around 85%. Diagnosis confirmed by elevated MetHb on co-oximetry. Treated with methylene blue.
• Viral Gastroenteritis / Influenza:
  • Diff: Normal COHb levels. Symptoms persist outside the home and do not resolve with ambient fresh air.
• Acute Coronary Syndrome (ACS):
  • Diff: May be triggered by CO toxicity in underlying CAD, but isolated ACS lacks co-oximetry elevation of COHb and basal ganglia involvement.

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Management

• Immediate Step:
  • Remove patient from exposure source immediately; secure airway, breathing, circulation (ABC approach).
• First-Line Therapy (Mild to Moderate):
  • 100% High-Flow Normobaric Oxygen ($NB{O}_2$) via non-rebreather mask (NRB) or endotracheal tube.
  • Mechanism: Reduces COHb half-life from ~300 minutes (on room air) to ~60–90 minutes.
  • Continue until the patient is asymptomatic and $\text{COHb}<3\text{–}5\%$.
• Second-Line / Definitive for Severe Cases: Hyperbaric Oxygen Therapy (HBO):
  • Mechanism: Reduces COHb half-life to ~20–30 minutes and facilitates tissue-level oxygenation independent of Hb.
  • Indications for HBO ($100\%O_2\text{at }2.5\text{–}3\text{ atm}$):
    • COHb > 25% in any patient.
    • COHb > 15% in pregnant patients (fetal Hb has higher affinity for CO; prolonged fetal half-life).
    • Loss of consciousness / syncope.
    • Severe neurologic deficits (AMS, seizures, coma).
    • Evidence of myocardial ischemia (ECG changes, chest pain, elevated troponin).
    • Severe persistent metabolic acidosis ($\text{pH}<7.1$ or high lactate) refractory to $NB{O}_2$.

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Complications

• Delayed Neuropsychiatric Syndrome (DNS):
  • Develops 2 to 40 days after initial apparent full recovery.
  • Manifests as cognitive impairment, memory loss, parkinsonism, psychosis, personality changes, ataxia, or dementia.
• Myocardial Infarction & Arrhythmias:
  • Long-term increased cardiovascular mortality due to hypoxic myocardial injury.
• Cerebral Edema & Permanent Basal Ganglia Necrosis.
• Intrauterine Fetal Demise (IUFD) / severe teratogenic encephalopathy in pregnancy.