Toxicology — Carbon Monoxide Poisoning

Carboxyhemoglobin Formation and Oxygen Deprivation

• CO binds to the iron (Fe²⁺) moiety within the heme pocket of hemoglobin with binding affinity approximately 210-250 times greater than oxygen, forming carboxyhemoglobin (COHb)
• This preferential binding occurs because CO binds to the sixth coordination position of iron with strong σ-bonding and backbonding interactions, creating an energetically favorable complex
• Formation of COHb not only removes binding sites for oxygen but also causes a leftward shift of the oxygen-hemoglobin dissociation curve through allosteric effects on remaining oxygen-binding subunits, further impairing oxygen release to tissues
• The result is histotoxic hypoxia where arterial oxygen saturation (measured by standard pulse oximetry) may appear falsely normal (typically reads ~85% when COHb is 15-30%) because pulse oximeters cannot distinguish between oxyhemoglobin and carboxyhemoglobin; functional oxygen delivery to tissues becomes severely compromised despite adequate hemoglobin saturation readings
• This explains why patients may have pulse oximetry readings of 94-98% while being critically hypoxic—a critical diagnostic trap

Mitochondrial Cytochrome c Oxidase Inhibition

• CO also directly inhibits cytochrome c oxidase (Complex IV) in the mitochondrial electron transport chain by binding to the ferrous heme a₃ component
• This blocks aerobic ATP production even when oxygen is present and sufficient, creating a second independent mechanism of cellular energy failure distinct from simple oxygen deprivation
• The combination of blocked oxygen transport (via hemoglobin) and blocked oxygen utilization (via cytochrome oxidase) produces severe metabolic derangement
• This explains why patients can deteriorate even with supplemental oxygen alone if carboxyhemoglobin levels remain elevated

Oxidative Stress and Cellular Injury

• Reperfusion injury during reoxygenation generates excessive reactive oxygen species (ROS) including superoxide (O₂⁻), hydroxyl radical (OH•), and hydrogen peroxide (H₂O₂)
• These free radicals damage cellular lipid membranes through lipid peroxidation, denature proteins through oxidative modification, and cause DNA strand breaks through multiple mechanisms
• The white matter of the brain is particularly vulnerable due to high lipid content and relatively limited antioxidant defenses compared to gray matter
• This oxidative injury continues even after CO has been eliminated, explaining delayed neuropsychiatric sequelae that emerge days to weeks after apparent recovery

Inflammatory Cascade Activation

• CO exposure triggers activation of the innate immune system through danger-associated molecular patterns (DAMPs) released by hypoxic/injured cells
• Nuclear factor-kappa B (NF-κB) pathway activation leads to upregulation of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) and activation of leukocyte infiltration into affected tissues
• Microglial activation in brain tissue produces additional inflammatory mediators and perpetuates neuronal injury even after the initial hypoxic insult has resolved
• This neuroinflammation appears central to the pathophysiology of delayed neuropsychiatric syndrome

Tissue-Specific Vulnerability

• The brain and heart are most vulnerable due to high metabolic rates (accounting for 20% and 10% of total body oxygen consumption respectively despite comprising only 2% and 0.5% of body weight)
• The basal ganglia (particularly the globus pallidus), white matter, and hippocampus show preferential involvement in CO poisoning-related brain injury
• Delayed injury to white matter tracts causes the characteristic delayed neuropsychiatric syndrome distinct from acute poisoning symptoms

Primary Sources of Carbon Monoxide Exposure

Vehicle Exhaust and Motor-Related Exposure

• Motor vehicles remain the single most common unintentional source of CO in developed nations, especially in enclosed spaces (garages, tunnels)
• Both acute (exhaust piping into closed vehicle) and chronic (occupational exposure in parking attendants, mechanics) exposures occur
• The prevalence of catalytic converters has significantly reduced but not eliminated this source

Residential Heating and Combustion Appliances

• Faulty furnaces, kerosene heaters, wood stoves, and space heaters produce CO when combustion is incomplete or ventilation is inadequate
• Seasonal clustering occurs in winter months when heating systems are in use and homes are sealed
• Fire-related CO exposure causes significant poisoning during structural fires; firefighters are at particular occupational risk

Occupational Exposures

• Occupations with chronic low-level CO exposure include: metal welders, smelter workers, firefighters, miners, traffic police, parking garage attendants, and workers near furnaces
• Chronic occupational exposure may produce subtle neuropsychiatric effects distinct from acute poisoning

Consumer Products and Recreational Activities

• Portable generators used indoors (increasingly common during hurricanes/power outages), particularly when placed near windows or air intakes
• Charcoal grills and burning charcoal indoors, especially in enclosed spaces like tents or campers
• Boat engines and personal watercraft engines in enclosed spaces
• Improperly ventilated lanterns and camping stoves

Intentional Exposure (Suicide)

• CO exposure via vehicle exhaust has been a common method of suicide historically, though increasing use of catalytic converters has reduced lethality
• Represents approximately 10-15% of CO poisoning cases in emergency settings

Secondary Risk Factors Increasing Severity

• Pregnancy: Placental transfer of CO and carboxyhemoglobin persistence longer in fetus increases risk of fetal demise, teratogenicity, and developmental abnormalities
• Cardiovascular disease: Patients with coronary artery disease, myocardial infarction history, arrhythmias, or heart failure develop symptomatic disease at lower COHb levels
• Chronic pulmonary disease: COPD and asthma patients have reduced respiratory reserve
• Anemia: Reduced hemoglobin concentration limits oxygen-carrying capacity independent of CO binding
• Age extremes: Children have higher metabolic rates; elderly have reduced physiologic reserve

Cardinal Symptoms (Acute Exposure)

Headache

• Present in 80-90% of acute CO poisoning cases, often the most prominent symptom in mild-moderate exposure
• Typically described as diffuse, throbbing, and progressive, unresponsive to conventional analgesics
• Neurophysiologic basis: direct effect of CO on cerebral metabolism combined with increased intracranial pressure from cerebral edema
• May be the only symptom at COHb levels of 10-20%

Dizziness and Vertigo

• Results from brainstem and cerebellar hypoxia
• Often accompanies headache and may progress to frank ataxia in moderate poisoning
• Can lead to falls and traumatic injuries, particularly in elderly patients

Nausea and Vomiting

• Present in 40-50% of cases, occurs through direct chemoreceptor trigger zone stimulation and cerebral hypoxia
• Not specific to CO poisoning and contributes to misdiagnosis (attributed to gastroenteritis, migraine, etc.)

Chest Pain and Dyspnea

• Occurs particularly in patients with underlying coronary disease or cardiomyopathy
• In pure CO poisoning without preexisting cardiac disease, dyspnea may be relatively absent despite significant hypoxia—another diagnostic trap
• When present, chest pain often prompts premature diagnosis of acute coronary syndrome, delaying appropriate CO-specific treatment

Confusion and Altered Mental Status

• Ranges from subtle impaired cognition to frank confusion, delirium, or loss of consciousness
• Results from diffuse cerebral hypoxia and metabolic derangement
• May manifest as personality changes, poor judgment, or combativeness that relatives report as "unusual behavior"
• In severe exposure, rapid progression to loss of consciousness with seizures

Syncope

• Occurs in moderate-to-severe poisoning due to combined cerebral hypoxia and cardiac dysfunction
• May be accompanied by seizures or aspiration
• Indicates severe exposure and poor prognosis

Physical Examination Findings

Vital Sign Abnormalities

• Tachycardia: Usually present, reflecting sympathetic response to tissue hypoxia; may be absent in severe poisoning
• Tachypnea: Typically present but variable; normal respiratory rate does NOT exclude CO poisoning
• Hypertension or hypotension: Hypertension from sympathetic activation common; hypotension indicates severe poisoning or cardiogenic shock
• Hypothermia: May occur due to altered metabolic rate and hypothalamic dysfunction

Characteristic but Uncommon Physical Findings

• "Cherry-red" lips and skin: Classically taught but actually rare, present in <10% of cases; when present indicates severe poisoning with high COHb levels; results from bright pink carboxyhemoglobin pigment
• Cyanosis: Notably ABSENT in most cases despite severe tissue hypoxia, another classic trap—patients appear deceptively well despite critical illness
• Pulmonary edema: Manifests as bilateral crackles; may develop acutely in severe poisoning
• Altered mental status ranging from confusion to unresponsiveness: Severity correlates imperfectly with COHb levels

Neurological Examination Findings

• Pyramidal signs: Weakness, hyperreflexia, clonus indicating upper motor neuron involvement from white matter injury
• Extrapyramidal signs: Parkinsonism, rigidity, bradykinesia appearing during acute phase or after delayed onset
• Cerebellar signs: Ataxia, dysarthria, dysmetria
• Cranial nerve abnormalities: Facial weakness, dysphagia may appear
• Cognitive changes: Impaired attention, memory deficits, slow thought process

Clinical Variants and Atypical Presentations

Chronic Low-Level Exposure

• Workers with occupational CO exposure may present with insidious cognitive decline, personality changes, mood disturbances, or subtle memory problems
• These patients may never have acute symptoms prompting diagnosis
• Affects judgment, work performance, and safety

Myocardial Infarction Presentation

• Patients with underlying coronary disease may present primarily with chest pain, ECG changes, and elevated troponins
• CO exposure unmasking critical coronary stenosis or precipitating demand ischemia
• Troponin elevation occurs in 5-20% of moderate-to-severe poisoning even without coronary disease, from direct myocardial hypoxia

Asymptomatic or Minimally Symptomatic Exposure

• Some patients with moderate COHb levels (25-35%) may report minimal symptoms, especially young healthy individuals
• Creates dangerous false reassurance about severity
• These patients remain at risk for delayed complications

Clinical Suspicion and History

Key Historical Elements

• Source identification: Ask specifically about potential CO sources—home heating systems, vehicle exhaust, generators, charcoal grills, fires
• Temporal clustering: Multiple household members or coworkers with similar symptoms simultaneously (flu-like illness in winter → consider CO before influenza)
• Symptom progression: Relationship between symptom onset and time spent in certain locations (symptoms improve when leaving home → source in home)
• Season: Winter heating season increases incidence substantially
• Recent events: Power outages, hurricanes (generator use), house fires, vehicle maintenance in enclosed spaces
• Timeline: Acute exposure typically causes rapid symptom onset within minutes to hours; chronic occupational exposure produces gradual symptom evolution

Diagnostic Tests

Carboxyhemoglobin (COHb) Level - Gold Standard

• Testing method: Co-oximetry (multi-wavelength spectrophotometry) is the only accurate way to measure COHb; standard pulse oximetry cannot differentiate oxyhemoglobin from carboxyhemoglobin and provides falsely reassuring readings
• Normal values: <2% in non-smokers; <3% in smokers (due to ambient CO and smoking exposure)
• Interpretation by severity:
• Mild poisoning: COHb 10-20% (typically causes headache, mild symptoms)
• Moderate poisoning: COHb 20-40% (confusion, significant symptoms)
• Severe poisoning: COHb 40-60% (loss of consciousness, seizures, cardiac dysfunction)
• Critical poisoning: COHb >60% (usually fatal without immediate intervention)
• Critical pearl: There is NO RELIABLE CORRELATION between COHb level and symptom severity or prognosis at presentation; patients with COHb 15% may have severe symptoms while COHb 40% patients may appear relatively well
• Half-life: In room air: 4-6 hours; on 100% oxygen: 60-90 minutes; on hyperbaric oxygen: 15-30 minutes (this is why COHb measurement must occur at time of diagnosis—levels fall rapidly during transport)
• Timing issue: COHb must be measured immediately upon arrival; delaying transport or observation periods allows COHb decline, potentially underestimating exposure

Laboratory Studies Supporting the Diagnosis

Arterial or Venous Blood Gas

• pH: May show metabolic acidosis (elevated lactate from anaerobic metabolism and mitochondrial dysfunction)
• PCO₂: Usually normal to low (if tachypneic)
• PO₂: Often paradoxically NORMAL or elevated even with severe poisoning because dissolved oxygen in plasma is adequate; this is the key trap—normal PO₂ does not exclude CO poisoning
• HCO₃⁻: Usually normal unless metabolic acidosis present

Serum Lactate

• Elevated lactate indicates anaerobic metabolism and tissue hypoxia, supporting severity assessment
• Lactate >5 mmol/L associated with worse outcomes and increased risk of complications
• Does not prove CO poisoning but provides evidence of hypoxia when COHb delayed

Cardiac Biomarkers

• Troponin I or T: Elevated in 5-20% of moderate-to-severe CO poisoning, indicating direct myocardial injury or demand ischemia
• Elevation associated with increased risk of arrhythmias and complications
• Presence may indicate need for extended cardiac monitoring

Complete Blood Count

• Primarily to assess baseline hemoglobin for oxygen-carrying capacity
• Anemia worsens CO poisoning effects due to reduced functional hemoglobin

Electrocardiogram (ECG)

• Acute changes: ST-segment depression, T-wave inversion, prolonged QT interval (indicates myocardial hypoxia)
• Arrhythmias: Atrial fibrillation, premature ventricular contractions, bradycardia, sinus node dysfunction
• Findings: ECG abnormalities present in 10-30% of moderate-to-severe poisoning
• May show pattern mimicking acute myocardial infarction (especially in inferior distribution)

Neuroimaging

Computed Tomography (CT) Head

• Acute phase: Often normal in first 24 hours despite symptomatic poisoning
• When present: Bilateral globus pallidus hypodensity/necrosis (characteristic finding if present), though may take days to appear
• Use: Primarily to exclude other causes of altered mental status (hemorrhage, stroke, trauma)
• Sensitivity/specificity: CT limited for diagnosing CO poisoning; absence of findings does NOT exclude diagnosis

Magnetic Resonance Imaging (MRI) Brain

• **Acute phase (24-72 hours

Immediate stabilisation (before any confirmatory test)

• Remove from the source and give 100% oxygen: high-flow non-rebreather at 15 L/min is the single most important first action; oxygen competitively displaces CO from the heme iron and shortens COHb half-life from hours to roughly an hour. The 2017 ACEP Clinical Policy on acute CO poisoning supports immediate normobaric oxygen for all suspected cases — do not wait for a co-oximetry result.
• Airway and circulation: intubate and ventilate with FiO₂ 1.0 for coma, seizures, or airway burns. Follow AHA ACLS for arrest; treat hypotension with isotonic crystalloid and vasopressors as needed.
• Continuous cardiac monitoring, ECG, and troponin in anyone with chest pain, syncope, age >40, or known coronary disease.

Duration and endpoint

• Normobaric 100% oxygen is continued until symptoms resolve and COHb normalises (commonly to <5%); a longer course is customary in pregnancy because fetal hemoglobin binds CO more avidly and fetal COHb clears more slowly.

Escalation — hyperbaric oxygen (HBO)

• HBO at 2.5–3 atmospheres dissolves enough oxygen in plasma to support tissues independent of hemoglobin, accelerates CO clearance from cytochrome c oxidase, and reduces leukocyte adhesion implicated in delayed neurologic injury.
• The Undersea and Hyperbaric Medical Society lists CO poisoning as an approved indication, typically for syncope or any loss of consciousness, neurologic deficit, seizure, myocardial ischemia or arrhythmia, persistent metabolic acidosis, markedly elevated COHb, and pregnancy with fetal distress or significant maternal exposure. ACEP notes the evidence base is insufficient to define precise selection criteria — consult toxicology/poison control (1-800-222-1222).

Concomitant cyanide (enclosed-space fire)

• Hydroxocobalamin (Cyanokit) IV is the antidote of choice; sodium thiosulfate may be added.

Contraindicated / avoid

• Sodium nitrite in smoke-inhalation victims — induced methemoglobinemia further cripples oxygen carriage atop existing COHb.
• Methylene blue has no role (it treats methemoglobinemia).
• Untreated pneumothorax must be decompressed before HBO.
• Never titrate therapy to pulse oximetry, and never discharge a patient back to an uninspected source.

Neurologic

• Delayed neuropsychiatric syndrome (DNS): the signature complication — lucid recovery followed days to weeks later by cognitive impairment, memory loss, parkinsonism, mutism, gait apraxia, urinary incontinence, or affective change. Mechanism is ongoing lipid peroxidation, leukocyte-mediated neuroinflammation, and demyelination of periventricular white matter. Signalled by bilateral globus pallidus lesions and confluent white-matter hyperintensity on MRI. Not an emergency but the reason for scheduled neurologic follow-up.
• Cerebral edema, seizures, coma: emergency; from global hypoxic-ischemic injury. Signalled by declining GCS, posturing, or status epilepticus.

Cardiac

• Myocardial ischemia/infarction and arrhythmia: emergency; demand ischemia plus direct myocyte cytochrome inhibition. Signalled by ST-segment depression, T-wave inversion, or troponin elevation; myocardial injury after CO poisoning predicts increased long-term mortality.
• Cardiogenic shock and pulmonary edema: emergency; stunned myocardium with hypoxic capillary leak. Signalled by hypotension with bilateral crackles and diffuse infiltrates.

Metabolic and muscular

• Rhabdomyolysis with acute kidney injury: from prolonged immobility on a hard surface plus muscle hypoxia; signalled by markedly elevated CK, dark urine, urine dipstick positive for blood without red cells.
• Persistent lactic acidosis: reflects mitochondrial failure; in a fire victim, a very high lactate with normal PaO₂ should raise concern for coexisting cyanide toxicity — an emergency.

Obstetric

• Fetal hypoxia and demise: emergency; fetal hemoglobin binds CO more tightly and fetal COHb peaks later and clears more slowly than maternal. Signalled by non-reassuring fetal heart tracing; maternal COHb underestimates fetal risk.

Complications of treatment

• Hyperbaric oxygen: middle-ear and sinus barotrauma (most common — tympanic membrane rupture), pulmonary barotrauma/pneumothorax on ascent, CNS oxygen-toxicity seizures, claustrophobia, and reversible myopia. Transporting an unstable patient to a chamber is itself a hazard.
• Prolonged high-FiO₂ therapy: absorption atelectasis and hyperoxic lung injury.
• Sodium nitrite given for presumed cyanide causes methemoglobinemia that compounds impaired oxygen delivery.

• The single best next step is 100% oxygen by non-rebreather, given before the COHb result returns. Any stem with a winter space heater, a generator after a storm, or a running car in a garage is testing this reflex.
• Pulse oximetry is the classic trap: it reads carboxyhemoglobin as oxyhemoglobin, so SpO₂ is falsely normal. PaO₂ is also normal because dissolved oxygen is unaffected. Order co-oximetry — it is the only test that distinguishes the two.
• Multiple people (or the family pet) sick in the same house with "flu" in winter, improving away from home, is the classic vignette. Cherry-red skin is a buzzword but is rare and usually postmortem — do not require it.
• COHb level does not predict severity or outcome. Disposition and HBO referral hinge on syncope, neurologic deficit, ischemic ECG/troponin, acidosis, and pregnancy — the UHMS indications — not on the number alone.
• The association examiners love: bilateral globus pallidus necrosis on CT/MRI, and delayed neuropsychiatric syndrome appearing days to weeks after apparent full recovery (parkinsonism, incontinence, memory loss). Always arrange neurologic follow-up.
• Pregnancy is a special case: fetal hemoglobin holds CO more avidly and clears it more slowly, so treat longer and at a lower threshold than maternal symptoms suggest, per UHMS.
• House-fire victim with profound lactic acidosis despite oxygen → suspect concurrent cyanide toxicity; give hydroxocobalamin. The distractor is sodium nitrite, which is harmful in smoke inhalation because induced methemoglobinemia further reduces oxygen carriage. Methylene blue is another distractor — it has no role in CO poisoning.
• Do not discharge back to the source. Involve poison control and require appliance inspection and CO detectors; recurrence is a preventable cause of death.