LabMed

Toxicity Associated with Carbon Monoxide

At a Glance

Carbon monoxide gas is odorless, tasteless, colorless, and nonirritating. The product of incomplete combustion of carbonaceous material, carbon monoxide is most commonly found in cigarette smoke, automobile exhaust, and improperly ventilated home heating units. Endogenous carbon monoxide is produced in the metabolic conversion of heme to biliverdin, thus, it increases in cases of hemolytic anemia.

Acute carbon monoxide poisoning is usually suspected on the basis of suggestive history, whereas the diagnosis of chronic exposure is notoriously difficult. Clinical features of carbon monoxide poisoning are highly variable and nonspecific. Mild to moderate exposure is often misdiagnosed as viral illness because of the symptoms of headache (most common symptom), malaise, nausea, and dizziness. Physical findings are mostly restricted to mental status changes, ranging from mild confusion to seizure or coma.

Although carbon monoxide poisoning classically presents with “cherry red cyanosis,” this finding is not sensitive or specific. Furthermore, standard (two-wavelength) pulse oximeters give falsely elevated oxygenation levels because of their inability to distinguish oxyhemaglobin from carboxyhemoglobin; however, a pulse CO-oximeter measures absorption at additional wavelengths and can make the distinction (although with lower sensitivity and specificity than conventional blood gas analysis). Other manifestations of carbon monoxide poisoning include myocardial ischemia, ventricular arrhythmias, pulmonary edema, and profound lactic acidosis.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

The gold standard for confirmatory testing is carboxyhemoglobin by arterial blood gas - a carboxyhemoglobin result of >10% is suggestive of carbon monoxide poisoning. Screening options include pulse CO-oximetry, breath CO measurement, or venous carboxyhemoglobin sampling. CO-oximeters have lower sensitivity and specificity when compared to breath CO devices, an alternative point-of-care testing option. Venous carboxyhemoglobin samples may be used as a screening tool in cases of large numbers of potential carbon monoxide poisoning victims if pulse CO-oximetry and breath CO monitoring is unavailable.

Once the diagnosis of carbon monoxide poisoning is confirmed by arterial blood gas analysis for carboxyhemoglobin, electrocardiogram and cardiac biomarkers should be ordered to rule out myocardial ischemia as a sequelae of carbon monoxide poisoning. A head CT may be obtained as well to rule out other causes of mental status change and hemorrhagic infarction of the globus pallidus and/or deep white matter tracts, rare sequelae of carbon monoxide poisoning.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

Depending on the instrument, concentrations of carboxyhemoglobin by arterial blood gas analysis are overestimated by 1-4% at hydroxycobalamin (vitamin B12) concentrations up to 593 mmol/L.

What Lab Results Are Absolutely Confirmatory?

Arterial blood gas carboxyhemoglobin level is the gold standard for diagnosis.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

A fingerstick glucose may be useful to exclude hypoglycemia as a cause of the clinical symptoms. Measured oxygen saturation will help determine whether there is tissue hypoxia present in the patient. Additional toxicology screening may be useful, particularly in cases in which attempted suicide is suspected.

Up to 40% of patients with significant carbon monoxide exposure develop the syndrome of delayed neurologic sequelae (DNS), which may manifest anywhere between 3 and 240 days after exposure. DNS is characterized by variable degrees of cognitive deficits, personality changes, movement disorders, and focal neurological defects, which may persist for 1 year or longer. There is poor correlation between carboxyhemoglobin levels and development of DNS, although most cases are associated with loss of consciousness during the acute exposure.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

Reference values for carboxyhemoglobin in rural nonsmokers are about 0.5%, for urban nonsmokers 1-2%; and for smokers, 5-6%. Values may be increased by about 3% in hemolytic anemias. Headache and mental status changes roughly correlates to a level of 30-50%. Carboxyhemoglobin greater than 60% corresponds to loss of consciousness and is rapidly fatal in some cases.

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