Carbon Monoxide Poisoning

Laboratory Diagnosis

• Laboratory CO-oximetry is the gold standard for diagnosing carbon monoxide poisoning, as it directly measures carboxyhemoglobin (COHb) levels through spectrophotometry of blood samples, with COHb levels indicating exposure, including >3-4% in nonsmokers and >10% in smokers 1
• Venous and arterial COHb levels are similar and can be considered equivalent for clinical purposes, and COHb levels may be low or normal at presentation if there was a delay between exposure and testing or if oxygen treatment was already initiated 1
• Blood samples for COHb measurement are stable during transport 1

Pulse CO-oximetry

• Pulse CO-oximetry (fingertip) uses multiple wavelengths and has a sensitivity of 61% for COHb ≥10% and 83% for COHb ≥15%, but if used for initial diagnosis, confirm with laboratory CO-oximetry before hyperbaric oxygen therapy 1

Arterial Blood Gas Analysis

• Caution should be exercised when using older blood gas machines without CO-oximetry capability, as they calculate oxygen saturation based only on PaO₂ and pH, and will not account for COHb, reporting falsely normal saturation, whereas modern blood gas analyzers with integrated CO-oximetry overcome this limitation 1

Clinical Presentation

• Symptoms of carbon monoxide poisoning include headache, dizziness, nausea/vomiting, confusion, fatigue, chest pain, shortness of breath, and loss of consciousness, with headache being the most common symptom 1
• "Cherry red" skin coloration is rare and typically only seen with lethal COHb levels, and less than half of fatal CO poisoning cases show this sign, making it an unreliable indicator 1
• Standard pulse oximetry cannot differentiate between COHb and O₂Hb, and will report falsely normal SpO₂ until COHb exceeds 40%, and COHb levels do not reliably predict symptoms or outcomes, and are used primarily to confirm exposure 1

Treatment

• Administer 100% normobaric oxygen to any person suspected of CO poisoning while awaiting confirmation, as high oxygen environments can help weaken the CO-hemoglobin bond, facilitating CO removal 1
• The American Heart Association recommends first-line treatment for symptomatic methemoglobinemia with methylene blue (1-2 mg/kg of 1% solution), with a strength of evidence supported by 2 and 3
• Methylene blue treatment is recommended, despite its potential to interfere with pulse oximetry readings and co-oximetry measurements, as noted by the American Journal of Hematology and Circulation 2 and 3

Diagnostic Considerations

• Pulse oximetry has limitations, including inability to distinguish between different types of hemoglobin, and is affected by factors such as poor peripheral perfusion, motion artifacts, ambient light interference, dark skin pigmentation, and nail polish or artificial nails, as noted by the European Respiratory Journal 4

• Normal SpO2 ranges for different age groups are:

  Age Group	SpO2 Range
  18-24 years	96.9% (±0.40%)
  25-34 years	96.7% (±0.7%)
  35-44 years	96.7% (±0.6%)
  45-54 years	96.5% (±1%)
  55-64 years	95.1% (±0.7%)
  >64 years	95.5% (±1.4%)

• Pulse oximetry cannot detect carboxyhemoglobin or methemoglobin, and readings may be falsely elevated in carbon monoxide poisoning or tend toward 85% regardless of actual saturation in methemoglobinemia, as reported by the European Respiratory Journal and the British Thoracic Society 4, 5, 6
• Clinicians should not misinterpret "normal" or "near-normal" pulse oximetry readings as excluding methemoglobinemia, and should consider methemoglobinemia when there's a discrepancy between clinical cyanosis and pulse oximetry readings, as recommended by the American Journal of Hematology 2
• Modern arterial blood gas analyzers using co-oximetry and spectrophotometry can directly measure concentrations of different hemoglobin species, providing a complete and precise evaluation, as recommended by the American Thoracic Society 1
• Meticulous attention to blood sampling technique and daily equipment calibration are required to ensure precise and reproducible measurements, as suggested by the American Association for Respiratory Care and the American College of Chest Physicians 6, 1