How to diagnose impaired oxygen binding to hemoglobin with increased carbon monoxide (CO) binding?

Diagnosing Impaired Oxygen Binding to Hemoglobin with Increased Carbon Monoxide Binding

Laboratory CO-oximetry is the gold standard for diagnosing carbon monoxide poisoning, as it directly measures carboxyhemoglobin (COHb) levels through spectrophotometry of blood samples. 1

Diagnostic Testing Algorithm

Primary Diagnostic Method

1. Blood CO-oximetry
   • Obtain venous or arterial blood sample for laboratory spectrophotometry
   • Directly measures concentrations of different hemoglobin species:
     • Oxyhemoglobin (O₂Hb)
     • Deoxyhemoglobin
     • Carboxyhemoglobin (COHb)
     • Methemoglobin (MetHb)
   • COHb levels indicating exposure:

     • 3-4% in nonsmokers

     • 10% in smokers 1, 2

Important Clinical Considerations

• Venous and arterial COHb levels are similar and can be considered equivalent for clinical purposes 1, 2
• Blood samples for COHb measurement are stable during transport 2
• 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

Limitations of Standard Pulse Oximetry

• Standard two-wavelength pulse oximeters (660 and 990 nm) cannot differentiate between COHb and O₂Hb 1
• Will report falsely normal or near-normal oxygen saturation in CO poisoning 1, 3
• Example: A patient with 40% COHb and PaO₂ 100 mmHg would show SpO₂ of 97-98%, despite only 60% of hemoglobin being available to carry oxygen 1

Alternative/Supplementary Testing Methods

1. Pulse CO-oximetry (fingertip)

   • Available since 2005, uses multiple wavelengths
   • Sensitivity: 61% for COHb ≥10%; 83% for COHb ≥15% 4
   • Specificity: 86% for COHb ≥10%; 81% for COHb ≥15% 4
   • If used for initial diagnosis, confirm with laboratory CO-oximetry before hyperbaric oxygen therapy 1

2. Arterial Blood Gas Analysis

   • Caution: Older blood gas machines without CO-oximetry capability calculate oxygen saturation based only on PaO₂ and pH
   • These calculations will not account for COHb and report falsely normal saturation 1
   • Modern blood gas analyzers with integrated CO-oximetry overcome this limitation 3

3. Environmental CO Measurement

   • Ambient CO levels measured by emergency personnel can confirm exposure source 1

Clinical Correlation

Key Symptoms to Evaluate

• Headache (most common symptom)
• Dizziness
• Nausea/vomiting
• Confusion
• Fatigue
• Chest pain
• Shortness of breath
• Loss of consciousness 1

Important Pitfalls to Avoid

1. Do not rely on skin color

   • "Cherry red" skin coloration is rare and typically only seen with lethal COHb levels 1
   • Less than half of fatal CO poisoning cases show this sign 1

2. Do not rely on standard pulse oximetry

   • Cannot differentiate between COHb and O₂Hb
   • Will report falsely normal SpO₂ until COHb exceeds 40% 1, 3

3. Do not assume COHb levels correlate with symptoms

   • COHb levels do not reliably predict symptoms or outcomes 1, 2
   • Used primarily to confirm exposure 1

4. Do not delay oxygen therapy

   • Administer 100% normobaric oxygen to any person suspected of CO poisoning while awaiting confirmation 1

Physiological Basis

• CO binds to hemoglobin with an affinity approximately 240 times greater than oxygen 5, 6
• As COHb concentration increases, oxygen affinity of remaining hemoglobin increases (left-shifted oxygen dissociation curve) 5, 7
• This further impairs oxygen delivery to tissues by making the remaining hemoglobin release oxygen less readily 7
• High oxygen environments can help weaken the CO-hemoglobin bond, facilitating CO removal 6

By following this diagnostic approach, clinicians can accurately identify and quantify impaired oxygen binding to hemoglobin due to increased carbon monoxide binding, enabling appropriate treatment decisions.