In carbon monoxide poisoning, why can pulse oximetry be unreliable?

• A. It accurately detects carboxyhemoglobin levels.
• B. It cannot distinguish carboxyhemoglobin from oxyhemoglobin, so readings may be normal.
• C. Only arterial blood gas can detect CO exposure.
• D. It is unaffected by CO exposure.

Answer: (B)

Pulse oximetry can be unreliable in carbon monoxide poisoning because it cannot distinguish carboxyhemoglobin from oxyhemoglobin. The oximeter uses two wavelengths to estimate how much hemoglobin is bound to oxygen, and carboxyhemoglobin absorbs light in a way that mimics oxyhemoglobin. So it can read a normal or even high oxygen saturation even though tissues aren’t getting oxygen effectively. In reality, carbon monoxide binds Hb with high affinity and shifts the oxygen dissociation curve to the left, reducing oxygen delivery to tissues. To accurately detect CO exposure, you need co-oximetry (multiple-wavelength analysis) on an arterial blood sample, which can differentiate carboxyhemoglobin from oxyhemoglobin and measure other hemoglobin species.