Which statement describes a limitation of pulse oximetry in carbon monoxide exposure?

• A. It overestimates SpO₂ due to CO binding
• B. It cannot distinguish carboxyhemoglobin from oxyhemoglobin, leading to falsely normal SpO₂
• C. It measures PaO₂ directly
• D. It is unaffected by CO levels

Answer: (B)

Pulse oximetry can be misleading in carbon monoxide exposure because it can’t distinguish carboxyhemoglobin from oxyhemoglobin. The device estimates oxygen saturation by using two light wavelengths to compare how much red and infrared light is absorbed by hemoglobin. Carboxyhemoglobin and oxyhemoglobin absorb these wavelengths in a very similar way, so the monitor interprets CO-bound hemoglobin as if it were oxygen-bound hemoglobin. The result is a normal or near-normal SpO2 reading even though the actual oxygen-carrying capacity of the blood is reduced and tissues may be hypoxic. To accurately assess CO exposure, you’d use co-oximetry, which measures carboxyhemoglobin levels and provides a true picture of oxygen delivery. PaO2, the amount of dissolved oxygen, is not what pulse oximetry measures, and the presence of CO can render SpO2 readings misleading despite normal values.