A man is found unresponsive with a cyanotic face, but the pulse oximeter shows 98%. Which is the most likely explanation?

• A. Increased body temperature
• B. His extremities are cold
• C. Carbon monoxide poisoning
• D. Severe pulmonary edema

Answer: (C)

The key idea is that pulse oximetry can be misleading in carbon monoxide poisoning. A normal or near-normal SpO2 can hide serious hypoxia because the device cannot tell carboxyhemoglobin from oxyhemoglobin; both absorb light in a way that the oximeter interprets as oxygenated blood. If a person is unresponsive with a cyanotic face but the SpO2 reads about 98%, carbon monoxide binding to hemoglobin is the most plausible cause. The body’s tissues aren’t getting enough oxygen because CO blocks hemoglobin’s ability to carry and release oxygen, so despite a high reading, there’s tissue hypoxia. The other options don’t fit this mismatch: temperature effects or cold extremities don’t routinely produce a normal SpO2 with cyanosis, and severe pulmonary edema would typically lower the SpO2.