A firefighter rescued from a house fire has a headache, dizziness, and confusion. His ABG shows PaO2 of 98 mmHg on room air, yet his pulse oximeter reads 92%. His skin appears bright red. The discrepancy between the normal PaO2 and the low saturation is best explained by:

• A Carbon monoxide impairs alveolar gas exchange, lowering the diffusion capacity
• B Carboxyhaemoglobin reduces the amount of oxygen dissolved in plasma
• C Carbon monoxide shifts the oxyhaemoglobin dissociation curve to the right
• D Pulse oximeters overestimate saturation when carboxyhaemoglobin is present ✓

Explanation

PaO2 measures only dissolved oxygen and is unaffected by carbon monoxide, which occupies haemoglobin binding sites but does not alter the small dissolved fraction. Standard pulse oximetry uses two wavelengths that cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so it reports a falsely elevated saturation, not a falsely low one. The curve shifts LEFT, not right, because carboxyhaemoglobin increases affinity of remaining sites. Co-oximetry is required for a true reading.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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