Which statement is true regarding carbon monoxide poisoning in smoke inhalation?

• A. Inhalation of smoke rarely accompanied by burns
• B. CO poisoning if COHb > 20%
• C. Pt is always without airway obstruction
• D. COHb is not diagnostic

Answer: (B)

Carbon monoxide poisons by binding to hemoglobin to form carboxyhemoglobin, which both reduces the blood’s oxygen-carrying capacity and shifts the oxygen dissociation curve to the left, meaning less oxygen is released to tissues. In smoke inhalation, this hypoxic effect can occur even if there aren’t obvious external burns, so recognizing CO exposure is crucial.

Measuring COHb with CO-oximetry provides direct evidence of CO binding. A carboxyhemoglobin level above about 20% is a clinically meaningful threshold indicating poisoning in the setting of smoke inhalation, and it tends to correlate with the onset of symptoms and risk of hypoxic injury. This is why that statement is considered true: it reflects a level at which CO exposure is toxic and requires treatment (typically 100% oxygen, with consideration of hyperbaric oxygen for more severe cases).

Other statements don’t fit because airway injury can occur with inhalation even without burns, and COHb can be diagnostic with the right testing (pulse oximetry alone can be misleading since it can’t distinguish COHb from oxyhemoglobin).