A patient presents with bradypnea but normal pulse oximetry. This pattern is suspicious for what poisoning?

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Multiple Choice

A patient presents with bradypnea but normal pulse oximetry. This pattern is suspicious for what poisoning?

In carbon monoxide poisoning, tissues become hypoxic even when the pulse oximeter reads normal. CO binds tightly to hemoglobin, forming carboxyhemoglobin, which both reduces the blood’s oxygen-carrying capacity and shifts the oxygen dissociation curve to the left, making it harder for oxygen to release to tissues. Standard pulse oximetry can’t distinguish carboxyhemoglobin from oxyhemoglobin, so the device often shows a normal or near-normal oxygen saturation despite true hypoxia. That mismatch—low tissue oxygen delivery with a deceptively normal SpO2—is why bradypnea with a normal pulse oximetry pattern is suspicious for carbon monoxide poisoning. Confirm with co-oximetry measurement of carboxyhemoglobin and treat with 100% oxygen (and consider hyperbaric therapy if indicated). Cyanide poisoning can present with normal SpO2 because oxygen is still present in the blood, but cells can’t use it, leading to lactic acidosis. Methemoglobinemia often gives a misleading but consistently low reading around 85% that may not improve much with oxygen. Oxygen toxicity is related to excessive inspired oxygen and has different clinical features.

In carbon monoxide poisoning, tissues become hypoxic even when the pulse oximeter reads normal. CO binds tightly to hemoglobin, forming carboxyhemoglobin, which both reduces the blood’s oxygen-carrying capacity and shifts the oxygen dissociation curve to the left, making it harder for oxygen to release to tissues. Standard pulse oximetry can’t distinguish carboxyhemoglobin from oxyhemoglobin, so the device often shows a normal or near-normal oxygen saturation despite true hypoxia. That mismatch—low tissue oxygen delivery with a deceptively normal SpO2—is why bradypnea with a normal pulse oximetry pattern is suspicious for carbon monoxide poisoning. Confirm with co-oximetry measurement of carboxyhemoglobin and treat with 100% oxygen (and consider hyperbaric therapy if indicated).

Cyanide poisoning can present with normal SpO2 because oxygen is still present in the blood, but cells can’t use it, leading to lactic acidosis. Methemoglobinemia often gives a misleading but consistently low reading around 85% that may not improve much with oxygen. Oxygen toxicity is related to excessive inspired oxygen and has different clinical features.

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