Differential Diagnosis for Normal SpO2 on Pulse Oximeter but Hypoxemia
- Single Most Likely Diagnosis
- Carbon monoxide poisoning: This condition can cause a normal SpO2 reading on a pulse oximeter due to the similar light absorption properties of carboxyhemoglobin and oxyhemoglobin, despite the presence of hypoxemia.
- Other Likely Diagnoses
- Methemoglobinemia: Similar to carbon monoxide poisoning, methemoglobinemia can lead to a falsely normal SpO2 reading due to the altered state of hemoglobin, which affects the accuracy of pulse oximetry.
- Anemia: Severe anemia can result in a normal SpO2 reading, as the pulse oximeter measures the saturation of hemoglobin that is present, not the total amount of hemoglobin.
- Do Not Miss Diagnoses
- Cardiac shunt (e.g., patent foramen ovale): A right-to-left cardiac shunt can cause hypoxemia despite a normal SpO2 reading on a pulse oximeter, as the desaturated blood bypasses the lungs.
- Pulmonary arteriovenous malformation: Similar to a cardiac shunt, a pulmonary arteriovenous malformation can cause hypoxemia due to the bypassing of oxygenated blood, which may not be detected by pulse oximetry.
- Rare Diagnoses
- Sulfhemoglobinemia: This rare condition, caused by the presence of sulfhemoglobin in the blood, can lead to a falsely normal SpO2 reading due to its similar light absorption properties.
- Dyshemoglobinemia (e.g., due to certain drugs or toxins): Certain substances can alter the state of hemoglobin, affecting the accuracy of pulse oximetry and leading to a normal SpO2 reading despite hypoxemia.