Anesthesiologists commonly use local anesthetics to anesthetize the airway and pharynx in preparation for awake fiber-optic intubations or endoscopies. A rare though potentially serious toxicity associated with the use of local anesthetics is methemoglobinemia (MHbemia). Anesthesiologists should understand the pathophysiology, presentation, diagnosis, and treatment of MHbemia to prevent and/or manage potential problems.

In the case above, the patient experienced arterial desaturation to 90% on a FiO₂ of 100% oxygen. The differential diagnosis for a low pulse oximetry measurement (SpO₂) included: pulse oximetry artifact, hypoxemia, abnormal hemoglobin variants, sulfhemoglobinemia, and MHbemia. Pulse oximetry relies on measurement of absorbed light from two diodes, one emitting light at 660 nm (red) and another emitting light at 940 nm (near-infrared). It is well known that pulse oximetry is distorted by artifacts arising from motion, ambient light, poor perfusion, and injected dyes including indigo carmine and methylene blue.

Hypoxemia is the most critical cause for low SpO₂ and must be investigated urgently. Hypoxemia may result from hypoxia, hypoventilation, shunting, ventilation/perfusion mismatching, decreased partial pressure of oxygen in mixed venous blood, and rarely, diffusion abnormalities. Quickly ruling out these causes of a low SpO₂ is paramount to caring for any patient. In this case, the patient was ventilated manually with 100% FiO₂. The tube placement was verified to be correct with a fiber-optic scope; auscultation, and presence of end-tidal CO₂. The nasotracheal endotracheal tube was suctioned and the breathing circuit checked for kinks. New-onset diffusion abnormality was highly unlikely.