A 16 year-old male with a mandibular fracture is scheduled to undergo surgical repair. Awake fiberoptic intubation is performed using benzocaine for topical anesthesia. Twenty minutes after the start of the procedure, oxygen saturation is noted to decrease to 88%.
Which of the following is the MOST likely cause of the hypoxemia?
a) Pneumothorax
b) Bronchospasm
c) Methemoglobinemia
d) Carboxyhemoglobinemia
e) Right endobronchial intubation
Answer c.
Excessive benzocaine administration can lead to methemoglobinemia. With clinically significant methemoglobinemia, pulse oximetry readings will tend to approximate 85% because of the equal absorption by methemoglobin for both infrared and red light. Although right endobronchial intubation can occur (and should be ruled out along with other causes of hypoxemia), it is unlikely in this situation where the fiberoptic technique has been used to visualize position of the endotracheal tube.