A 54 year-old male has a peripheral nerve block prior to hand surgery. The pulse oximetry reading changes from 98% on 2L/min oxygen via nasal cannula to 85%. Blood is aspirated from an arterial line in place and is noted to be chocolate colored. An ABG is checked and while the SaO2 is less than 90%, the PaO2 is measured to be greater than 70 mmHg. Which of the following local anesthetics is most likely implicated in this clinical scenario?

A. Bupivacaine
B. Ropivacaine
C. Prilocaine

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Correct Answer: C


This scenario is suggestive of methemoglobinemia. The following four anesthetics have been reported as possibly causing methemoglobinemia: prilocaine (ortho-toluidine is responsible for hemoglobin oxidation), benzocaine, lidocaine, and tetracaine. Local anesthetic-related methemoglobinemia is a clinically important problem. It is diagnosed with an elevated direct co-oximeter measurement of methemoglobin (>1 – 2%), but it can also present as a drop in the oxygen saturation (though the pulse oximeter typically grossly underestimates the degree of hypoxia), chocolate-colored blood, and a discrepancy between SaO2 and PaO2. The treatment for methemoglobinemia is IV methylene blue.

Of note, prilocaine should not be used in infants less than 6 months (unless for transcutaneous anesthesia), pregnant patients, and patients with glucose-6-phosphate dehydrogenase deficiency.

Guay, J. Methemoglobinemia Related to Local Anesthetics: A Summary of 242 Episodes. Anesth Analg 2009;108:837-845.

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