Baylor University Medical Center Proceedings April 2014, Volume 27, Number 2 | Page 61

Figure 2. Hemoglobin extinction curves. Pulse oximetry uses the 660 and 940 nm wavelengths. As carboxyhemoglobin and oxyhemoglobin absorb equally at 660 nm, they both read the same oxygen saturation on a conventional pulse oximeter. From Huford W, Kratz A (2). Copyright © 2004 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society. equates to an oxygen saturation of 85% (5). Therefore, the pulse oximeter will display an oxygen saturation of approximately 85% regardless of the patient’s real oxygenation status. Newer oximeters called co-oximeters measure absorption at multiple wavelengths, which circumvents the issue seen with traditional pulse oximeters. Co-oximeters will determine the percentage of hemoglobin converted to methemoglobin and carboxyhemoglobin and also will provide an accurate estimate of the true oxygen saturation state of hemoglobin. However, these co-oximeters are not widely used (5). Most cases of methemoglobinemia will resolve within 24 to 36 hours. In severe cases, particularly when the methemoglobin blood level is above 30%, prompt treatment with methylene blue is advised. Methylene blue is quickly reduced by NADPH methemoglobin reductase to leukomethylene blue, which then reduces methemoglobin to hemoglobin. This is called the methylene blue rescue pathway. This rescue pathway relies on an intact glucose-6-phosphate dehydrogenase (G6PD) system. In the rare event that the patient’s status worsens after April 2014 methylene blue administration, G6PD deficiency should be strongly suspected. Benzocaine is still a widely used anesthetic. There are several different formulations, the spray form being the most commonly used. Benzocaine sprays are marketed under different brand names such as Hurricaine, Cetacaine, Exactacain, and Topex (1). The benzocaine concentration in these formulations ranges from 14% to 20%. The benzocaine dosage that can produce methemoglobinemia in adults has been estimated to be approximately 300 mg, with initial onset of symptoms within 20 to 60 minutes. The patient presented was exposed to benzocaine during the second emergent intubation. It is unknown whether the anesthetic utilized in the first intubation included benzocaine. However, the benzocaine was administered to a mucosa that was likely traumatized during the first intubation, potentially increasing its absorption and causing a greater oxidative burden. This patient likely had a gastrointestinal perforation from her prior episode of acute appendicitis. After the second intubation, it was difficult to wean her off the ventilator. ABG analysis showed her methemoglobin level rising to 45%. The methemoglobin level quickly declined over a period of 2.5 hours after receiving methylene blue. Unfortunately, the patient developed significant hemodynamic instability likely due to multiple factors, ultimately developed multiorgan failure and anoxic brain injury, and subsequently died. 1. 2. 3. 4. 5. US Food and Drug Administration. FDA continues to receive reports of a rare, but serious and potentially fatal adverse effect with the use of benzocaine sprays for medical procedures [Safety announcement, April 7, 2011]. Available at http://www.fda.gov/Drugs/DrugSafety/ucm250040. htm. Hurford WE, Kratz A. Case 23-2004. A 50-year-old woman with low oxygen saturation. N Engl J Med 2004;351(4):380–387. Chung NY, Batra R, Itzkevitch M, Boruchov D, Baldauf M. Severe methemoglobinemia linked to gel-type topical benzocaine use: a case report. J Emerg Med 2010;38(5):601–606. Janssen WJ, Dhaliwal G, Collard HR, Saint S. Clinical problem-solving. Why “why” matters. N Engl J Med 2004;351(23):2429–2434. El-Husseini A, Azarov N. Is threshold for treatment of methemoglobinemia the same for all? A case report and literature review. Am J Emerg Med 2010;28(6):748.e5–748.e10. Methemoglobinemia precipitated by benzocaine used during intubation 135