The Poison Review

Case Report: Is threshold for treatment of methemoglobinemia the same for all? A case report and literature review.  El-Husseini A, Azarov N.  Amer J Emerg Med 2010;28:748.e5-748.e10

Abstract

This puzzling and ultimately unsatisfying case report is redeemed by some spectacular clinical pictures illustrating the findings in methemoglobinemia.  A 78-year-old woman hospitalized for a CVA workup underwent transesophageal echocardiography, receiving benzocaine and lidocaine during the procedure.  One hour later she became confused, dyspneic and cyanotic.  Work-up revealed cyanosis unresponsive to 100% oxygen, pO2 of 70% on 2L/NC (76% on 100% O2),and an ABG on 100% o@ showing PaO2 of 391 mm Hg and oxygen saturation of 98%.  A methemoglobin level was drawn, and methylene blue administered.  The patient improved rapidly, with pulse oximetry increasing to 98% and mental status changes resolving. The measured methemoglobin level was 10.8%.

There are two main problems with this paper.  First, the gap between the PaO2 on ABG (98%) and the pulse oximetry (70-76%) is not completely explained by a methemoglobin level of 10.8%.  What accounted for the additional 10 or 15% discrepancy.  One has to wonder if there specimens were drawn at different times, if there was a lab error, or if  two different processes were going on. (Was the patient wheezing? There’s not mention of a lung exam when she became symptomatic.)

The second problem is that the authors never answer the question posed in their title.  Is the threshold for treating methemoglobinemia the same for all? Should the threshold be based on a level, symptoms, or some combination of factors. Having introduced this topic, the authors should have taken care to discuss it more thoroughly.