Benzocaine
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Methaemoglobinaemia following topical administration: 2 case reports Two patients developed methaemoglobinaemia after receiving topical benzocaine spray to anaesthetise the posterior region of the pharynx prior to transoesophageal echocardiography. A 36-year-old woman received 14% benzocaine spray; an unknown number and duration of sprays were administered. She also received midazolam, butamben and tetracaine. After completion of echocardiography, she developed severe cyanosis. Arterial blood gas analysis revealed the following: pH 7.49, PaO2 315mm Hg, PaCO2 35.5mm Hg and SaO2 100%. Oxygen saturation by pulse oximetry decreased from 93% on 3 L/min of oxygen to 84% on 6 L/min of oxygen. A nonrebreather mask was used, but her oxygen saturation remained low. Her HR decreased from 103 to 84 beats/min. A methaemoglobin level of 39% and a total haemoglobin level of 8.9 g/dL were revealed by co-oximetry. Her facial colour went from pale to blue, showing severe cyanosis. IV methylthioninium chloride [methylene blue] was administered. After about 1 hour, she had a methaemoglobin level of 6.3% and a haemoglobin level of 9.7 g/dL. About 3 hours later, her methaemoglobin level had normalised to 0.6% and her haemoglobin level was 10.1 g/dL. Arterial blood gas analysis revealed the following: pH 7.54, PaO2 54mm Hg, PaCO2 31.8mm Hg and SaO2 91%. While breathing room air, her oxygen saturation was 99%, and her colour became pink. After 8 days, she was discharged without further complications. A 57-year-old man received 14% benzocaine spray in multiple lengthy sprays, along with midazolam, butamben and tetracaine. After echocardiography, arterial blood gas analysis revealed the following: pH 7.43, PaO2 117mm Hg, PaCO2 31.7mm Hg and SaO2 99%. His oxygen saturation by pulse oximetry was 90% on 2 L/min of oxygen and remained the same with a nonrebreather mask. Co-oximetry revealed a methaemoglobin level of 45.8% and a total haemoglobin level of 9.8 g/dL. His arterial blood was chocolate brown, and his appearance went from dark to dusky. His mucosal membranes showed severe cyanosis and his tongue turned blue. He received IV methylthioninium chloride. After about 1 hour, his methaemoglobin and haemoglobin levels were 11.7% and 10 g/dL, respectively. About 1 hour later, his methaemoglobin and haemoglobin levels were 4.2% and 10.3 g/dL, respectively. His oxygen saturation was 97% on room air. His cyanosis resolved. He had no further complications and, after 18 days, was discharged. Author comment: "[T]he Naranjo adverse drug reaction probability scale . . . indicated a probable relationship (score of 7) between the administration of the benzocainecontaining spray and the development of methemoglobinemia in both patients." Throm MJ, et al. Benzocaine-induced methemoglobinemia in two patients: interdisciplinary collaboration, management, and near misses. Pharmacotherapy 27: 801091174 1206-1214, No. 8, Aug 2007 - USA
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