Benzocaine-induced methemoglobinemia: Case presentations (Part 2)
The patient was found to have profound peripheral and central cyanosis. Her cardiac and respiratory exams were normal. Her blood pressure was 107/73 mmHg, heart rate was 96 beats/min and O2 saturation by pulse oximetry was 82% to 89% on 100% O2 by a nonrebreathing mask. Her chest and cardiovascular exam were normal but she was dyspneic, drowsy and confused. Flumazenil (0.2 mg intravenously) and naloxone hydrochloride (0.4 mg intravenously) were administered without a significant response. The O2 was briefly discontinued without a significant change in her O2 saturation or clinical condition. The patient denied any chest pain or shortness of breath, but did have a significant headache and stated she felt generally unwell. Her chest x-ray and electrocardiogram were unremarkable.
With no change in her O2 saturation with or without O2, as well as the significant peripheral and central cyanosis, the diagnosis of methemoglobinemia was considered. An arterial blood gas was requested. The arterial blood was chocolate brown coloured which further suggested the diagnosis of methemoglobinemia. Methylene blue was ordered as the blood gas was processed. Methemoglobinemia was confirmed with arterial blood gas analysis with an MHg level of 48.7% (normal 0.0% to 1.5%). The patient was treated with 1.5 mg/kg of methylene blue given intravenously as an intravenous push into an intravenous line with normal saline running at 200 mL/h over a period of 5 min. Within 5 min of methylene blue administration, the patient started feeling better and her colour improved. Her headache resolved over approximately 30 min to 45 min.