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Pulse oximetry is widely used perioperatively to detect respiratory events. Abnormal haemoglobins such as methemoglobin can result in inaccurate readings. Here we present the clinical challenges of managing a patient with congenital methemoglobinemia who went for surgery involving injection of patent blue dye and subsequently had a prolonged duration of poor saturation readings post-operatively. Differentials include desaturation due to respiratory causes, worsening of methemoglobinemia and delayed excretion of patent blue dye. noma. She had a history of congenital methemoglobinemia with a resting peripheral blood oxygen saturation level (SpO 2) of 92-93% on room air. Baseline methemoglobin levels were not available. Aside from a greyish hue appearance, she was otherwise asymptomatic with good exercise tolerance. Preoperative hemoglobin level was 14.7 g/dL. General anesthesia was induced with intravenous propofol 100 mg, rocuronium 30 mg, dexamethasone 8 mg and target controlled infusion (TCI) of remifentanil at an effect-site concentration (Ce) of 3 ng/ml. Following endotracheal intubation, anesthesia was maintained with desflurane and TCI remifentanil at a Ce of 1-2 ng/ ml. Intravenous cefazolin 2 g was given prior to surgical incision for antibiotic prophylaxis with repeated 1 g dosing every 4 hours. An intra-arterial line was inserted to facilitate hemodynamic monitoring and repeated arterial blood gas (ABG) analysis. Baseline methemoglobin level sent off at the start of surgery was 21.7% (normal range: 0-1.5%). Due to the chronicity and asymptomatic nature of the underlying condition, it was elected not to treat the methemoglobinemia with methylene blue. The patient tolerated induction of anesthesia well with stable hemodynamics and a SpO2 of 94-96% on a fraction of inspired oxygen (FiO2) of 50%. However, 10 minutes after 2.5% patent blue V dye (Guerbet, B.P. 57400, Roissy, France) 1.5 ml had been injected subareolarly and subdermally on each side for sentinel lymph *Corresponding author: Priscilla Hui Yi PHOON, MBBS, MMed (Anaesthesia), FANZCA, MClinUS, Department of Anaesthesiology, Singapore General Hospital, 1 Hospital Drive, Singapore 169608; Department of Cardiothoracic Anaesthesia, National Heart Centre, Singapore, Tel: +65-63214220 CasE REPORT

Low Saturation Readings in a Patient with Congenital Methemoglobinemia Exposed to Patent Blue Dye: A Case Report