Methaemoglobinemia: a diagnosis of surprise with recent literature review and management

Abstract

Abstract Background Methaemoglobinemia (MetHb) is a rare entity in clinical practice which often goes undiagnosed and keeps both the anaesthesiologist and attending surgeon under tension during surgery on seeing dark or chocolate-coloured blood in the surgical field. A low oxygen saturation (SpO2) will further panic us to search for a cause but may end futile. To add further, SpO2 may not rise significantly with adequate oxygenation and may not reach 100 with a fraction of inspired oxygen (FiO2) of 1 which keeps us searching further for a cause. An arterial blood gas (ABG) finally clinches our diagnosis. It is often missed in the pre-anaesthetic evaluation due to its rarity and the patient being asymptomatic most of the time. Case presentation We present a case of a 61-year-old man, a reformed smoker and hypertensive on regular medication was evaluated for laparoscopic partial nephrectomy for right renal cell carcinoma. MetHb was diagnosed preoperatively in the midst of the COVID pandemic when we had all our patients’ room air SpO2 recorded and thus helped us in the smooth and hassle-free management with vitamin C preoperatively for 5 days and an uneventful perioperative period. Conclusions MetHb is an uncommon and potentially reversible cause of hypoxia. A simple bedside SpO2 evaluation may give a hint to the diagnosis along with a high haematocrit which urges us to order for an ABG when no other cause is attributable. A preoperative diagnosis can lead to an effective and simple management with vitamin C which often reduces methaemoglobin to significantly low levels and to have a favourable outcome. According to the literature, any level of less than 20% does not have much clinical significance in asymptomatic patients and surgery need not be deferred.

Keywords

• Methaemoglobinemia
• Anaesthesia
• Oxygen saturation
• Arterial blood gas
• Vitamin C
• Pulse oximeter