Saturation Gap During Anesthesia in Glucose-6-Phosphate Dehydrogenase Deficiency and Methemoglobinemia: A Case Report and Review of Perioperative Management

The coexistence of glucose-6-phosphate dehydrogenase (G6PD) deficiency and methemoglobinemia may produce misleading pulse oximetry readings, creating a significant diagnostic challenge during the perioperative assessment of oxygenation. Methemoglobinemia results from the oxidation of hemoglobin iron to the ferric state, impairing oxygen delivery and altering light absorption, often leading to falsely low oxygen saturation values. A 20-year-old man with confirmed G6PD deficiency and suspected chronic methemoglobinemia underwent an elective electrophysiologic study for recurrent supraventricular tachycardia. On arrival in the operating room, pulse oximetry showed a saturation of 45% on room air, despite preserved clinical status. Arterial blood gas analysis revealed a partial pressure of oxygen (PaO₂) exceeding 400 mmHg, confirming adequate arterial oxygenation and establishing a saturation gap. General anesthesia was induced and maintained with propofol and remifentanil without complications. Due to the risk of hemolysis associated with methylene blue in G6PD deficiency, conservative management was adopted, with hyperbaric oxygen therapy available as a contingency strategy. The intraoperative course was uneventful, with stable hemodynamics and no evidence of tissue hypoxia or hemolysis. This case demonstrates that severe pulse oximetry desaturation does not necessarily reflect true hypoxemia. Recognition of a saturation gap is essential when oximetry findings are discordant with clinical presentation, particularly in patients with G6PD deficiency, in whom standard therapies may be contraindicated. Accurate evaluation requires the integration of clinical assessment with arterial blood gas analysis. This report highlights the diagnostic pitfalls and implications for safe perioperative management.