Congenital methaemoglobinaemia detected by preoperative pulse oximetry

Methaemoglobinaemia is an unusual cause of cyanosis whether it is congenital or acquired. Hence, the diagnosis may not be immediately obvious and appropriate treatment may be delayed. The case described shows that it should be considered when pulse oximetry and arterial blood gas analysis appear to give conflicting results. A healthy 24-yr-old woman was found to have a pulse oximeter reading of 82% prior to induction of anaesthesia for minor surgery. Clinical examination confirmed cyanosis but no other abnormality was detected. She had no important medical history and was not receiving any medications. Arterial blood gas analysis with the patient breathing air showed PaO2 12.03 kPa (90 mmHg). Co-oximeter analysis of this sample revealed a methaemoglobin content of 13.4% and she was subsequently found to have congenital methaemoglobin reductase deficiency. Anaesthesia was induced and maintained with incremental doses of propofol and fentanyl. A spontaneously breathing technique with oxygen in nitrous oxide was employed uneventfully. No specific treatment for methaemoglobinaemia was given. Perioperative pulse oximetry is one of the major advances in patient monitoring in recent years but unexpected results should not be accepted uncritically. A knowledge of the working principles of oximetry is essential to enable appropriate management in the presence of dyshaemoglobins.