A case of previously undiagnosed methemoglobinemia exhibiting sustained low SpO2 value at the induction of general anesthesia
Ono, R.; Tanigami, H.; Kagawa, K.; Shimaoka, M.; Sonoda, S.; Iura, A.; Yasue, Y.; Hamabe, N.; Ishii, A.
Masui. Japanese Journal of Anesthesiology 62(6): 710-713
2013
ISSN/ISBN: 0021-4892 PMID: 23814999 Document Number: 666261
A 70-year-old woman was scheduled to undergo surgery for removal of thyroid tumor under general anesthesia. A routine preoperative evaluation confirmed that the patient was stable with no signs of cyanosis and dyspnea. However, during pre-oxygenation as well as mechanical ventilation with 100% oxygen, she showed sustained low SpO2 values (i.e., 91%). Arterial blood gas analysis at FIO2 of 1.0 showed an oxygen partial pressure (PaO2) of 297 mmHg. Unexpectedly, the analysis revealed methemoglobinemia (MetHb concentration: 15%) causing a discrepancy between the low SpO2 and normal PaO2 values in this patient. Methemoglobinemia is an uncommon cause of cyanosis; however, anesthesiologists should be aware that some drugs used during perioperative period (e.g., local anesthetics) can cause methemoglobinemia. While our case was a mild one and the patient recovered with no complications, methemoglobinemia levels above 30% could cause tissue hypoxemia and, thereby, requiring a treatment with methylene blue or ascorbic acid.