[Patent blue-induced false-methemoglobinemia]

We experienced an anesthetic case, in which interference in oxygen saturation values by patent blue was mistaken for nitroglycerin-induced methemoglobinemia. A 40-year-old woman underwent mastectomy for breast cancer under general anesthesia. Nitroglycerin was infused to produce hypotensive anesthesia. Patent blue was locally injected around the cancer by a surgeon and surgery was started. A few minutes later, oxygen saturation by finger pulse oximeter suddenly decreased from 100% to 90%. And then, blood gas analysis was performed to confirm this change. The decrease of oxygen saturation and the increase of methemoglobin were also observed in the finding of blood gas analysis. At first, we judged these changes as methemoglobinemia by nitroglycerin infusion. Finally, however, we concluded that all these changes were attributable to the interference by patent blue on oxygen saturation value because we found that there had been some inconsistent findings as methemoglobinemia. The fact seems that nothing happened to the patient during the episode. This case showed us that anesthesiologists should understand the principle of hematological monitors and the meaning of the data obtained by monitors.