Pulse oximetry in methaemoglobinaemia. Failure to detect low oxygen saturation.

The results of pulse oximetry saturation in a patient with a high level of methaemoglobinaemia, who subsequently underwent intravenous methylene blue treatment, are presented. The reasons for the erroneously low values after treatment are explained. Pulse oximeters currently available are not helpful in patients treated with methylene blue and should be used with caution in patients who present with cyanosis of unknown origin.