The Science Journal of the American Association for Respiratory Care
Case report: A 57 year-old black female with metastatic stage IV leiomyosarcoma developed altered mental status, tingling, and lethargy following a chemotherapy regimen consisting of mesna, ifosfamide, adriamycin, and dacarbazine. Upon exam, the patient was in no apparent respiratory distress but there was a marked ?chocolate? hue to the patient's lips and mucous membranes. The Sp02 while breathing room air was 88-94% and this remained unchanged despite the application of a 6 liter nasal cannula. The arterial blood gas (ABG) sample was visibly dark brown and revealed: pH 7.49/PaC02 29 torr/Pa02 56 torr with a measured Sa02 of 46.2% and a methemoglobin level (MetHgb) of 49.8%. 100% oxygen was given and repeat ABG revealed: pH 7.51/PaCO2 27 torr/Pa02 346 torr with a measured Sa02 of 42.2% and a MetHgb of 56.6%. The patient was transferred to the Intensive Care Unit and intubated for airway protection. Intravenous methylene blue was administered and an ABG thirty minutes later showed: pH 7.49/PaCO2 27 torr/PaO2 475 torr with a measured Sa02 of 67.5% and a MetHgb of 31.3%. Over the next twelve hours the methemoglobin level decreased to 4.2% and within 24 hours it was within normal limits. The patient was extubated and transferred back to the general care unit.
Discussion: Methemoglobin is an abnormal hemoglobin incapable of carrying oxygen because the iron molecule is in the ferric [Fe 3+] rather than in the ferrous [Fe 2+] form. Methemoglobinemia is rare and its development is usually secondary to certain drugs administered to the patient. In this case, no drugs reported to be related to MetHgb were administered but we suspect ifosfamide since it is closely related to cyclophosphamide, a drug known to produce MetHgb. Methemoglobinemia presents two important problems for respiratory care practitioners (RCPs): First, MetHgb impairs oxygen delivery because increased levels of methemoglobin decrease the overall oxygen supply and because the oxygen dissociation curve is shifted to the left, which inhibits the release of oxygen from the hemoglobin molecule. Increasing Fi02 will increase dissolved 02 content but this is no substitute for methylene blue administration to eliminate MetHgb. A second problem for RCPs is that standard pulse oximeters have difficulty distinguishing MetHgb from oxyhemolglobin. Note that in this patient, pulse oximetry showed near normal values for SpO2 even though the measured SaO2 was less than 50%. Pulse oximetry thus cannot be relied upon to rule out MetHgb.