The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts


Melinda A. Hester1, Frank M. Freihaut1, Craig A. Piquette1

Introduction: Methemoglobinemia occurs when oxidation of the hemoglobin (Hb) molecule converts ferrous to ferric Hb, which is incapable of binding or carrying oxygen. Normal oxidative stressors can result in MetHb formation <2%. Acquired increased MetHb is caused by drug or chemical exposure, including cetacaine,lidocaine,benzocaine,nitrates and nitrites. Levels <10% have mild symptoms and can self resolve. As levels reach 30%-50% respiratory failure, hypoxia, and cardiac arrhymthias can occur which require supportive care, and the introduction of the antidote, methylene blue. Prompt treatment and removal of the causative agent before the level reaches 30% have shown successful outcomes. Levels >70%, with associated ischemic effects (seizures, coma, and dysrhythmias), can be fatal.

Case Summary: A 22 yr. old patient who was asymptomatic prior to the administration of cetacaine spray in conjunction with trans esophageal ECHO(TEE) testing, experienced dyspnea, confusion, bradycardia and decreased SpO2 shortly after completion of the TEE. The respiratory therapist observed cyanosis and SpO2 readings of 85%,which were refractory to increased FiO2. Arterial blood was noted to be "chocolate brown" and additional co-oximetry testing was requested which revealed a MetHb of 33%. The antidote of methylene blue was administered at 2mg/kg over 5 minutes, and repeated 1 hour later at 1mg/kg. The MetHb level decreased to normal, symptoms resolved, and extubation followed within 12 hrs. No negative outcomes were noted.

Discussion: There are several clues to early discovery of methemoglobinemia. The discrepancy between SpO2, SaO2 and PaO2 is a significant key which should prompt co-oximetry testing. MetHb absorbs light at a different frequency than normal Hb thus pulse-ox readings are inaccurate when MetHb is present. Many ABG analyers report functional Hb (Hb which is capable of carrying O2), not fractional Hb (all Hb including aberrant Hbs which are incapable of carrying O2). Co-oximetry must be ordered to confirm diagnosis. Another clue is the characteristic "chocolate-colored" arterial blood, which does not change when exposed to air. The presence of cyanosis which does not respond to increased FiO2, may help differentiate a diagnosis of pulmonary embolis verses methhemoglobinemia. Prompt recognition of symptoms, co-oximetry measurement of MetHb, identification and removal of the cause, and timely introduction of methylene blue are key to successful outcomes.