Outpatient transesophageal echocardiography was performed in a 69-year-old
man with a history of aortic valve repair. During the procedure the patient
became markedly cyanotic and hypotensive. Oxygen saturation measured by pu
lse oximetry decreased from 97% to the mid-80s. The man's condition continu
ed to deteriorate. On transfer to a critical care unit, blood analysis by c
o-oximetry showed methemoglobin saturation of 67.8%. The patient's conditio
n improved significantly after intravenous administration of methylene blue
1 mg/kg. With increasing numbers of outpatient procedures performed under
topical anesthesia, measures should be in place to deal with a potential li
fe-threatening adverse event such as methemoglobinemia.