"Critical care MD here - this is an uncommon but very well known complication of some meds, every case I've ever seen has been from a topical anesthetic, usually cetacaine.
What makes it such a tricky emergency is that usually the patients had the cetacaine as prep for an endoscopic procedure, usually a scope or ultrasound in the esophagus, for which they also received light sedation. So the typical case is, I'll get a call from the post op floor that a patient came back from an EGD (stomach scope) and is lethargic but otherwise "fine". The patient has bright blue tongue and lips, fingernails etc, but the tricky thing is, as noted below, the pathology is that the hemoglobin hinds too tightly to oxygen, and thus doesn't "drop it off" at the cells. So, the oxygen sats are 100%.
So you have a patient that is acutely dying of oxygen starvation, with all your equipment telling you the last thing to worry about is oxygen. And they just came back from a procedure where they got sedation, which is a much much more common cause of post op crashing. So you have maybe 3 - 4 minutes to figure it out and get the methylene blue in them, and the methylene blue has to be sprinted up from the hospital pharmacy.
Then to make it even trickier, when you give the methylene blue, the patient will get pink and revive in seconds, but the methylene blue fools with the oxygen sensors, so now as the patient is recovering, all the alarms start screaming as the O2 sats drop to 30% (normal is 98-100, 80 or below is usually a death or near death state).
They are also rare, so in 10 years at a major city's biggest hospital I saw three cases. Just something you always hafta have in mind when getting a call for a rapid response critical care consult."