We went to the cardiologist for Cordelia's follow-up visit, and as suspected she's fine. The doctor couldn't find any abnormality in the EKG or the ultrasound. Like Dave said, it was just a startup problem -- "Device driver not initialized" or some such.
We do have to hold off on stimulant decongestants for the next six months or so, though, just in case. Obligingly enough, Cordelia came down with a cold last night. Fortunately we can still give her nasal decongestant spray, bar none my favourite cold remedy, so we did that and soon enough she was sleeping soundly.