Yes, a HR of 150 in a 15 year old should set off alarm bells all over the place, both literally and figuratively. Given the patient's history of Crohn's - likely on some form of immunosupressant therapy and therefore less likely to mount a febrile response - and a ruptured appy, I'd be very suspicious of sepsis and make sure I brought it to someone's attention - even if it's been documented as being communicated and even if the overall PEWS score hadn't changed. (I'm Canadian too, and our hospital has been using B-PEWS
for probably 5 years.) I might preface my reporting of it with, "I'm perhaps way off in left field, but I'm concerned that this boy's HR was already high at 120 yesterday, and today it's persisting in the 150 range, with no other obvious changes. Is there something I'm missing?" Sometimes they're a little dry and some fluid brings it down. But in retrospect this boy was looking a bit shocky and there should have been more of an assessment done by all involved, not just you, the float nurse. Many people don't realize that what signals impending shock in adults isn't present in children until the very last minute. They maintain their cardiac output by driving up their HR - BP doesn't fall until very late in the process. Most 15 year-olds behave more like adults, but there are always those outliers.
As far as his discharge, deterioration and subsequent readmission being all your fault, it's not. There are other people in the chain of survival besides the nurse. Physicians are responsible for their own assessments; that isn't something they can legally delegate and if this boy was not assessed by a physician prior to discharge, that needs to be addressed. No peds physician worth the title would ignore that HR. The fact that you were floated out of your own area of expertise is also a system factor that can't be ignored. Yes, this situation gives the hospital a black eye, but it's a shared black eye so don't be so anxious to claim it! Learn from this and move on.