We heard back from the vet with the final autopsy results on Jack. He did have inflammatory bowel disorder, as we suspected, but other than that, he was perfectly healthy. A consult with an anesthesiologist turned up that it was most likely a case of “epinephrine reversal.” The way the vet described it, the pre-anesthetic sedative they gave him had an adverse effect that basically took down his blood pressure in a wildly unexpected way. It happens in animals that get really stressed out - for example, it’s common with horses because horses get stressed easily. It’s virtually unheard of in cats. That, and Jack wasn’t exhibiting any stress-related behavior. Aside from him just not liking the IV and catheter (who does?), he was fine.
There really isn’t any way we could have detected it or avoided it. He needed to be checked out so we could help him with his condition and treat it correctly, and this was the only way to do that. Even if we had skipped it now, who’s to say it wouldn’t happen in some future medical scenario.
The doctor is continuing to look into ways to prevent it from happening to other animals, but it looks like… well, “it just happens.”