The patients have heart rate monitors with GPS signalers that signal the cryonics company as soon as the patient flatlines.
Oh wow, this is massively obvious in retrospect. Does anyone know if anything like this actually exists?
No. Ben Best has been trying to build a device like this for years (link from downthread); people very often say to him that it should be easy, but he never sees a finished prototype.
Check out the latest episode of Castle (Headcase) to see Cryonics covered in mainstream fiction in a not entirely terrible manner. The details are not exactly accurate but probably not more inaccurate than similar fictionalised coverage of most other industries. In fact there is one obvious implementation difference that the company in Castle uses which is how things clearly ought to be:
Amulets of Immortality
It is not uncommon for cryonics enthusiasts to make 'immortality' jokes about their ALCOR necklaces but the equivalent on the show make the obvious practical next step. The patients have heart rate monitors with GPS signalers that signal the cryonics company as soon as the patient flatlines. This is just obviously the way things should be and it is regrettable that the market is not yet broad enough for 'obvious' to have been translated into common practice.
Other things to watch out for: