I have been wondering about that 8 min target for some time now, trying to understand why 8, instead of 5 or 10. I couldn't find a reason at all, our limits are quite different. Until I did my A-CPR refresher course this summer (at a new hospital, mind you). It appears 8 min is the time you can leave a heart alone (no cpr, no oxygen), and still expect resuscitation to be successful. It has to do with it filling with blood, stretching of fibres and so on. You probably knew this already, but maybe that's where the 8 min come from?