Uh, the survival statistics for opiate overdoses are very very minute, even if paramedics can arrive to help. Permanent brain damage occurs in a matter of minutes once the brain's oxygen supply has been cut off.
You are much less likely to overdose on insufflated than injected heroin because you may just fall asleep before you do a lethal amount. Also, with insufflation, it is virtually impossible not to be able to judge how high you are and whether dosing again is safe. All in all, insufflation is not a real danger as compared to IV.
Falling unconscious takes time, so even if you IV it, you will probably not go unconscious right away, and if you do, you aren't dead yet. When you fall out, you don't know that you are going to or that you already have fallen out until you wake up. If you are so tired that you can't keep your eyes open or stand up, your lips are turning blue, your face is pale, and your breathing gradually slows down, you probably have overdosed.
I personally have fallen out on sniffed and injected heroin. With sniffing, I fell asleep for about 10 minutes, then woke up to not knowing what had happened. With injection, I fell out about 6 minutes after and remained unconscious for 30 minutes, only to wake up lying on the floor. Also I was heavily sweating and nauseous.