I have never tried this but it was suggested to me:
we had a cat that got caught up with my husband's back tire, it was pretty obvious the cat was not going to make, I called a vet and asked them what to do..
They gave me two suggestions:
one to bring her in and they would put the cat down, well I really couldn't do that, at that time I had just lost my job and was looking for another and my husband wasn't working, he was trying to get disability. I had explained this to them and they understood
so they gave me a second suggestion, to put the cat into a box and have the exhaust from a car shoot into the box, they told me this will put the cat to sleep and adventually die. I didn't have the heart to do it but the cat ended up dieing within in a few hours anyway...
I'm not trying to sound mean in anyway, it was something that was suggested to me.....