What is the procedure in a shelter euthanasia room?

i worked in a shelter for a year and it is a touchy subject. there is no difference between what animals are chosen and who goes. some shelters have a "3 day" policy they come in and they have 3 days to go. but there are many rescues who come in and get animals by the bundle. and other shelters are non-euthinasian. they will keep the animals for forever until they are adopted, or live their whole lives there. its not easy to choose the animals to be put down. our shelter all depended if we were over crowded. and we did everything we could before we put them down. we mostly did the sick ones who had no chance of coming back. and other times we had no choice but to choose the ones who went. perfectly good dogs can go. which i was strongly against but i couldnt do anything about it. but the procedure we did at our shelter for euthinasia is we walked the dogs back to the room we spent time with them held them. there is a shot you give them and it relaxes all of their muscles and they go into a calm and submissive place. we let them calm down for a minute and let the medicine sink in. then we take the euthinasia medicine and inject it straight into the heart. so it takes them quickly as opposed to going through their whole body and making them suffer. they take about 2 breaths and then they are gone. i say all of this because you want to know. it is a very sad subject. its a very hard thing to do. i cried many times honestly. but we made it very comfortable for them. if you have any other questions i will be glad to answer. i hope this has helped you.