I talked about giving Moose heated fluids for his sub acute body temperature when he was sick. I'm going to expand on this a little, because it has many good applications.
When a cat is very ill, it is not uncommon for them to be unable to maintain body temperature. It is unfortunately sometimes a sign that the cat is dying, but some cats can be saved by helping to maintain that body temp. Injury, illness and hypothermia call be reasons why the body temp can drop. Many vets have machines that warm the fluids, but obviously most people who might give fluids at home do not.
You can warm the fluids several ways. You can place the iv bag on a heating pad, making sure the fluid is warm, but not hot. Test this like you would a baby bottle on the inside of your arm. it should just feel a little warm (yep, some real science there..eh?)
you can also take a large syringe such as a 140cc, fill it, remove the needle and place it in the microwave for about 20 seconds. Put a new needle on and give the fluids. This is my favorite technique...it's quick and simple.
Subcute fluids are invaluable. They should be your first line of emergency treatment next to obvious bleeding or need for immobilization. Fluids can help stave off what is a huge killer in any emergency situation and that is SHOCK....shock can occur from many things, injury being the most common, but fluids can buy your cat time to get to vet. It's not foolproof, nothing is, but it certainly can help.
I feel like this is the #1 thing you should have in a pet first aid kit. Ringer's lactate IV fluids are available by prescription from your vet, or can be ordered with a prescription online as well as venosets and needles.
IF you don't know how to give fluids, ask your vet to show you. It isn't hard and with a little practice and some stratigcally located plant hooks and a hanger, you can even manage by yourself.
The picture is Pudge. Pudge died when she was 14 after a bout with liver cancer. She was born on my living room floor 18 years ago, along with her two sisters and her brother. We kept her one sister Patches who passed about 6 months ago from old age, as well as her mother. My sister took her other sister Sprite and Sprite is still hanging around, doing the old cat thing. Their brother I adopted out to a coworker, who let him outside and he died when he was 2 of feline leukemia. That news made me very mad and very sad at the same time, since this woman had promised she woiuld keep him inside. Sometimes people just don't realize the bad diseases there are out there that there are no cures for.