You want what's called a living will - it specifically directs what happens to you in exactly the situation you described.
You may also want to give someone you trust power of attorney which allows them to act on your behalf, but the living will is the ultimate say-so, and overrides POA. POA tends to end when you're incapacitated, so if you want them to be able to act on your behalf in a non-life threatening but still dangerous situation, make sure the POA is "Durable power of attorney."
Just make sure these say exactly what you want, because they're legally binding.
I would add, "Healthcare Power of Attorney" here. There are different flavors of POA, but having the living will, advance healthcare directive, and healthcare POA meets most of the objective.
All of these (if relevant in your state) could probably be downloaded from the secretary of state website. You can fill them out by hand or print one up.
You will need to sign these with a notary public. That will cover the extended life support situation.
Forgot to at you
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