It should just last for a little while instead of going nuntil you stop it.
that sounds better
Have it when you try to use the Paint Can (Default 'E' as according to the game) in a minigame with painting disabled for a quick use thing.
That sounds rather easy to do, it could also put in the chatbox "MEDIC!"
How will we know when to call though, no health bar.
And how will people, mainly someone who is a Medic, know where you are calling from?