At a non-towered airport in the US, what should I do if I hear a medevac flight waiting to take off while I'm turning base? I'm in the left downwind at my favorite non-towered airport. Just while I'm making the turn to base, I hear a radio call on CTAF: "Such-and-such Traffic, Medevac 123, holding short runway 31." They don't make any requests or say anything else; they merely announce that they're there, waiting for me to land and get clear.

Since they're a medevac flight, should I abort my approach and let them take off immediately, or is it fine if I simply land and vacate the runway as soon as practical?

If I abort the approach, what's the best way to do that, given that I'm on the base leg now? Continue past the runway and make an upwind leg? Just turn right and exit the pattern completely, and then come back once the medevac flight is out of the way?

And whatever the right thing to do is, what radio call should I make? I assume that there's something more standard than "Medevac 123, Diamond 456, I'm just gonna continue past the runway and make an upwind leg so that you can take off right away."

(This happened in a 2-seater I was riding in today. In this case, the pilot decided to simply land and get clear of the runway quickly.)
The most obvious thing to me would be to just talk to them:


  Medevac 123, this is the Diamond on base for 31, do you need to expedite your departure?


If they say no, then just continue and land as normal. If they say yes, then I'd get out of their way:


  Medevac 123, roger, we'll extend our base and fly a wide upwind to let you  out


I wouldn't worry about standard phraseology too much, the critical point is to make sure you both understand each other and if there's no well-established phrase for what you need then just say whatever seems clearest.

Extending base to upwind has the advantage that it keeps you in the pattern so you can see what's going on as the medevac flight departs and you don't waste too much time going around the pattern again. (Although unless I have an emergency of my own I'm more than happy to make whatever detour is necessary to help out a medevac flight.)

Turning out right puts the pattern behind you and if you turn too early then you're potentially turning into other traffic on a long final, so it wouldn't be my first choice. But I don't think there's any standard way to break off from base and there are lots of possible variables (terrain, traffic, airspace etc.) so it's up to the pilot to decide what's safest at that particular moment.