My script is this (it's long, I know. But there's no way to do this well and be brief):
[Although unlikely to happen] OR [Due to your / your loved one's severe illness], one topic we need to discuss is what to do if you/they suddenly have a life threatening event, such as your heart stops beating, or you are having trouble breathing on your own. We need to review that now, because should it happen someone will need to make a quick decision about what to do and may not have time to contact family or you may be too ill to discuss it yourself. You may have completed a form designating someone else to make decisions for you, but since you are awake and alert, it's important that I and your family know what's important to you.
If something very severe like this were to occur, there are several options about what we could do. One option is to treat you as aggressively as possible using medications, machines, and procedures to try to save your life. This might require using a breathing machine to help you breath, and trying to get your heart started with electrical shocks should it stop. If this happens, you will likely be unconscious either from your severe illness, or from medications we need to give you. You would likely be in an ICU for an extended period of time. Although some people can make a full recovery from a serious problem like this, many never fully recover -- some brain damage can happen, and a long hospital stay can make you very weak. Some patients survive, but are unable to go back home because of their illness. But some recover well and can have a good life.
The other option is to focus on comfort care. Should you become suddenly ill like this, instead of using machines and procedures to try to save your life we focus on making sure you are comfortable, using pain medications, relaxing medications, or anything else we think would maximize your comfort. We would treat you aggressively and fully for everything else -- just focus on comfort if it appeared that something very acute and serious was happening.
A third, middle option is to try "heroic" measures for a short period of time. If we can turn things around quickly and it appears you'll make a good recovery, then that's great. If it looks like something terrible has happened and that it will impact your life going forward, we would then focus on comfort. A reasonable amount of time to consider would be 48-72 hours.
There is no right answer here -- each person has their own desires and needs. Some people reach a stage of life where they feel they have enjoyed their life, want to continue if the quality of life is good, but should something catastrophic happen, would want to pass away comfortably. Other people feel that fighting for every day of life is important, or have some future event that they really want to be present for. No decision you make is permanent -- you can change your mind any time. Have you talked about these types of decisions with your PCP or loved ones? What's important to you?