You may have enough confidence in the judgment of the person you appoint to make health decisions that you just give them the power and trust to their good judgment. This is not a bad choice considering how medicine continues to evolve ever more rapidly. Detailed instructions about what to do in a number of situations have two main problems.
First, detailed instruction sets cannot cover all of the myriad health situations we human beings encounter. Conditions that used to mean imminent death are now either curable or controllable enough to be chronic conditions. Unless you can predict the future more accurately than most, it is a good idea to give general decision powers in addition to specific instructions.
It is a good idea to choose a decision maker who knows you and your wishes well. Just because something is written down does not mean it will be read at a time of stress. It is also a good idea to choose someone who not only cares about you, but has a steady head as well. Pick someone who will ask questions, but does not think their internet search is the equivalent of a medical degree.
As with a living will make sure you have given a copy of your health care power of attorney to all of your medical providers and the decision maker. With electronic medical records once you have handed the document over it should appear in your medical record whenever needed. If you think you will remember to carry it with you when you go to the hospital you might be wrong.