If you become demented, how do you prevent a DNR from being signed on you by the one appointed as personal representative?

A DNR doesn't mean they'll let you die of a cold. It means they won't try to resuscitate you if you die.

First, though, fill out a medical diregtive that says what treatment you do and don't want for medical care. Just because you state that you want them to do everything they can to keep you alive doesn't mean they will. If you're dying, you're dying. Doctors ultimately will make the call when they decide you can't be resuscitated. If you requested CPR in your medical diregtive, know that your ribs will be shattered as they do it, and you likely won't recover anyway.

Finally, don't appoint a POA who won't respect your wishes.

Typically the person that you choose to be your representative is someone that you trust to make the decisions that you would make for yourself OR that would make the decisions that are in your BEST interest in the absence of the ability to make a choice that they know you would want if the situation warranted it.

Meaning- in the absence of a living will/advance directive - your POA/representative would KNOW your wishes and be able to execute what you would want for yourself, but you would TRUST them to make the correct decisions for your quality of life if they KNEW that any other choice would leave you in agonizing pain, in misery, with no quality of life or...were you in your right mind - wishing that you were dead.

So a better question might be - how do you ensure that your end of life planning is done to afford your representative a clear directive of your wishes? If you do not want to give them any leeway to make any other choice - as Alva said, you have to spell it out, in no uncertain terms - that you want ALL heroic measures done in every circumstance, no matter what the outcome. That you do not care what your quality of life will be after. That you want them to try every single time.

That being said - it might be a good idea for you to genuinely understand what that really means. As Grandma mentioned - heroic measures can often be incredibly horrifying for the extremely elderly, frail, or unhealthy. Broken ribs, damage to internal organs, damage to the brain due to extended loss of oxygen, or additional or new disabilities can all be a product of just CPR. Additionally, at what point is the line? If someone is already on life support- but with clear certainty that they are in an irreversible coma - but go into cardiac arrest for example - are heroic measures called for?

In 1990, Terri Schiavo collapsed and went into a persistent vegetative state at the age of 41. Her brain was severely damaged due to oxygen deprivation. Her entire brain, not just part of it. That's young.....like YOUNG. At that age - a lot of people haven't even thought about end of life plans, whether they want heroic measures if something happens to them - of course they do! Why wouldn't they? Except...there are always going to be situations where heroic measures don't make sense. FIFTEEN years later, when the courts and her parents were done arguing about keeping her alive, her husband - her legal guardian - was finally allowed to make the choice that he knew Terri would have made had she been able to do so. Her parents maintained that she could have recovered and that was one of the reasons she was kept alive so long. Her autopsy revealed that her brain damage was so severe that recovery was impossible.

So while I fully support anyone's right to make decisions about their own life and how they want to live it, or as their end of life comes - not live it, I do think that very careful consideration should be given to things like very absolute requirements.

Both mine and my DH's advance directives and living wills (which we have had since we were 24 and 30 respectively) are written in such a way that it is clear that we want life-saving measures, but if there is no expectation of survival or quality of life upon survival we don't wish to have heroic measures because we don't wish to have life prolonged on life support indefinitely if there are on natural or organic signs of life and the only life is from a machine.

But you will have to be very clear in what you want, or your POA will have the ability to speak on your behalf if you do not leave clear wishes.

Sign a very strong advance directive that says that you wish to have all heroic measures available to you regardless of your mental capacity or lack of it, that you expect your MPOA to direct that all heroic measures be carried out until, despite all measures, you are no longer alive.

This will suffice in so far as ANYTHING will suffice, for, if you are severely mentally incapacitated AND severely physically incapacitated, then at some point the medical community, and the person you appoint to carry forth your wishes will come to the conclusion that further intervention is tantamount to torture, and such "heroic" measures will no longer be entertained.

Prolonging life with dementia at play is nothing short of cruelty, in my opinion. Imagine being kept alive with feeding tubes and artificial means, only to have no faculties left, lying there in bed all day? Why??

You make the decisions now, when you are cognizant.
Complete a POST indicating what measures you want taken.
(POLST goes by other names in some states, it is more detailed than a DNR)
Please keep in mind CPR does not always work.
If it does you will most likely have broken ribs, broken sternum and possibly punctured lung(s) from the broken ribs. And there is no telling what brain damage may have occurred due to lack of oxygenated blood circulating.
Honestly ...if I did not have my POLST or advance directives and I did have dementia I would not want any measures taken to extend my "life". I would hope that a family member would NOT allow CPR, intubation, IV's or tube feeding.

You get a Medical Directive done which lists everything you do and don't want.

There comes a time, though, when even a doctor will say "its time". You really don't want measures done to prolong a life of Dementia and pain, do u. CPR, in the elderly, cracks bones. A DNR allows you to die a natural death.

Create a living will/advanced directive (like 5 wishes) and go over it with the person you choose to be your medical decision maker. Obviously don't pick a person that doesn't share your beliefs.
But nobody in the medical profession is walking away from caring for people based on a DNR alone, there would have to be a reason to opt for hospice or allowing a natural end to life. And even a frail, confused elder can ask for treatment even if there is a DNR in place, I know because that's what happened with my uncle (unfortunately).

Short answer, you can't. Once you're no longer able to act in your own self interest, someone does it for you, so get that done now.

Might want to visit a facility and see some folks who are being kept alive because they don't have an advance directive. Every UTI, flu, cough is treated with an intent to cure. With the meds we have now, we can keep people alive long past the time when nature might have taken them. Can't do the same with their brains, though.

I think the better question is why would you want to?