One of the components of an estate plan is the advanced directive. This works with the powers of attorney for health care to ensure that you get treatments you are comfortable with when you aren't able to make decisions on your own.
When you are drawing up the forms that will outline your wishes, you might decide to include instructions about life support. While some people think that this means being kept alive through artificial means, there is much more to life support.
Kidney dialysis is a form of life support because without it, the patient will die. This is necessary when the kidneys aren't functioning properly to be able to filter toxins out of the blood. Those toxins will accumulate without the artificial filtering that occurs during dialysis.
Life-sustaining hydration and nutrition are also forms of life support. This might be necessary if you are unable to eat and drink on your own. Doctors might use this to allow the intestines heal or to give the body a rest. In some cases, doctors are unwilling to withdraw this support if there is not another indication that the person is going to perish quickly.
Cardiopulmonary resuscitation and ventilation are two other issues to consider. These are often what people are referring to when they say that they don't want to be resuscitated.
It is important that you clearly state your wishes and speak to your medical team about them so that you make sure everyone is on the same page. There is no sense in you having to deal with unwanted treatments just because you can't speak up for yourself at the time.
Source: Cleveland Clinic, "Understanding Life Support Measures," accessed Sep. 06, 2017