In the past, living wills were the most common documents used to provide guidance in the event of a catastrophic health event that left an individual unable to express their wishes. The event in question is usually one that would be terminal but could be averted through life-saving measures, even though the individual would likely be left in a persistent vegetative state as a result. The living will was meant as a tool the person could put in place beforehand to navigate these situations, but the document had its limitations. It only addressed certain scenarios.
An advance healthcare directive, however, allows the person to name an agent who can act on his or her behalf in similar situations (where a persistent vegetative state is likely) as well as instances where the principal is facing a myriad of other terminal conditions or is in the final stages of a deadly illness.