These critical documents about your preferences for end-of-life care don't always work as planned. More flexibility might be the answer.

My father was in a coma, hooked up to a ventilator, and I had to make a tough call.

His living will expressed his desires for a few black-and-white situations: He didn't want to be kept alive if he was terminally ill, or in an irreversible vegetative state. But the situation I faced wasn't so simple. The neurologist said he would wake up from the coma, but there was a good chance he would have severe brain damage. How much of a chance? The doctors couldn't say.

Doctors and nurses say my heart-wrenching experience is typical of the complexity of real-life bedside decisions. An estimated 25% to 30% of Americans have filled out living wills, documents that spell out wishes for medical treatment. But ethicists say the typically simplistic documents aren't the solution many hoped they would be. Life-prolonging medical technology has far outstripped doctors' ability to predict outcomes. The hardest choices center on when quality of life will be so diminished that death is preferable.

As such, some health organizations are trying to improve living wills, allowing for more flexibility and nuance. Some ethicists, meanwhile, are de-emphasizing living wills altogether and focusing on appointing a trusted family member or friend as your health-care agent.

"Most of us have come to the conclusion that the way to get over the vagueness is to get someone to speak for you," says Robert M. Veatch, a professor of medical ethics at Georgetown University's Kennedy Institute of Ethics in Washington, D.C.

Living wills were created in the 1960s and gained national attention in the 1970s when a young woman, Karen Ann Quinlan, following alcohol and drug use at a party, was left in a vegetative state, raising alarms about medical technology keeping people alive in hopeless circumstances.

"We had a naive view that if you had a document, that would solve the problem," says Daniel Callahan, co-founder and president emeritus of the Hastings Center, a Garrison, N.Y., nonprofit that was an early champion of living wills. "In practice," he says, "all sorts of problems arise" that aren't spelled out in the documents.

When Paul Shalline, an active 86-year-old who regularly bested his grandchildren at ping pong, was unable to communicate after a severe stroke in March, treatment decisions fell to his daughter, Robin. Ms. Shalline, a 57-year-old teacher from Monkton, Vt., says her father had a living will but had never talked to her about his wishes. "There is so much gray area," she says. "You'd hope the living will would spell it all out, but it doesn't." 

His living will called for withdrawing life support if there was no reasonable expectation of regaining a "meaningful quality of life" but didn't describe what that meant, she says. Ms. Shalline, when told by doctors that her father could be blind in one eye, unable to feed himself and might never walk again, made the decision to withdraw the ventilator based on "what I knew about his life." Mr. Shalline, who loved Wiffle ball and had recently helped build a staircase, was "proud of his 'physicalness,' " she says. He died March 18.

It is hard enough, under the best of circumstances, to know what your family member would want in a particular situation. But add to that the fact that even top doctors can't predict outcomes very well.

Lee H. Schwamm, vice chairman of the neurology department at Massachusetts General Hospital in Boston, where Mr. Shalline was treated, says that even when he thinks he can predict a patient's outcome after a stroke, he is wrong 15% to 20% of the time on major outcome measures, such as whether a patient will be able to walk again. "I've never seen a living will—and I've seen a lot—that speaks to this question of diagnostic uncertainty," says Dr. Schwamm.

Living Documents

You can get a living will from a lawyer or download it from the Internet. Many focus on permanent comas and clearly hopeless conditions. Florida's statute-suggested living will, for example, directs life-prolonging treatments to be stopped if there is "no reasonable medical probability" of recovery from a terminal condition or persistent vegetative state. Florida, like most states, allows you to write your own living will; a few states, such as New Hampshire, specify that living wills must use a state-approved form. (A bill now being considered in New Hampshire would make the state form optional.)

A number of efforts have been made to improve on the standard-style living will. A document available online from Lifecare Directives LLC, Las Vegas, for example, spells out several levels of cognitive decline from coma to mental "confusion" that require 24-hour supervision, and asks if you would want life support if your brain failed that much. The document also gives you an option to say whether you want doctors to be "positively certain," "certain to a high degree" or "reasonably certain" that you will never recover before pulling the plug.

A simpler but also innovative approach is the popular Five Wishes living will. Five Wishes is written at a sixth- to seventh-grade level, says Paul Malley, president of Aging with Dignity, a nonprofit that distributes the document. Despite its simplicity, the Five Wishes living will addresses issues many others don't—for example, asking if you want pain medication to relieve suffering even if it makes you sleepy. It also has a blank space where people can specify a state in which they wouldn't want to be kept alive.

"Some people have a phrase that pops out in their mind: 'If I'm in the same condition as Aunt Mary,' " Mr. Malley says. Originally written in 1997, the Five Wishes will has been available online in an interactive format since last year.

Open to Interpretation

The problem with living wills is that most people can't articulate what they want, says ethicist Angela Fagerlin, co-director of the University of Michigan-affiliated Center for Bioethics and Social Sciences in Medicine in Ann Arbor. And even if they can, family members often don't properly interpret those wishes.

In a 400-patient study published in 2001, Dr. Fagerlin and colleagues found that family members who were presented with nine hypothetical scenarios correctly predicted patient wishes about 70% of the time, whether or not the patient had filled out a living will.

Can you forgo such documents completely? Mr. Callahan, who championed living wills in their early days, says he doesn't have one, preferring instead to give decision-making power to his wife, to whom he has said simply, "When in doubt, don't treat."

A health-care agent—a trusted family member, for instance—could supplant the need for a living will. Under the legal doctrine of "substituted judgment," health-care agents must try to make the decision you would if you could, says Alan Meisel, the director of the Center for Bioethics and Health Law at the University of Pittsburgh. Anything—a phone conversation, a list of instructions or a formal living will—can be used as evidence of your wishes, he adds.

As for my father, we postponed the decision, and he woke up, sharp as a tack, able to make his own decisions.

A version of this article appeared June 11, 2012, on page R5 in the U.S. edition of The Wall Street Journal, with the headline: A New Look at Living Wills.