Such medical wishes and plans for end-of-life care, called "advance directives," outline what can and cannot be done if patients are unable to decide for themselves. The most famous examples are so-called living wills.

But the restrictions are debated among doctors, said the survey, published in the Annals of Surgery.

"(Surgeons) feel the advance directive basically ties their hands behind their back, and they're not given the tools to get them through the surgery," said Margaret Schwarze, an assistant professor at the University of Wisconsin School of Medicine and Public Health, who was one of the survey's authors.

She and her colleagues asked 912 surgeons who regularly perform risky operations 14 questions on how they discuss a patient's advance directives and whether the directives influence their decision to operate.

More than four out of every five surgeons discussed which forms of life support the patients would like to limit. But only about half asked specifically about the patient's advance directive, which can include restricting the use of feeding tubes and ventilators to keep a person alive.

"I think some surgeons just don't discuss advance directives because they think it's so irrelevant," Schwarze added.

More than half the surgeons said they would not operate if an advance directive limited what could be done to keep a patient alive after surgery.

The researchers said such instructions can also cause tension between the surgeon and the patient because it shows the patient may be unwilling to accept the therapies that come with high-risk operations.

Heart surgeons were much more likely than brain surgeons to decline an operation -- perhaps because brain surgeons may see removing life support as a reasonable decision, since their surgical complications can cause brain damage.

It's not uncommon for heart surgeons to use techniques such as CPR after an operation and restrictive instructions may get in their way, Schwarz added.

"I think it's important for patients to discuss their values and goals with surgeons before a big operation," Schwarz added in an email to Reuters Health.

"It's also incredibly important to discuss this with family members or someone who may need to make decisions."

Although the use of advance directives has increased in recent years, a study earlier this year found that U.S. laws set up too many roadblocks for many people to put them into place.

SOURCE: bit.ly/t9SVLr

(Reporting from New York by Andrew Seaman; editing by Elaine Lies)

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