Filed at 1:02 a.m. ET

DALLAS (AP) -- Despite liberalized guidelines that shorten pre-operation fasting times, many patients undergoing elective surgery still are instructed to go without food and liquids for excessive periods, according to a new study.

Some patients fasted for 20 or more hours before going under anesthesia and suffered from irritability, severe hunger, headaches, dehydration and lowered blood sugar, the study found.

``Obviously this isn't good for patients,'' said Jeannette Crenshaw, co-author of the study published in the May issue of the American Journal of Nursing.

Most of 155 patients interviewed after surgery said they were told not to ingest anything after 12 a.m. on the day of surgery, which the American Society of Anesthesiology no longer believes is necessary for most patients undergoing elective surgery.

In 1999, the ASA released new guidelines allowing clear liquids, such as black coffee, tea, apple juice or carbonated beverages, up to two hours before elective surgery, a light breakfast such as tea and toast six hours before the procedure, and a heavier meal if the surgery is eight hours away.

Crenshaw said shortened fasts generally do not increase the risk of vomiting during surgery, or the incidence of pulmonary aspiration, a rare and sometimes fatal complication in which stomach contents enter the lungs.

The researchers interviewed patients at Presbyterian Hospital in Dallas at the end of 2001 and early this year. On average, the patients refrained from consuming liquids for 12 hours and solids for 14 hours. One patient had nothing to drink for as long as 20 hours and two patients went without food for 37 hours.

Diana Mason, a registered nurse and editor-in-chief of the American Journal of Nursing, said there is often a lag between the release of published guidelines on patient care and their implementation.

``Hospitals need to change the status quo and initiate fasting policies that are congruent with, in this case, the ASA recommendations,'' she said.

Crenshaw said Presbyterian Hospital is revising its fasting policy to include the ASA guidelines, which apply only to healthy patients undergoing elective surgery.

Some hospitals have feared that relaxing fasting guidelines would impair their flexibility to schedule surgeries, said Dr. Mark Warner, chair of anesthesiology at the Mayo Clinic in Rochester, Minn.

But operations are more likely to be delayed than to be rescheduled for much earlier than originally set, he said.

And consuming clear liquids several hours before surgery helps flush out acids from the stomach, Warner said. In addition, a cup of black coffee or tea will help some people avoid headaches and nausea caused by caffeine withdrawal.

Ray Thibodeau fasted from food and liquids for more than 20 hours when he had cataract surgery, which was unexpectedly delayed from about 8 a.m. to 4 p.m.

``I was clearly not suffering malnutrition,'' said the Doylestown, Pa., man. ``But the mental process takes over. I could have eaten the back side of a mule, I was so hungry.''

He said some liquids the day of the surgery would have eased his discomfort.