Sleeping on the back may be good for babies but could be dangerous for some adults, a new study at Barnes-Jewish Hospital shows.

Doctors at the hospital screened 2,500 patients who came in for surgery to see how many have a sleep disorder known as obstructive sleep apnea. Nearly a quarter of the patients were at high risk for the disorder, which causes people to stop breathing for short periods during sleep, and didn''t know it. This summer, the hospital began treating such patients differently after surgery.

Sleep apnea happens when soft tissues in the back of the throat narrow and close the airway repeatedly during sleep, causing people to stop breathing. As the oxygen levels in the blood drop and carbon dioxide rises, the body gets the signal to wake and start breathing again. The arousals last only a few seconds and people usually aren''t aware that they''ve awakened.

While it''s necessary to resume breathing, the constant arousals mean sleep is disrupted multiple times each night, said Dr. Stephen P. Duntley, director of the Washington University Sleep Medicine Center at the Chase Park Plaza. Some people wake up "every minute or so. It''s not refreshing sleep."

The condition has been linked to high blood pressure that is resistant to treatment, diabetes, heart disease, stroke, abnormal heart rhythms and sudden death. People with sleep apnea are at higher risk of having auto accidents too, Duntley said.

People with obstructive sleep apnea are more likely to die of heart disease while sleeping, while other cardiac patients usually die in the morning, said Dr. Rajeshkumar Bhalodia of St. Louis University. Bhalodia and a team of researchers screened a group of people who either had a history of heart attacks or had blocked arteries. They found that 40 percent of the heart disease patients had previously undiagnosed sleep apnea.

The researchers further examined 134 people with the disorder while they slept in a sleep lab. They found that people with severe apnea have prolonged episodes of low oxygen during REM sleep — the stage when people dream. The low oxygen levels were also associated with a higher frequency of an abnormal heart rhythm. Those factors may account for the higher risk people with sleep apnea have of dying in their sleep, Bhalodia said.

The sleep disorder also can be dangerous for surgery patients as they attempt to wake up from anesthesia and for a few days after surgery. Anesthesia drugs, painkillers and sedatives make it harder for the body to wake itself up, so a patient may slip into respiratory arrest.

People coming to the hospital for surgery are generally not in good health to begin with, and apnea plus disruptions to a person''s normal sleep routine can make things worse, Duntley said.

"You''re frequently sleep deprived in the hospital. It''s not a good place to sleep," he said.

The doctors at Barnes-Jewish Hospital recognized that the sleep disorder could pose a problem for vulnerable patients, but since most people are unaware they have sleep apnea, the surgical staff at the hospital had no idea how many people could benefit from special handling. The hospital put together a task force to study the problem.

The effort was prompted when a healthy man died after an elective surgery to repair a minor injury. The man went into respiratory arrest, slipped into a coma and later died, said Dr. Kevin Finkle, a Washington University anesthesiology resident at the hospital. Doctors suspect that undiagnosed sleep apnea may have caused the death, he said.

Finkle was part of the task force led by Dr. Michael Avidan and registered nurse Colleen Becker.

An estimated 4 percent of adults and 1 percent of children in the United States have sleep apnea, but most aren''t aware of it. Age and obesity contribute to the problem, so more people are likely to develop the sleep disorder as the population continues to get older and heavier, Finkle said. About half of obese people have the sleep disorder, he said. Other risk factors include a thick neck, frequently waking up during the night, morning headaches, drowsiness during the day and snoring. Men and people over 40 are more likely to have sleep apnea, as are people who smoke or drink alcohol heavily.

The task force screened 2,500 surgical patients at Barnes-Jewish Hospital over the first six months of this year and found that 23 percent of them were at high risk of having sleep apnea. The researchers sent some of the high risk patients home with a device called an ARES Unicorder. The small device straps onto the forehead and contains an oxygen sensor, a gyroscope for determining if the person is sleeping on his or her back or side, a microphone to detect snoring and tubes that fit in the nose to detect whether the patient stops breathing.

The device showed that 76 percent of the people at high risk for sleep apnea had the sleep disorder.

"More or less if you suspect a patient has sleep apnea, they do have it," Finkle said.

The researchers estimate that about 5,000 surgical patients at Barnes-Jewish Hospital each year have undiagnosed sleep apnea.

The hospital now screens all patients slated for surgery for risk of sleep apnea during their preoperative examination. High-risk patients get purple plastic wristbands marked "OSA Risk," and their paperwork is stamped in purple. Those measures alert doctors and nurses that the patient needs special treatment.

Such patients are given different sedatives and supplementary oxygen. They are positioned on their sides instead of the back since sleeping on the back makes the condition worsen, and their oxygen levels are closely monitored. The patients are also advised to go to a sleep lab for testing to see if they really do have sleep apnea.

In May, the American Society of Anesthesiologists issued a set of guidelines that recommends similar measures to those enacted at Barnes-Jewish Hospital. The society said it had not surveyed members to see how many hospitals have adopted the guidelines. Barnes-Jewish is the first hospital in the Metro area to take such precautions, Finkle said.

"I wouldn''t be surprised if people''s lives have been saved because of this," he said.