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Release: May 20, 2002

Study looks at residual effect of anesthetics used for sedation

A study conducted at the University of Chicago by a researcher who has since moved to the University of Iowa suggests that patients who receive sedation for an outpatient medical procedure would do well to heed their physicians' warnings to take it easy for the rest of the day.

Medical procedures where patients are sedated for a short period of time in an outpatient setting are becoming increasingly common. Sedation reduces patients' discomfort and anxiety, making procedures such as endoscopy less unpleasant. The anesthetic drugs used in these cases are designed to wear off quickly after the procedure so that patients can resume normal activities as soon as possible. However, the drugs do impair a person's abilities and reaction times, and determining exactly how long their effects last has important safety implications.

"The effects of the anesthetics that we use are similar to alcohol in the way they impair a person's abilities," said J. Lance Lichtor, M.D., UI professor of anesthesia and principal author of the study. "So, just as we tell people that they shouldn't drink and drive, the same advice goes for people who have undergone sedation for an outpatient procedure."

In the past, determining how long an anesthetic affects patients has been based on tests that measure patients' reaction times and physical coordination. The results of these so-called psychomotor tests suggest that the effects of commonly used anesthetic drugs wear off within two hours.

However, Lichtor and his colleagues were concerned that psychomotor tests are not sufficiently sensitive to accurately measure the duration of the anesthetics' effects. Psychomotor tests measure the responses of a person actively concentrating on a test at hand. The researchers felt that a passive sleep latency test, which measures a patient's sleepiness, might better reflect how long patients are affected by the drugs.

How fast a person can react after hearing a sound is not typically why people get into accidents, Lichtor explained. A more common situation might be an accident occurring when a stoplight goes unnoticed because of a person's sleepiness.

"Instead of having people respond to a stimulus, we put them in a quiet room, turn off the lights, and tell them to try and fall asleep," Lichtor said.

Using electroencephalogram (EEG) electrodes to monitor changes in the electrical activity in the brain, sometimes called brain waves, the researchers were able to measure sleep latency, or how long it takes for a person to fall asleep. Normally, a healthy non-sedated adult takes about ten minutes or more in a quiet dark room to drop off.

Lichtor and his colleagues tested sleep latency in 12 healthy young volunteers with normal sleep habits who had been given various combinations of three anesthetic drugs, propofol, fentanyl and midazolam. These drugs are commonly used to produce sedation. Every two hours after the injection the volunteers completed several psychomotor tests and then lay down in a dark room. The researchers used EEG measurements to time how long it took the volunteers to fall asleep.

Two hours after being sedated, volunteers performed worse than normal on some of the psychomotor tests. For example, auditory reaction times were longer than normal for each of four different drug combinations. However, other psychomotor tests such as visual reaction times were not significantly affected. After four hours, none of the psychomotor tests could detect any impairment in the volunteers.

In contrast, the sleep latency test found increased sleepiness in the volunteers for at least twice as long after sedation for all drug combinations.

"We showed significant differences for up to four hours," Lichtor said. "And there were some people who were still falling asleep quickly up to eight hours after the anesthetic."

The researchers also found that the duration of an anesthetic's effect varied with different drug combinations. In particular, the effect of propofol on its own or in combination with fentanyl was very short-lived whereas the midazolam combinations, which are commonly used for this type of sedation, appeared to have a longer lasting effect on patients and left people very sleepy for a long period of time after they regained consciousness.

Although physicians currently tell patients not to do anything for the rest of the day after light sedation, previous studies have not really backed up that advice. Lichtor explained that this study does reinforce the recommendations, and demonstrates that even when a person's reaction times and coordination appear to be back to normal, they are still measurably "under the influence" of the anesthetic.

The study appeared in the April issue of the journal Anesthesiology. Lichtor's colleagues were Richard Alessi, M.D., staff anesthesiologist, Northern CO Anesthesia Professional Consultants, Fort Collins, Colo., and Bradford Lane, pain clinic manager, University of Chicago.

The study was funded in part by the Society for Ambulatory Anesthesia and by the National Institutes of Health.

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