Few studies have looked at the link between dreaming and the depth of anesthesia, and those that have investigated the topic have yielded inconclusive results, lead author Dr. Kate Leslie, from Royal Melbourne Hospital, and colleagues note.

They point out that dreams experienced under anesthesia can be distressing to patients, some of whom may think their dream was actual awareness resulting from inadequate anesthesia.

In their study, the researchers assessed the association in 300 consecutive healthy patients who were undergoing elective surgery that required general anesthesia. The Bispectral Index, a measure of the anesthetic effect on the brain, was used to gauge the depth of anesthesia during surgery and the subjects were interviewed about their dreams postoperatively.

Twenty-two percent of patients reported dreaming. No significant difference in the median Bispectral Index value was noted between dreamers and nondreamers, the report indicates.

Correlates of dreaming under anesthesia included younger age, male gender, frequent dream recall at home, receiving propofol maintenance or regional anesthesia, and opening eyes sooner after surgery.

Dreams under anesthesia were similar to those during sleep, were typically pleasant, and the content was not related to surgery, the authors note.

"The similarities between most anesthetic dreams and the dreams of sleep onset suggest that anesthetic dreaming occurs during recovery when patients are in a lightly sedated or physiologic sleep state," Dr. Leslie's team concludes. "Our results are therefore reassuring to patients and their carers, who occasionally may equate dreaming with inadequate anesthesia."

Anesthesiology 2007;106:33-42.