Patients treated with regional anesthesia often require concomitant medication for comfort and sedation. Propofol is widely used for this purpose, but studies upon awareness or post-operative retrograde amnesia are limited. So we designed this study to investigate the frequency of retrograde amnesia on various effect-site concentrations of propofol by using target controlled infusion (TCI).

Ninety patients (ASA I, II) undergoing orthopedic or urogenital surgery with spinal, epidural or axillary block were randomly assigned to one of six groups. Target concentrations of propofol in the six groups were respectively 1.0, 1.2, 1.4, 1.6, 1.8, or 2.0 µg/ml. When the effect site concentration reached the target concentration in each group, we waited for 5 minutes and then lowered the target concentration to 0.5 µg/ml. When the effect site concentration returned to 0.5 µg/ml, we again waited for 5 minutes and then showed the patient number and picture cards and asked the patient to memorize them. Subsequently the target concentration was returned to the original level. In the post anesthesia care unit, retrograde amnesia was checked 2 hours after eye opening and then rechecked 24 hours later.

The percentages of retrograde amnesia checked 2 hours after eye opening were 27%, 27%, 40%, 40%, 40%, and 53% (number card) and 27%, 27%, 40%, 47%, 53%, and 63% (picture card) for propofol dose of 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 µg/ml. A significant correlation was found between the propofol concentration and the percentage of retrograde amnesia for picture but not for the numbered cards. The percentages of retrograde amnesia observed at 24 hours after eye opening were similar to the 2 hour results.

When propofol is used for sedation at 1.0 2.0 µg/ml, retrograde amnesia increases in proportion to the effect site concentration. Many patients can recall intra-operative awareness, and thus we cannot be assured of effective retrograde amnesia. Therefore although a patient is under deep sedation, careful consideration of awareness and recall is needed.