MIDLATENCY AUDITORY-EVOKED POTENTIALS AND EXPLICIT AND IMPLICIT MEMORY IN PATIENTS UNDERGOING CARDIAC-SURGERY

Background: A high incidence of intraoperative awareness during cardia c surgery has been reported. Midlatency auditory evoked potentials (ML AEP) have been used recently as an indicator of awareness. In the curr ent study, memory for information presented during anesthesia was inve stigated using MLAEP as one experimental indicator in 45 patients sche duled for elective cardiac surgery. Methods: In all patients general a nesthesia was maintained using high-dosage fentanyl (1.2 mg . h-1). In addition, the patients of group 1 (n = 10) received flunitrazepam (1. 2 mg . h-1), the patients of group 2 (n = 10) isoflurane (0.6-1.2 vol% ), and the patients of group 3 (n = 10) propofol (4-8 mg . kg-1 . h-1) . Group 4 (n = 15) served as a control, and those patients were assign ed randomly to one of the anesthetic regimes. After sternotomy and bef ore cardiopulmonary bypass, an audiotape, which included an implicit m emory task, was presented to the patients of groups 1-3. Auditory evok ed potentials were recorded while the patients were awake and during g eneral anesthesia immediately before and after the audiotape presentat ion. Latencies of the brain stem peak V and the early cortical potenti als Na and Pa were measured. Results. Three to 5 days postoperatively no patient had a clear explicit memory of intraoperative events. Howev er, there were statistically significant differences in the incidence of implicit recall among the groups. Five patients in the flunitrazepa m-fentanyl group, 1 patient in the isoflurane-fentanyl group, 1 patien t in the propofol-fentanyl group, and no patient in the control group showed an implicit memory of the intraoperative tape message. In the a wake state, MLAEP showed high peak-to-peak amplitudes and a periodic w aveform. In the patients with implicit memory postoperatively, MLAEP c ontinued to show this pattern during general anesthesia. The early cor tical potentials Na and Pa did not increase in latency or decrease in amplitude before or after the audiotape presentation. In contrast, in the patients without implicit memory, MLAEP waveform was severely atte nuated or abolished. Na and Pa showed marked increases in latencies an d decreases in amplitudes or were completely suppressed. In 9 patients , including all patients (7 of 9) with implicit memory, Pa latency inc reased less than 12 ms, and 21 of 23 patients without implicit memory showed a Pa latency increase of greater than 12 ms during anesthesia a nd the audiotape presentation. Therefore, the Pa latency increase of g reater or less than 12 ms may provide sensitivity of 100% and specific ity of 77% in distinguishing patients with implicit memory from patien ts without implicit memory postoperatively. Conclusions. When the earl y cortical potentials of MLAEP are preserved during general anesthesia , auditory information may be processed and remembered postoperatively by an implicit memory task.