Investigation of automatic memory during general anesthesia for elective surgery using the process dissociation procedure

Background: This prospective study evaluated memory function
during general anesthesia for elective surgery and its relation
to depth of hypnotic state. The authors also compared
memory function in anesthetized and nonanesthetized subjects.
Methods: Words were played for 70 min via headphones to 48
patients (aged 18–70 yr) after induction of general anesthesia
for elective surgery. Patients were unpremedicated, and the
anesthetic regimen was free. The Bispectral Index (BIS) was
recorded throughout the study. Within 36 h after the word
presentation, memory was assessed using an auditory word
stem completion test with inclusion and exclusion instructions.
Memory performance and the contribution of explicit and implicit
memory were calculated using the process dissociation
procedure. The authors applied the same memory task to a
control group of nonanesthetized subjects.
Results: Forty-seven patients received isoflurane, and one
patient received propofol for anesthesia. The mean ( SD) BIS
was 49 9. There was evidence of memory for words presented
during light (BIS 61–80) and adequate anesthesia (BIS 41–60)
but not during deep anesthesia (BIS 21–40). The process dissociation
procedure showed a significant implicit memory contribution
but not reliable explicit memory contribution (mean
explicit memory scores 0.05 0.14, 0.04 0.09, and 0.05
0.14; mean automatic influence scores 0.14 0.12, 0.17 0.17,
and 0.18 0.21 at BIS 21–40, 41–60, and 61–80, respectively).
Compared with anesthetized patients, the memory performance
of nonanesthetized subjects was better, with a higher
contribution by explicit memory and a comparable contribution
by implicit memory.
Conclusion: During general anesthesia for elective surgery,
implicit memory persists even in adequate hypnotic states, to a
comparable degree as in nonanesthetized subjects.