Implicit and explicit memory during TCI of propofol and propofol/remifentanil.

Dr Lequeux PY, Dr Velghe-Lenelle CE, Pr Cantraine F, Pr Levarlet M, Pr Barvais L.

Department of anaesthesiology, Erasmus hospital, ULB, Belgium. (808,route de Lennik. 1070 Brussels)

Background & Goal of the study: Episodes of implicit memory have been described during propofol anaesthesia¹. Propofol is often associated with an opiate like remifentanil which markedly decreases the concentration of propofol associated with loss of consciousness (LOC). This could result in a higher incidence of implicit memory.

Material & Methods, 70 patients were enrolled in 2 successive studies: 20 in a control group (CG), 20 in a reference group (RG), 20 in a propofol group (PG) and 10 in a propofol/remifentanil group (PRG). LOC was obtained by progressive stepwise increases of 0.5 µg/ml of the target blood concentration of propofol using a Diprifusor TCI system. A tape containing 30 words repeated three-times was played either before the start of anaesthesia in the CG, or when LOC was reached in the PG and PRG or never in the RG. Thereafter, general anaesthesia was continued as required for surgery. In the PRG, a TCI of remifentanil with a target blood concentration of 8 ng/ml was started before the TCI of propofol and the stepwise increases of the target blood concentration of propofol to obtain LOC were 0.2 µg/ml. Three different memory tests (word stem completion test, free recall test and forced-choice recognition test²) were performed between 2 and 4 hours after recovery to investigate explicit and implicit memory in the 4 groups. Student’s t-test and Mann-Whitney test were used for statistical analysis.

Results: The four groups were comparable regarding age, sex, duration of surgery and interval recovery-test. Explicit and implicit memories were evidenced in the CG (with a statistical difference compare to the RG in the 3 tests : p<0.01). No implicit nor explicit memory was evidenced in the PG or PRG (no statistical difference compare to the RG : p > 0.05).

Conclusions: In our group of young ASA I/II patients, no explicit or implicit memory was evidenced when the effect site concentration of propofol (used alone or with high effect site concentration of remifentanil) was maintained at the level associated with LOC.

Reference:

1. Munte S et al. Anesthesiology 2000;90:733-738

2. Light LL et al. J exp psychol learn mem cogn 1987;13(4): 534-541